build/torch-cuda/interface.py

# Copyright (c) 2025, Jay Shah, Ganesh Bikshandi, Ying Zhang, Vijay Thakkar, Pradeep Ramani, Tri Dao.
# [2025-07-04] Version in Cute-DSL, for Hopper and Blackwell. You'll need install nvidia-cutlass-dsl==4.2.0.

import os
import math
from dataclasses import dataclass
from functools import lru_cache
from typing import Optional, Tuple, Callable

import torch


import cuda.bindings.driver as cuda

import cutlass
import cutlass.cute as cute
from cutlass import Int32, Float32
from quack.compile_utils import make_fake_tensor as fake_tensor
from .cache_utils import get_jit_cache
from .testing import is_fake_mode


if os.environ.get("CUTE_DSL_PTXAS_PATH", None) is not None:
    from . import cute_dsl_ptxas  # noqa: F401

    # Patch to dump ptx and then use system ptxas to compile to cubin
    cute_dsl_ptxas.patch()


from . import utils
from . import fa_logging
from .cute_dsl_utils import (
    to_cute_tensor, to_cute_aux_tensor, get_aux_tensor_metadata, get_broadcast_dims,
)
from .flash_fwd import FlashAttentionForwardSm80
from .flash_fwd_sm90 import FlashAttentionForwardSm90
from .flash_fwd_sm100 import FlashAttentionForwardSm100, DescaleTensors
from .flash_fwd_sm120 import FlashAttentionForwardSm120
from .flash_fwd_sm120_tma import FlashAttentionForwardSm120Tma
from .flash_bwd_preprocess import FlashAttentionBackwardPreprocess
from .flash_bwd import FlashAttentionBackwardSm80
from .flash_bwd_sm90 import FlashAttentionBackwardSm90
from .flash_bwd_sm100 import FlashAttentionBackwardSm100
from .flash_bwd_sm120 import FlashAttentionBackwardSm120
from .flash_bwd_postprocess import FlashAttentionBackwardPostprocess
from .flash_fwd_combine import FlashAttentionForwardCombine
from .flash_fwd_mla_sm100 import FlashAttentionMLAForwardSm100

from .block_sparsity import (
    BlockSparseTensorsTorch,
    get_sparse_q_block_size,
    to_cute_block_sparse_tensors,
    normalize_block_sparse_config,
    normalize_block_sparse_config_bwd,
)

def _parse_arch_str(arch_str):
    """Parse arch string (e.g. 'sm_80', 'sm_90a', '80', '100') to int (e.g. 80, 90, 100)."""
    import re
    match = re.match(r"^(?:sm_?|SM_?)?(\d+)(\d)([af]?)$", arch_str)
    if not match:
        raise ValueError(f"Invalid arch format: {arch_str}")
    major, minor, _ = match.groups()
    return int(major) * 10 + int(minor)


@lru_cache(maxsize=None)
def _get_device_arch():
    """Cached device arch check.

    Override with FLASH_ATTENTION_ARCH (e.g. 'sm_80' or '80') to select which
    kernel path to use (SM80/SM90/SM100/SM120) independently of the compilation
    target (CUTE_DSL_ARCH).

    For CPU-only compilation (no GPU), set both:
      FLASH_ATTENTION_ARCH=sm_80  (kernel selection)
      CUTE_DSL_ARCH=sm_80         (compilation target)
    """
    arch_override = os.environ.get("FLASH_ATTENTION_ARCH", None)
    if arch_override is not None:
        return _parse_arch_str(arch_override)
    major, minor = torch.cuda.get_device_capability()
    return major * 10 + int(minor)


def _validate_head_dims(head_dim: int, head_dim_v: int, compute_capability: int, alignment: int) -> None:
    """Validate head dimension constraints based on compute capability."""
    is_deepseek_shape = head_dim == 192 and head_dim_v == 128
    is_deepseek_mla_absorbed_shape = head_dim == 64  and head_dim_v == 512
    is_standard_range = 8 <= head_dim <= 128 and 8 <= head_dim_v <= 128

    is_sm90_range = 8 <= head_dim <= 256 and 8 <= head_dim_v <= 256
    if compute_capability == 9:
        assert is_sm90_range and head_dim % alignment == 0 and head_dim_v % alignment == 0, (
            f"(head_dim, head_dim_v)=({head_dim}, {head_dim_v}) is not supported on SM90. "
            f"head_dim and head_dim_v must be between 8 and 256 and divisible by {alignment}."
        )
    elif compute_capability in [10, 11]:
        assert (is_standard_range or is_deepseek_shape or is_deepseek_mla_absorbed_shape) and head_dim % alignment == 0 and head_dim_v % alignment == 0, (
            f"(head_dim, head_dim_v)=({head_dim}, {head_dim_v}) is not supported on SM100/SM110. "
            f"head_dim and head_dim_v must be between 8 and 128 and divisible by {alignment}, or (192, 128) for DeepSeek."
        )


@dataclass(frozen=True)
class FwdConfig:
    m_block_size: int
    n_block_size: int
    mma_pv_is_rs: bool
    intra_wg_overlap: bool


def _tile_size_fwd_sm90(head_dim, head_dim_v, is_causal, is_local, sparse_block_size_q=None):
    """Return FwdConfig for SM90 forward.

    Tile sizes and flags based on tile_size_fwd_sm90 in hopper/tile_size.h, adjusted
    for the Python kernel's different register/smem tradeoffs (benchmarked on H100 SXM).

    When sparse_block_size_q is set, tile_m must divide it. For head_dim <= 96 the
    optimal tile_m=192 is used when compatible, otherwise we fall back to 128.
    """
    if head_dim <= 64:
        # C++: 192×192 non-causal, 192×128 causal/local.
        # Python: 192×128 RS+OL is consistently best across seqlens.
        if sparse_block_size_q is not None and sparse_block_size_q % 192 != 0:
            return FwdConfig(128, 128, True, True)
        return FwdConfig(192, 128, True, True)
    elif head_dim <= 96:
        # C++: 192×144 noRS+OL for all cases.
        # Python: RS is catastrophic with 192× tiles (~300 vs ~600 TFLOPS).
        # noRS+OL is always required. Causal: 192×128 slightly better short seqlen.
        if sparse_block_size_q is not None and sparse_block_size_q % 192 != 0:
            return FwdConfig(128, 128, False, True)
        if is_causal or is_local:
            return FwdConfig(192, 128, False, True)
        else:
            return FwdConfig(192, 144, False, True)
    elif head_dim <= 128:
        return FwdConfig(128, 128, True, True)
    elif head_dim <= 192:
        tile_n = 96 if is_local else (128 if head_dim_v <= 128 else 112)
        return FwdConfig(128, tile_n, True, True)
    else:  # hdim 256
        tile_n = 64 if is_local else 80
        return FwdConfig(128, tile_n, True, True)

@dataclass(frozen=True)
class BwdConfig:
    m_block_size: int
    n_block_size: int
    num_stages_Q: int
    num_stages_dO: int
    num_stages_PdS: int
    SdP_swapAB: bool
    dKV_swapAB: bool
    dQ_swapAB: bool
    AtomLayoutMSdP: int
    AtomLayoutNdKV: int
    AtomLayoutMdQ: int
    num_wg: int = 2  # MMA warp groups (total threads = (num_wg + 1) * 128)
    dQ_single_wg: bool = False


def _tile_size_bwd_sm90(head_dim, head_dim_v, causal, local, sparse_block_size_q=None):
    """Return BwdConfig for SM90.

    Configs based on C++ FA3 hopper/flash_bwd_launch_template.h,
    benchmarked on H100 SXM.
    """
    if head_dim <= 64:
        # C++ FA3: 128, 128, 64, ..., 2, 2, true, false, false, 2, 1, 2, 2
        return BwdConfig(
            m_block_size=128, n_block_size=128,
            num_stages_Q=2, num_stages_dO=2, num_stages_PdS=2,
            SdP_swapAB=True, dKV_swapAB=False, dQ_swapAB=False,
            AtomLayoutMSdP=1, AtomLayoutNdKV=2, AtomLayoutMdQ=2,
        )
    elif head_dim <= 96:
        # C++ FA3: 64, 128, 96, dQ_swapAB=False
        return BwdConfig(
            m_block_size=64, n_block_size=128,
            num_stages_Q=2, num_stages_dO=2, num_stages_PdS=2,
            SdP_swapAB=True, dKV_swapAB=False, dQ_swapAB=False,
            AtomLayoutMSdP=1, AtomLayoutNdKV=2, AtomLayoutMdQ=1,
            dQ_single_wg=True,
        )
    elif head_dim <= 128:
        # C++ FA3: causal/local: 64, 128; non-causal: 80, 128 with dQ_swapAB
        is_causal_or_local = causal or local
        m_block_size = 64 if is_causal_or_local else 80
        if sparse_block_size_q is not None and sparse_block_size_q % m_block_size != 0:
            m_block_size = 64
        return BwdConfig(
            m_block_size=m_block_size,
            n_block_size=128,
            num_stages_Q=2, num_stages_dO=2, num_stages_PdS=2,
            SdP_swapAB=True, dKV_swapAB=False,
            dQ_swapAB=m_block_size % 64 != 0,
            AtomLayoutMSdP=1, AtomLayoutNdKV=2, AtomLayoutMdQ=1,
        )
    elif head_dim <= 192:
        hdimv128 = head_dim_v <= 128
        if hdimv128:
            return BwdConfig(
                m_block_size=64, n_block_size=96,
                num_stages_Q=2, num_stages_dO=2, num_stages_PdS=1,
                SdP_swapAB=False, dKV_swapAB=True, dQ_swapAB=False,
                AtomLayoutMSdP=1, AtomLayoutNdKV=2, AtomLayoutMdQ=1,
                num_wg=2,
            )
        else:
            return BwdConfig(
                m_block_size=64, n_block_size=96,
                num_stages_Q=2, num_stages_dO=1, num_stages_PdS=1,
                SdP_swapAB=False, dKV_swapAB=True, dQ_swapAB=False,
                AtomLayoutMSdP=1, AtomLayoutNdKV=2, AtomLayoutMdQ=1,
                num_wg=2,
            )
    else:
        # hdim 256
        return BwdConfig(
            m_block_size=64, n_block_size=64,
            num_stages_Q=1, num_stages_dO=1, num_stages_PdS=1,
            SdP_swapAB=False, dKV_swapAB=False, dQ_swapAB=False,
            AtomLayoutMSdP=1, AtomLayoutNdKV=1, AtomLayoutMdQ=1,
        )



def maybe_contiguous(x):
    return x.contiguous() if x is not None and x.stride(-1) != 1 else x


def _validate_tensor(t, name, expected_shape, expected_dtype, expected_device):
    assert t.shape == expected_shape, f"{name} shape {t.shape} != expected {expected_shape}"
    assert t.dtype == expected_dtype, f"{name} dtype {t.dtype} != expected {expected_dtype}"
    assert t.device == expected_device, f"{name} device {t.device} != expected {expected_device}"
    if not is_fake_mode():
        assert t.is_cuda, f"{name} must be on CUDA"

torch2cute_dtype_map = {
    torch.float16: cutlass.Float16,
    torch.bfloat16: cutlass.BFloat16,
    torch.float32: cutlass.Float32,
    torch.float8_e4m3fn: cutlass.Float8E4M3FN,
    torch.float8_e5m2: cutlass.Float8E5M2,
}


def num_splits_heuristic(total_mblocks, num_SMs, num_n_blocks, max_splits):
    # If num_n_blocks is too small, use 1 split. For example, we never split for hdim = 128 and seqlen_k = 512.
    if num_n_blocks <= 4:
        return 1

    # NOTE: We should revisit this heuristic after persistence is supported for split KV.
    # Sometimes, it's ideal to over-schedule splits for better efficiency.
    return min(num_SMs // total_mblocks, max_splits, num_n_blocks)


def _resolve_causal_local_window(causal, window_size_left, window_size_right, mask_mod=None):
    """Resolve causal/local/window settings into canonical form.

    Returns (causal, local, window_size_left, window_size_right).
    """
    if mask_mod is not None:
        return False, False, window_size_left, window_size_right
    if causal:
        window_size_right = 0
    if window_size_left is not None and window_size_right is not None and window_size_left + window_size_right < 0:
        window_size_left = None
        window_size_right = None
    if window_size_left is not None or window_size_right is not None:
        if window_size_left is None and window_size_right == 0:
            causal, local = True, False
            window_size_right = None
        else:
            causal, local = False, True
    else:
        local = False
    return causal, local, window_size_left, window_size_right


def _flash_attn_fwd(
    q: torch.Tensor,
    k: torch.Tensor,
    v: torch.Tensor,
    qv: Optional[torch.Tensor] = None,
    cu_seqlens_q: Optional[torch.Tensor] = None,
    cu_seqlens_k: Optional[torch.Tensor] = None,
    seqused_q: Optional[torch.Tensor] = None,
    seqused_k: Optional[torch.Tensor] = None,
    max_seqlen_q: Optional[int] = None,
    max_seqlen_k: Optional[int] = None,
    min_seqlen_k: Optional[int] = None,
    page_table: Optional[torch.Tensor] = None,
    softmax_scale: Optional[float] = None,
    causal: bool = False,
    softcap: Optional[float] = None,
    window_size_left: Optional[int] = None,
    window_size_right: Optional[int] = None,
    learnable_sink: Optional[torch.Tensor] = None,
    tile_mn: Optional[Tuple[int, int]] = None,
    mma_pv_is_rs: Optional[bool] = None,
    intra_wg_overlap: Optional[bool] = None,
    num_threads: int = 384,
    num_splits: int = 1,
    pack_gqa: Optional[bool] = None,
    _arch: Optional[int] = None,
    score_mod: Optional[Callable] = None,
    mask_mod: Optional[Callable] = None,
    block_sparse_tensors: Optional[BlockSparseTensorsTorch] = None,
    return_lse: bool = False,
    out: Optional[torch.Tensor] = None,
    lse: Optional[torch.Tensor] = None,
    aux_tensors: Optional[list[torch.Tensor]] = None,
    q_descale: Optional[torch.Tensor] = None,
    k_descale: Optional[torch.Tensor] = None,
    v_descale: Optional[torch.Tensor] = None,
    gather_kv_indices: Optional[torch.Tensor] = None,
    dropout_p: float = 0.0,
    dropout_seed: Optional[int] = None,
) -> Tuple[torch.Tensor, torch.Tensor]:
    """Forward pass for FlashAttention.

    Args:
        ...
        score_mod: A callable that takes the attention scores and applies a modification.
        mask_mod: A callable that takes token position information and selectively masks
        block_sparse_tensors: A tuple of tensors used for block sparsity.
        return_lse: Whether to return the log softmax of the attention scores. If set to True will always calculate
            The returned LSE supports taking gradient.
        out: Optional pre-allocated output tensor. If None, will be allocated internally.
        lse: Optional pre-allocated log-sum-exp tensor. If None, will be allocated when needed.
        aux_tensors: Some score_mods will want to read from global aux_tensors. This is how we thread them through to the inner kernel.
        dropout_p: Dropout probability (0.0 = disabled).
        dropout_seed: RNG seed for dropout mask (auto-generated if None and dropout_p > 0).
    """
    q, k, v = [maybe_contiguous(t) for t in (q, k, v)]
    q_descale, k_descale, v_descale = [maybe_contiguous(t) for t in (q_descale, k_descale, v_descale)]
    num_head, head_dim = q.shape[-2:]
    if cu_seqlens_q is None:
        batch_size, seqlen_q = q.shape[:2]
        total_q = batch_size * seqlen_q
    else:
        batch_size = cu_seqlens_q.shape[0] - 1
        seqlen_q = None
        total_q = q.shape[0]
    if page_table is not None:
        assert cu_seqlens_k is None, "page_table is not supported with cu_seqlens_k"
        assert page_table.dtype == torch.int32, "page_table must be int32"
        assert page_table.stride(-1) == 1, "page_table must be contiguous in the last dimension"
        max_num_pages_per_seq = page_table.shape[1]
        assert page_table.shape == (batch_size, max_num_pages_per_seq)
        num_pages, page_size = k.shape[:2]
        seqlen_k = num_pages * page_size
    else:
        num_pages, page_size = None, None
        seqlen_k = k.shape[-3]
    num_head_kv = k.shape[-2]
    head_dim_v = v.shape[-1]
    if cu_seqlens_k is None:
        if page_table is None:
            assert k.shape == (batch_size, seqlen_k, num_head_kv, head_dim)
            assert v.shape == (batch_size, seqlen_k, num_head_kv, head_dim_v)
        else:
            assert k.shape == (num_pages, page_size, num_head_kv, head_dim)
            assert v.shape == (num_pages, page_size, num_head_kv, head_dim_v)
    else:
        assert k.shape == (seqlen_k, num_head_kv, head_dim)
        assert v.shape == (seqlen_k, num_head_kv, head_dim_v)
        assert cu_seqlens_k.shape == (batch_size + 1,), (
            "cu_seqlens_k must have shape (batch_size + 1,)"
        )

    if cu_seqlens_q is not None:
        assert cu_seqlens_q.shape == (batch_size + 1,), (
            "cu_seqlens_q must have shape (batch_size + 1,)"
        )
    assert seqused_q is None or seqused_q.shape == (batch_size,), (
        "seqused_q must have shape (batch_size,)"
    )
    assert seqused_k is None or seqused_k.shape == (batch_size,), (
        "seqused_k must have shape (batch_size,)"
    )
    assert q.dtype in [torch.float16, torch.bfloat16, torch.float8_e4m3fn, torch.float8_e5m2], (
        "inputs must be float16, bfloat16, fp8 e4m3fn, or fp8 e5m2"
    )
    assert q.dtype == k.dtype == v.dtype, "inputs must have the same dtype"
    for t in [cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k]:
        if t is not None:
            assert t.dtype == torch.int32, (
                "cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k must be int32"
            )
            assert t.stride(0) == 1, (
                "cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k must be contiguous"
            )
    if learnable_sink is not None:
        assert learnable_sink.shape == (num_head,)
        assert learnable_sink.dtype == torch.bfloat16, "learnable_sink must be bfloat16"

    if not is_fake_mode():
        assert all(
            t is None or t.is_cuda
            for t in (
                q,
                k,
                v,
                q_descale,
                k_descale,
                v_descale,
                cu_seqlens_q,
                cu_seqlens_k,
                seqused_q,
                seqused_k,
                page_table,
                learnable_sink,
            )
        ), "inputs must be on CUDA device"
    arch = _get_device_arch() if _arch is None else _arch
    assert arch // 10 in [8, 9, 10, 11, 12], "Unsupported compute capability. Supported: 8.x, 9.x, 10.x, 11.x, 12.x"
    assert num_head % num_head_kv == 0, "num_head must be divisible by num_head_kv"
    alignment = 16 // q.element_size()
    if arch // 10 not in [8, 12]:
        _validate_head_dims(head_dim, head_dim_v, arch // 10, alignment)
    if softmax_scale is None:
        softmax_scale = 1.0 / math.sqrt(head_dim) if qv is None else 1.0 / math.sqrt(head_dim + head_dim_v)
    if softcap == 0.0:
        softcap = None
    qhead_per_kvhead = num_head // num_head_kv
    if pack_gqa is None:
        pack_gqa = qhead_per_kvhead > 1

    is_fp8 = q.dtype in (torch.float8_e4m3fn, torch.float8_e5m2)
    if is_fp8 and (q.requires_grad or k.requires_grad or v.requires_grad):
        raise NotImplementedError("FA4 CuTe FP8 backward is not supported yet (forward-only).")
    out_torch_dtype = torch.bfloat16 if is_fp8 else q.dtype
    device = q.device
    q_batch_seqlen_shape = (batch_size, seqlen_q) if cu_seqlens_q is None else (total_q,)
    lse_shape = (batch_size, num_head, seqlen_q) if cu_seqlens_q is None else (num_head, total_q)
    requires_grad = q.requires_grad or k.requires_grad or v.requires_grad

    if out is None:
        out = torch.empty(
            *q_batch_seqlen_shape, num_head, head_dim_v, dtype=out_torch_dtype, device=device
        )
    else:
        _validate_tensor(out, "out", (*q_batch_seqlen_shape, num_head, head_dim_v), out_torch_dtype, device)

    if lse is None:
        lse = (
            torch.empty(lse_shape, dtype=torch.float32, device=device)
            if requires_grad or return_lse
            else None
        )
    elif lse is not None:
        _validate_tensor(lse, "lse", lse_shape, torch.float32, device)

    if is_fp8:
        for t, name in ((q_descale, "q_descale"), (k_descale, "k_descale"), (v_descale, "v_descale")):
            if t is not None:
                _validate_tensor(t, name, (batch_size, num_head_kv), torch.float32, device)
    else:
        assert q_descale is None and k_descale is None and v_descale is None, (
            "q_descale/k_descale/v_descale are only supported for FP8 inputs"
        )

    dtype = torch2cute_dtype_map[q.dtype]
    if is_fp8:
        assert arch // 10 == 10, "FP8 is only supported on SM100 (compute capability 10.x) for FA4 CuTe."
    use_block_sparsity = block_sparse_tensors is not None

    causal, local, window_size_left, window_size_right = _resolve_causal_local_window(
        causal, window_size_left, window_size_right, mask_mod
    )

    requested_use_clc_scheduler = utils._get_use_clc_scheduler_default()
    requested_disable_2cta = utils._get_disable_2cta_default()

    current_stream = cute.runtime.make_fake_stream(use_tvm_ffi_env_stream=True)

    # SM80/SM120: uses SM80 MMA, 128 threads (4 warps)
    if arch // 10 in [8, 12]:
        num_threads = 128

    fwd_cfg = FwdConfig(128, 128, True, True)  # default
    if tile_mn is None:
        if arch // 10 == 12:
            # SM120 tile sizes tuned for 99 KB SMEM capacity.
            # Dropout Philox PRNG causes register spilling at 128x128
            # non-causal. Split precompute/apply enables INT32/FP32
            # overlap; 128x64 avoids the spilling.
            if dropout_p > 0.0:
                if head_dim <= 64:
                    fwd_cfg = FwdConfig(128, 64, True, True)
                else:
                    fwd_cfg = FwdConfig(64, 64, True, True)
            elif head_dim <= 64:
                fwd_cfg = FwdConfig(128, 128, True, True)
            else:
                fwd_cfg = FwdConfig(128, 64, True, True)
        elif arch // 10 == 8:
            fwd_cfg = FwdConfig(128, 64, True, True)  # SM80, should tune
        elif arch // 10 == 9:
            sparse_q = get_sparse_q_block_size(block_sparse_tensors, seqlen_q)
            fwd_cfg = _tile_size_fwd_sm90(head_dim, head_dim_v, causal, local, sparse_block_size_q=sparse_q)
    else:
        fwd_cfg = FwdConfig(tile_mn[0], tile_mn[1], fwd_cfg.mma_pv_is_rs, fwd_cfg.intra_wg_overlap)
    tile_m, tile_n = fwd_cfg.m_block_size, fwd_cfg.n_block_size
    if mma_pv_is_rs is None:
        mma_pv_is_rs = fwd_cfg.mma_pv_is_rs
    if intra_wg_overlap is None:
        intra_wg_overlap = fwd_cfg.intra_wg_overlap

    # TODO: fix GQA + SplitKV + non-varlen
    if pack_gqa and num_splits != 1 and cu_seqlens_q is None:
        pack_gqa = False
    
    if pack_gqa and qv is not None and 128 % qhead_per_kvhead != 0:
        pack_gqa = False

    if max_seqlen_q is None:
        max_seqlen_q = seqlen_q if cu_seqlens_q is None else total_q
    if max_seqlen_k is None:
        max_seqlen_k = seqlen_k
    if cu_seqlens_k is None and seqused_k is None:
        min_seqlen_k = seqlen_k 
    seqlen_q_packgqa = max_seqlen_q * qhead_per_kvhead
    if arch // 10 == 10:
        q_stage = 2 if seqlen_q_packgqa > tile_m else 1
    else:
        q_stage = 1

    m_block_size_effective = q_stage * tile_m
    seqlen_k_loaded = max_seqlen_k if not local else max(0, min(max_seqlen_k, (window_size_right or max_seqlen_k) + (window_size_left or max_seqlen_k) + 1 + tile_m))
    num_m_blocks = (seqlen_q_packgqa + m_block_size_effective - 1) // m_block_size_effective
    total_mblocks = batch_size * num_head_kv * num_m_blocks
    num_n_blocks = (seqlen_k_loaded + tile_n - 1) // tile_n
    num_SMs = 132 if is_fake_mode() else torch.cuda.get_device_properties(device).multi_processor_count
    if num_splits < 1:
        num_splits = num_splits_heuristic(total_mblocks, num_SMs, num_n_blocks, 128)

    # SM120 does not support SplitKV in this kernel variant
    if arch // 10 == 12 and num_splits > 1:
        num_splits = 1

    # SplitKV uses float32 partial output, which doubles the O buffer size
    # in shared memory, causing OOM for diff-headdim (192, 128)
    if arch // 10 in [10, 11] and head_dim != head_dim_v and num_splits > 1:
        if num_n_blocks >= 64 and head_dim_v != 512:
            tile_n = 64
            num_n_blocks = (seqlen_k_loaded + tile_n - 1) // tile_n
            num_splits = num_splits_heuristic(total_mblocks, num_SMs, num_n_blocks, 128)
        else:
            num_splits = 1

    is_split_kv = num_splits > 1
    if is_split_kv:
        out_partial = torch.zeros(num_splits, *q_batch_seqlen_shape, num_head, head_dim_v, dtype=torch.float32, device=device)
        lse_partial = torch.full((num_splits, *lse_shape), float('-inf'), dtype=torch.float32, device=device)

    use_2cta_instrs = (
        arch // 10 in [10, 11]
        and not requested_disable_2cta
        and not causal
        and not local
        and not is_split_kv
        and cu_seqlens_q is None
        and seqused_q is None
        and not use_block_sparsity
        and page_size in [None, 128]
        and int(math.ceil(head_dim / 16) * 16) in [128, 192]
        and int(math.ceil(head_dim_v / 16) * 16) == 128
        and seqlen_q_packgqa > 2 * tile_m
        and (tile_m % qhead_per_kvhead == 0 or not pack_gqa)
    )

    if softcap is not None:
        assert score_mod is None, "softcap and score_mod cannot be used together"
        score_mod = utils.create_softcap_scoremod(softcap)
    elif score_mod is not None:
        if arch // 10 == 8:
            raise NotImplementedError("Custom user-provided score_mod is not supported on SM8x architectures.")
        
    # hash score and mask mods for compile cache
    score_mod_hash = utils.hash_callable(score_mod) if score_mod is not None else False
    mask_mod_hash = utils.hash_callable(mask_mod) if mask_mod is not None else False

    is_varlen = (
        cu_seqlens_q is not None
        or cu_seqlens_k is not None
        or seqused_q is not None
        or seqused_k is not None
    )

    # CLC regressed for varlen MHA and dense noncausal. Imbalanced varlen shapes
    # keep more K/V blocks in flight and hurt L2; dense noncausal mostly just
    # pays work-stealing overhead.
    is_varlen_mha = is_varlen and qhead_per_kvhead == 1
    is_dense_noncausal = not is_varlen and not causal and not local
    use_clc_scheduler = requested_use_clc_scheduler and not is_varlen_mha and not is_dense_noncausal

    if mask_mod is not None:
        if is_varlen:
            raise NotImplementedError(
                "mask_mod with aux_tensors is not yet supported for varlen sequences. This will be fixed in a future PR."
            )

    if use_block_sparsity:
        if is_varlen:
            raise NotImplementedError(
                "Block sparsity is not yet supported for varlen sequences. This will be fixed in a future PR."
            )
        # NB: pack_gqa requires block sparse head dim == 1 (broadcasted)
        if pack_gqa and block_sparse_tensors.mask_block_cnt.shape[1] != 1:
            pack_gqa = False
        if is_split_kv:
            raise NotImplementedError(
                "Block sparsity is not yet supported with SplitKV. TODO: partition sparse block lists per split."
            )

    # See get_broadcast_dims for why this is needed in compile key
    block_sparse_broadcast_pattern = None
    normalized_block_sparse_tensors = None
    q_subtile_factor = None
    if block_sparse_tensors is not None:
        if seqlen_q is None:
            raise ValueError("Block sparsity requires fixed-length sequences (seqlen_q must be known).")
        (
            normalized_block_sparse_tensors,
            block_sparse_broadcast_pattern,
            q_subtile_factor,
        ) = normalize_block_sparse_config(
            block_sparse_tensors,
            batch_size=batch_size,
            num_head=num_head,
            seqlen_q=seqlen_q,
            seqlen_k=seqlen_k,
            block_size=(tile_m, tile_n),
            q_stage=q_stage,
        )
    if aux_tensors is not None:
        aux_tensor_metadata = get_aux_tensor_metadata(aux_tensors)
    else:
        aux_tensor_metadata = None

    if qv is not None:
        assert arch // 10 in [10, 11], "only support Blackwell arch with qv"
        assert qv.shape[:-1] == q.shape[:-1]
        assert qv.shape[-1] == head_dim_v
        assert head_dim == 64 and head_dim_v == 512, "only support MLA weight absorbed shape with qv"
        assert not local, "local not yet supported with qv"
        assert page_table is None, "page table not yet supported with qv"
        assert q_descale is None and k_descale is None and v_descale is None, (
            "q_descale/k_descale/v_descale are not yet supported with qv"
        )

        assert not is_split_kv, "split kv not supported with qv"
        assert learnable_sink is None
        assert softcap is None
        assert score_mod is None
        assert mask_mod is None
        
        qv = maybe_contiguous(qv)

        gather_kv_length = 2048
        sparse_kv = gather_kv_indices is not None
        disable_sparse_kv_bitmask = False
        if sparse_kv:
            assert gather_kv_indices.shape[:-1] == q.shape[:-2]
            gather_kv_length = gather_kv_indices.shape[-1]
            assert gather_kv_length % 256 == 0
            if min_seqlen_k is None or causal:
                disable_sparse_kv_bitmask = False
            else:
                # seqlen_k_boundary = min_seqlen_k - max_seqlen_q + 1 if causal else min_seqlen_k
                seqlen_k_boundary = min_seqlen_k
                disable_sparse_kv_bitmask = seqlen_k_boundary >= gather_kv_length
    else:
        assert gather_kv_indices is None, "gather_kv_indices is only supported with qv"
        gather_kv_length = None
        sparse_kv = None
        disable_sparse_kv_bitmask = None

    compile_key = (
        dtype,
        head_dim,
        head_dim_v,
        qhead_per_kvhead,
        causal,
        score_mod_hash,
        mask_mod_hash,
        use_block_sparsity,
        block_sparse_broadcast_pattern,
        aux_tensor_metadata,
        lse is None,
        cu_seqlens_q is None,
        cu_seqlens_k is None,
        seqused_q is None,
        seqused_k is None,
        page_table is not None,
        window_size_left is not None,
        window_size_right is not None,
        learnable_sink is not None,
        q_descale is not None,
        k_descale is not None,
        v_descale is not None,
        tile_m,
        tile_n,
        q_stage,
        num_threads,
        is_split_kv,
        pack_gqa,
        arch,
        page_size not in [None, tile_n],  # paged KV non-TMA
        use_2cta_instrs,
        q_subtile_factor,
        mma_pv_is_rs,
        intra_wg_overlap,
        requested_use_clc_scheduler,
        qv is not None,
        gather_kv_length,
        sparse_kv,
        disable_sparse_kv_bitmask,
        fa_logging.get_fa_log_level(),
        dropout_p,
    )
    if compile_key not in _flash_attn_fwd.compile_cache:
        (
            cu_seqlens_q_tensor,
            cu_seqlens_k_tensor,
            seqused_q_tensor,
            seqused_k_tensor,
            learnable_sink_tensor,
        ) = [
            to_cute_tensor(t, assumed_align=4, leading_dim=0)
            if t is not None
            else None
            for t in (cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k, learnable_sink)
        ]
        page_table_tensor = (
            to_cute_tensor(page_table, assumed_align=4, leading_dim=1)
            if page_table is not None
            else None
        )
        q_tensor, k_tensor, v_tensor, o_tensor = [
            to_cute_tensor(t) for t in (q, k, v, out if not is_split_kv else out_partial)
        ]
        if is_split_kv:
            lse_tensor = to_cute_tensor(lse_partial, assumed_align=4)
        elif lse is not None:
            lse_tensor = to_cute_tensor(lse, assumed_align=4)
        else:
            lse_tensor = None

        q_descale_tensor = (
            to_cute_tensor(q_descale, assumed_align=4, leading_dim=1)
            if q_descale is not None
            else None
        )
        k_descale_tensor = (
            to_cute_tensor(k_descale, assumed_align=4, leading_dim=1)
            if k_descale is not None
            else None
        )
        v_descale_tensor = (
            to_cute_tensor(v_descale, assumed_align=4, leading_dim=1)
            if v_descale is not None
            else None
        )
        descale_tensors_tensor = (
            DescaleTensors(
                q_descale=q_descale_tensor,
                k_descale=k_descale_tensor,
                v_descale=v_descale_tensor,
            )
            if q_descale_tensor is not None
            or k_descale_tensor is not None
            or v_descale_tensor is not None
            else None
        )

        sparse_tensors = None
        if normalized_block_sparse_tensors is not None:
            sparse_tensors = to_cute_block_sparse_tensors(normalized_block_sparse_tensors)

        cute_aux_tensors = None
        aux_tensor_metadata = None
        if aux_tensors is not None:
            cute_aux_tensors = [to_cute_aux_tensor(buf) for buf in aux_tensors]

        qv_tensor = to_cute_tensor(qv) if qv is not None else None
        gather_kv_indices_tensor = to_cute_tensor(gather_kv_indices) if gather_kv_indices is not None else None

        if arch // 10 == 8:
            assert page_table is None, "paged KV not supported on SM 8.0"
            assert not is_split_kv, "SplitKV not supported on SM 8.0"
            fa_fwd = FlashAttentionForwardSm80(
                dtype,
                head_dim,
                head_dim_v,
                qhead_per_kvhead,
                is_causal=causal,
                is_local=local,
                pack_gqa=pack_gqa,
                tile_m=tile_m,
                tile_n=tile_n,
                num_stages=1,
                num_threads=num_threads,
                Q_in_regs=False,
                score_mod=score_mod,
                mask_mod=mask_mod,
                has_aux_tensors=aux_tensors is not None,
                p_dropout=dropout_p,
            )
        elif arch // 10 == 9:
            assert not is_split_kv, "SplitKV not supported on SM 9.0"
            fa_fwd = FlashAttentionForwardSm90(
                dtype,
                head_dim,
                head_dim_v,
                qhead_per_kvhead,
                is_causal=causal,
                is_local=local,
                pack_gqa=pack_gqa,
                tile_m=tile_m,
                tile_n=tile_n,
                # num_stages=1,
                num_stages=2,
                num_threads=num_threads,
                Q_in_regs=False,
                intra_wg_overlap=intra_wg_overlap,
                mma_pv_is_rs=mma_pv_is_rs,
                mask_mod=mask_mod,
                score_mod=score_mod,
                has_aux_tensors=aux_tensors is not None,
                q_subtile_factor=q_subtile_factor,
                paged_kv_non_tma=page_size not in [None, tile_n],
                p_dropout=dropout_p,
            )
        elif arch // 10 in [10, 11]:
            if qv is not None:
                fa_fwd = FlashAttentionMLAForwardSm100(
                    is_causal=causal,
                    use_cpasync_load_KV=sparse_kv,
                    topk_length=gather_kv_length,
                    is_topk_gather=sparse_kv,
                    pack_gqa=pack_gqa,
                    qhead_per_kvhead=qhead_per_kvhead,
                    nheads_kv=num_head_kv,
                    is_varlen_q=cu_seqlens_q is not None or seqused_q is not None,
                    disable_bitmask=disable_sparse_kv_bitmask,
                )
            else:
                fa_fwd = FlashAttentionForwardSm100(
                    head_dim,
                    head_dim_v,
                    qhead_per_kvhead=qhead_per_kvhead,
                    is_causal=causal,
                    is_local=local,
                    is_split_kv=is_split_kv,
                    pack_gqa=pack_gqa,
                    m_block_size=tile_m,
                    n_block_size=tile_n,
                    q_stage=q_stage,
                    is_persistent=not causal
                        and not local
                        and cu_seqlens_q is None
                        and seqused_q is None
                        and not is_split_kv,
                    score_mod=score_mod,
                    mask_mod=mask_mod,
                    has_aux_tensors=aux_tensors is not None,
                    paged_kv_non_tma=page_size not in [None, tile_n],
                    is_varlen_q=cu_seqlens_q is not None or seqused_q is not None,
                    q_subtile_factor=q_subtile_factor,
                    use_2cta_instrs=use_2cta_instrs,
                    use_clc_scheduler=use_clc_scheduler,
                    p_dropout=dropout_p,
                )
        elif arch // 10 == 12:
            # SM120 (Blackwell GeForce / DGX Spark): uses SM80 MMA with SM120 SMEM capacity.
            # Bundle-resolved dispatch combines four PRs:
            #   - #2348 paged KV → num_stages=2 in the non-TMA SM120 kernel
            #   - #2349 TMA forward → FlashAttentionForwardSm120Tma kernel
            #   - #2389 block-sparse SM80/SM120 → handled inside the non-TMA kernel's
            #     mainloop (flash_fwd.py), so block-sparse callers skip TMA
            #   - #2439 dropout → p_dropout flows through the non-TMA kernel only;
            #     the TMA kernel does not implement dropout yet
            # Dispatch order:
            #   1. TMA kernel when viable: no paged KV, no varlen, no block-sparse,
            #      no dropout
            #   2. Else non-TMA kernel: num_stages=2 when paged for page-table/MMA
            #      pipelining, else num_stages=1
            is_varlen = cu_seqlens_q is not None or cu_seqlens_k is not None
            use_tma_sm120 = (
                page_table is None
                and not is_varlen
                and not use_block_sparsity
                and dropout_p == 0.0
            )
            if use_tma_sm120 and FlashAttentionForwardSm120Tma.can_implement(
                dtype, head_dim, head_dim_v, tile_m, tile_n,
                num_mma_warps=4, kv_stages=2, is_causal=causal,
            ):
                fa_fwd = FlashAttentionForwardSm120Tma(
                    dtype,
                    head_dim,
                    head_dim_v,
                    qhead_per_kvhead,
                    is_causal=causal,
                    is_local=local,
                    pack_gqa=pack_gqa,
                    tile_m=tile_m,
                    tile_n=tile_n,
                    num_mma_warps=4,
                    kv_stages=2,
                    score_mod=score_mod,
                    mask_mod=mask_mod,
                    has_aux_tensors=aux_tensors is not None,
                )
            else:
                if page_table is not None:
                    assert seqused_k is not None, (
                        "Paged KV on SM120 requires seqused_k "
                        "(actual sequence lengths per batch)"
                    )
                    # num_stages=2 for paged KV: overlap page-table lookups with
                    # MMA. SMEM budget: sQ+sK+sV at tile_n=64 = 96KB ≤ 99KB SM120.
                    num_stages_sm120 = 2
                else:
                    num_stages_sm120 = 1
                fa_fwd = FlashAttentionForwardSm120(
                    dtype,
                    head_dim,
                    head_dim_v,
                    qhead_per_kvhead,
                    is_causal=causal,
                    is_local=local,
                    is_split_kv=is_split_kv,
                    pack_gqa=pack_gqa,
                    tile_m=tile_m,
                    tile_n=tile_n,
                    num_stages=num_stages_sm120,
                    num_threads=num_threads,
                    Q_in_regs=False,
                    score_mod=score_mod,
                    mask_mod=mask_mod,
                    has_aux_tensors=aux_tensors is not None,
                    p_dropout=dropout_p,
                )
        else:
            raise ValueError(
                f"Unsupported compute capability: {arch}. Supported: 8.x, 9.x, 10.x, 11.x, 12.x"
            )
        # TODO: check @can_implement
        if qv is not None:
            _flash_attn_fwd.compile_cache[compile_key] = cute.compile(
                fa_fwd,
                q_tensor,
                qv_tensor,
                k_tensor,
                v_tensor,
                o_tensor,
                lse_tensor,
                softmax_scale,
                cu_seqlens_q_tensor,
                cu_seqlens_k_tensor,
                seqused_q_tensor,
                seqused_k_tensor,
                gather_kv_indices_tensor,
                page_table_tensor,
                window_size_left,
                window_size_right,
                current_stream,
                options="--enable-tvm-ffi",
            )
        else:
            compile_args = [
                fa_fwd,
                q_tensor,
                k_tensor,
                v_tensor,
                o_tensor,
                lse_tensor,
                softmax_scale,
                cu_seqlens_q_tensor,
                cu_seqlens_k_tensor,
                seqused_q_tensor,
                seqused_k_tensor,
                page_table_tensor,
                window_size_left,
                window_size_right,
                learnable_sink_tensor,
                sparse_tensors,
                cute_aux_tensors,
                int(dropout_seed & 0xFFFFFFFF) if dropout_seed is not None and dropout_p > 0.0 else None,
                int((dropout_seed >> 32) & 0xFFFFFFFF) if dropout_seed is not None and dropout_p > 0.0 else None,
                current_stream,
            ]
            if arch // 10 in [10, 11]:
                compile_args.insert(-3, descale_tensors_tensor)
            _flash_attn_fwd.compile_cache[compile_key] = cute.compile(*compile_args, options="--enable-tvm-ffi")

    # In "fake mode", we will take torch fake tensors as input and the expected behaviors are:
    # - Use those fake metadata to populate compilation cache
    # - Return "fake" output tensors, which could be needed in follow-up fake operations
    # Thus, we skip the actual kernel invocation here.
    if not is_fake_mode():
        q_call, k_call, v_call = q.detach(), k.detach(), v.detach()
        qv_call = qv.detach() if qv is not None else None
        if is_fp8:
            # need uint8 workaround until we pin torch >= 2.11.0 where fp8 export is supported
            q_call = q_call.view(torch.uint8)
            k_call = k_call.view(torch.uint8)
            v_call = v_call.view(torch.uint8)
            if qv_call is not None:
                qv_call = qv_call.view(torch.uint8)
        descale_tensors = (
            DescaleTensors(q_descale=q_descale, k_descale=k_descale, v_descale=v_descale)
            if q_descale is not None or k_descale is not None or v_descale is not None
            else None
        )
        if qv is not None:
            _flash_attn_fwd.compile_cache[compile_key](
                q_call,
                qv_call,
                k_call,
                v_call,
                out.detach(),
                lse,
                softmax_scale,
                cu_seqlens_q,
                cu_seqlens_k,
                seqused_q,
                seqused_k,
                gather_kv_indices,
                page_table,
                window_size_left,
                window_size_right,
            )
        else:
            call_args = [
                q_call,
                k_call,
                v_call,
                out.detach() if not is_split_kv else out_partial,
                lse_partial if is_split_kv else lse,
                softmax_scale,
                cu_seqlens_q,
                cu_seqlens_k,
                seqused_q,
                seqused_k,
                page_table,
                window_size_left,
                window_size_right,
                learnable_sink,
            ]
            if arch // 10 in [10, 11]:
                call_args.append(descale_tensors)
            call_args.extend([
                normalized_block_sparse_tensors[:4] if normalized_block_sparse_tensors is not None else None,
                aux_tensors,
                int(dropout_seed & 0xFFFFFFFF) if dropout_seed is not None and dropout_p > 0.0 else None,
                int((dropout_seed >> 32) & 0xFFFFFFFF) if dropout_seed is not None and dropout_p > 0.0 else None,
            ])
            _flash_attn_fwd.compile_cache[compile_key](*call_args)
    if is_split_kv:
        _flash_attn_fwd_combine(
            out_partial,
            lse_partial.transpose(-1, -2),
            out,
            lse.transpose(-1, -2) if lse is not None else None,
            cu_seqlens_q,
            seqused_q,
        )
    return out, lse


_flash_attn_fwd.compile_cache = get_jit_cache("fwd")


def make_fake_bwd_tensors(dtype, has_gqa, varlen_q, varlen_k):
    sym = cute.sym_int
    # divisibility in elements: assumed_align_bytes = divisibility * dtype.width // 8
    # For 16-byte align: fp16/bf16 → divisibility=8, float32 → divisibility=4
    div = 128 // dtype.width  # 8 for fp16/bf16
    # Shared sym_ints for dimensions that must match across tensors
    b, seqlen_q, seqlen_k, h_q, d, d_v = sym(), sym(), sym(), sym(), sym(), sym()
    h_kv = h_q if not has_gqa else sym()
    seqlen_q_rounded, seqlen_k_rounded = sym(), sym()
    seqlen_q_d_rounded, seqlen_k_d_rounded, seqlen_k_dv_rounded = sym(), sym(), sym()
    total_q, total_k, total_q_rounded, total_k_rounded = sym(), sym(), sym(), sym()
    total_q_d_rounded, total_k_d_rounded, total_k_dv_rounded = sym(), sym(), sym()
    b_seqlenq = (b, seqlen_q) if not varlen_q else (total_q,)
    b_seqlenk = (b, seqlen_k) if not varlen_k else (total_k,)
    mQ = fake_tensor(dtype, (*b_seqlenq, h_q, d), divisibility=div)
    mO = fake_tensor(dtype, (*b_seqlenq, h_q, d_v), divisibility=div)
    mdO = fake_tensor(dtype, (*b_seqlenq, h_q, d_v), divisibility=div)
    mK = fake_tensor(dtype, (*b_seqlenk, h_kv, d), divisibility=div)
    mV = fake_tensor(dtype, (*b_seqlenk, h_kv, d_v), divisibility=div)
    mdQ = fake_tensor(dtype, (*b_seqlenq, h_q, d), divisibility=div)
    mdK = fake_tensor(dtype, (*b_seqlenk, h_kv, d), divisibility=div)
    mdV = fake_tensor(dtype, (*b_seqlenk, h_kv, d_v), divisibility=div)
    if not varlen_q:
        mLSE = fake_tensor(Float32, (b, h_q, seqlen_q), divisibility=1)
        mLSElog2 = fake_tensor(Float32, (b, h_q, seqlen_q_rounded), divisibility=4)
        mPdPsum = fake_tensor(Float32, (b, h_q, seqlen_q_rounded), divisibility=4)
        dQaccum = fake_tensor(Float32, (b, h_q, seqlen_q_d_rounded), divisibility=4)
    else:
        mLSE = fake_tensor(Float32, (h_q, total_q), divisibility=1)
        mLSElog2 = fake_tensor(Float32, (h_q, total_q_rounded), divisibility=4)
        mPdPsum = fake_tensor(Float32, (h_q, total_q_rounded), divisibility=4)
        dQaccum = fake_tensor(Float32, (h_q, total_q_d_rounded), divisibility=4)
    if not has_gqa:
        mdKaccum, mdVaccum = None, None
    else:
        if not varlen_k:
            mdKaccum = fake_tensor(Float32, (b, h_kv, seqlen_k_rounded), divisibility=4)
            mdVaccum = fake_tensor(Float32, (b, h_kv, seqlen_k_dv_rounded), divisibility=4)
        else:
            mdKaccum = fake_tensor(Float32, (h_kv, total_k_rounded), divisibility=4)
            mdVaccum = fake_tensor(Float32, (h_kv, total_k_dv_rounded), divisibility=4)
    return mQ, mK, mV, mO, mdO, mdQ, mdK, mdV, mLSE, mLSElog2, mPdPsum, dQaccum, mdKaccum, mdVaccum


def _compile_bwd_preprocess(
    dtype, head_dim, head_dim_v, m_block_size, has_cuseqlens_q, has_seqused_q, has_dlse,
):
    """Compile bwd preprocess kernel using cute fake tensors (no real GPU tensors needed)."""
    mQ, mK, mV, mO, mdO, mdQ, mdK, mdV, mLSE, mLSElog2, mPdPsum, mdQaccum, mdKaccum, mdVaccum = make_fake_bwd_tensors(
        dtype, has_gqa=True, varlen_q=has_cuseqlens_q, varlen_k=False
    )
    batch = mQ.shape[0] if not has_cuseqlens_q else cute.sym_int()
    batchp1 = cute.sym_int()
    mCuSeqlensQ = fake_tensor(Int32, (batchp1,), divisibility=1) if has_cuseqlens_q else None
    mSequsedQ = fake_tensor(Int32, (batch,), divisibility=1) if has_seqused_q else None
    mdLSE = fake_tensor(Float32, mLSE.shape, divisibility=1) if has_dlse else None
    fa_bwd_pre = FlashAttentionBackwardPreprocess(dtype, head_dim, head_dim_v, m_block_size)
    return cute.compile(
        fa_bwd_pre, mO, mdO, mPdPsum, mLSE, mLSElog2, mdQaccum, mCuSeqlensQ, mSequsedQ, mdLSE,
        cute.runtime.make_fake_stream(use_tvm_ffi_env_stream=True),
        options="--enable-tvm-ffi",
    )


def _bwd_preprocess(
    out, dout, dpsum, lse, lse_log2, dq_accum,
    cu_seqlens_q, seqused_q, dlse,
    dtype, head_dim, head_dim_v, m_block_size,
):
    """Backward preprocess: compute (o * dout).sum(dim=-1) - dLSE, lse * log2_e, and zero out dq_accum."""
    is_varlen = cu_seqlens_q is not None
    compile_key = (
        dtype, head_dim, head_dim_v, m_block_size, is_varlen, seqused_q is not None, dlse is not None,
    )
    if compile_key not in _bwd_preprocess.compile_cache:
        _bwd_preprocess.compile_cache[compile_key] = _compile_bwd_preprocess(*compile_key)
    if not is_fake_mode():
        _bwd_preprocess.compile_cache[compile_key](
            out, dout, dpsum, lse, lse_log2, dq_accum, cu_seqlens_q, seqused_q, dlse
        )


_bwd_preprocess.compile_cache = get_jit_cache("bwd_pre")


def _compile_bwd_postprocess(
    dtype, hdim, block_size, num_threads, atom_layout, swap_ab,
    has_cuseqlens_q, has_seqused_q,
    use_2cta_instrs, cluster_size, arch,
):
    """Compile bwd postprocess kernel using cute fake tensors."""
    mQ, mK, mV, mO, mdO, mdQ, mdK, mdV, mLSE, mLSElog2, mPdPsum, mdQaccum, mdKaccum, mdVaccum = make_fake_bwd_tensors(
        dtype, has_gqa=True, varlen_q=has_cuseqlens_q, varlen_k=False
    )
    batch = mQ.shape[0] if not has_cuseqlens_q else cute.sym_int()
    batchp1 = cute.sym_int()
    mCuSeqlensQ = fake_tensor(Int32, (batchp1,), divisibility=1) if has_cuseqlens_q else None
    mSeqUsedQ = fake_tensor(Int32, (batch,), divisibility=1) if has_seqused_q else None
    fa_bwd_post = FlashAttentionBackwardPostprocess(
        dtype, hdim, arch, block_size, num_threads, atom_layout, swap_ab,
        use_2cta_instrs=use_2cta_instrs,
        cluster_size=cluster_size,
    )
    return cute.compile(
        fa_bwd_post, mdQaccum, mdQ, Float32(0.0), mCuSeqlensQ, mSeqUsedQ,
        cute.runtime.make_fake_stream(use_tvm_ffi_env_stream=True),
        options="--enable-tvm-ffi",
    )


def _bwd_postprocess_convert(
    accum, output, scale,
    cu_seqlens, seqused,
    arch, dtype, hdim, block_size, num_threads,
    atom_layout, swap_ab,
    use_2cta_instrs=False, cluster_size=1,
):
    """Backward postprocess: convert float32 accumulator to bf16/fp16 output."""
    compile_key = (
        dtype, hdim, block_size, num_threads, atom_layout, swap_ab,
        cu_seqlens is not None, seqused is not None,
        use_2cta_instrs, cluster_size, arch,
    )
    if compile_key not in _bwd_postprocess_convert.compile_cache:
        _bwd_postprocess_convert.compile_cache[compile_key] = _compile_bwd_postprocess(*compile_key)
    if not is_fake_mode():
        _bwd_postprocess_convert.compile_cache[compile_key](
            accum, output, scale, cu_seqlens, seqused,
        )


_bwd_postprocess_convert.compile_cache = get_jit_cache("bwd_post")


def _flash_attn_bwd(
    q: torch.Tensor,
    k: torch.Tensor,
    v: torch.Tensor,
    out: torch.Tensor,
    dout: torch.Tensor,
    lse: torch.Tensor,
    softmax_scale: Optional[float] = None,
    causal: bool = False,
    softcap: float = 0.0,
    window_size_left: Optional[int] = None,
    window_size_right: Optional[int] = None,
    m_block_size: int = 64,
    n_block_size: int = 128,
    num_threads: int = 256,
    pack_gqa: bool = False,
    num_stages_Q: int = 2,
    num_stages_dO: int = 2,
    SdP_swapAB: bool = False,
    dKV_swapAB: bool = False,
    dQ_swapAB: bool = False,
    AtomLayoutMSdP: int = 2,
    AtomLayoutNdKV: int = 2,
    AtomLayoutMdQ: int = 2,
    V_in_regs: bool = False,
    cu_seqlens_q: Optional[torch.Tensor] = None,
    cu_seqlens_k: Optional[torch.Tensor] = None,
    seqused_q: Optional[torch.Tensor] = None,
    seqused_k: Optional[torch.Tensor] = None,
    max_seqlen_q: Optional[int] = None,
    max_seqlen_k: Optional[int] = None,
    deterministic: bool = False,
    dq: Optional[torch.Tensor] = None,
    dk: Optional[torch.Tensor] = None,
    dv: Optional[torch.Tensor] = None,
    score_mod: Optional[Callable] = None,
    score_mod_bwd: Optional[Callable] = None,
    mask_mod: Optional[Callable] = None,
    aux_tensors: Optional[list[torch.Tensor]] = None,
    block_sparse_tensors: Optional[BlockSparseTensorsTorch] = None,
    dlse: Optional[torch.Tensor] = None,
    dropout_p: float = 0.0,
    dropout_seed: Optional[int] = None,
) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
    arch = _get_device_arch()
    assert arch // 10 in [9, 10, 11, 12], "Unsupported compute capability. Supported: 9.x, 10.x, 11.x, 12.x"
    sparse_q = None
    if block_sparse_tensors is not None and arch // 10 == 9:
        sparse_q = block_sparse_tensors.block_size[0] if block_sparse_tensors.block_size is not None else 128

    num_head, head_dim = q.shape[-2:]
    head_dim_v = v.shape[-1]

    causal, local, window_size_left, window_size_right = _resolve_causal_local_window(
        causal, window_size_left, window_size_right
    )

    if arch // 10 == 12:
        # SM120: uses SM80 MMA with 99 KB SMEM, 128 threads (4 warps).
        m_block_size = 64
        n_block_size = 64
        if head_dim <= 64:
            num_stages_Q = 2
            num_stages_dO = 2
        else:
            num_stages_Q = 1
            num_stages_dO = 1
        SdP_swapAB = False
        dKV_swapAB = False
        dQ_swapAB = False
        AtomLayoutMSdP = 4
        AtomLayoutNdKV = 4
        AtomLayoutMdQ = 4
        V_in_regs = False
        cluster_size = 1
        use_2cta_instrs = False
        num_threads = 128
        dQ_single_wg = False
        assert not (block_sparse_tensors is not None), "Block sparsity backward not supported on SM 12.0"
        assert score_mod is None and score_mod_bwd is None, "score_mod backward not supported on SM 12.0"
        assert mask_mod is None, "mask_mod backward not supported on SM 12.0"
        assert deterministic is False, "deterministic backward not supported on SM 12.0"
    elif arch // 10 == 9:
        cfg = _tile_size_bwd_sm90(
            head_dim,
            head_dim_v,
            causal,
            local,
            sparse_block_size_q=sparse_q,
        )
        m_block_size = cfg.m_block_size
        n_block_size = cfg.n_block_size
        num_stages_Q = cfg.num_stages_Q
        num_stages_dO = cfg.num_stages_dO
        num_stages_PdS = cfg.num_stages_PdS
        SdP_swapAB = cfg.SdP_swapAB
        dKV_swapAB = cfg.dKV_swapAB
        dQ_swapAB = cfg.dQ_swapAB
        AtomLayoutMSdP = cfg.AtomLayoutMSdP
        AtomLayoutNdKV = cfg.AtomLayoutNdKV
        AtomLayoutMdQ = cfg.AtomLayoutMdQ
        num_threads = (cfg.num_wg + 1) * 128
        dQ_single_wg = cfg.dQ_single_wg
        cluster_size = 1
        use_2cta_instrs = False
        is_varlen = (
            cu_seqlens_q is not None
            or cu_seqlens_k is not None
            or seqused_q is not None
            or seqused_k is not None
        )
    else:
        m_block_size = 128
        n_block_size = 128
        dQ_swapAB = False
        dKV_swapAB = False
        AtomLayoutMdQ = 1
        AtomLayoutNdKV = 1
        requested_disable_2cta = utils._get_disable_2cta_default()
        disable_2cta = (
            requested_disable_2cta
            or score_mod is not None
            or score_mod_bwd is not None
            or mask_mod is not None
            or block_sparse_tensors is not None
        )
        cluster_size = 2 if head_dim >= 128 and not disable_2cta else 1
        use_2cta_instrs = cluster_size==2

    q, k, v, out, dout, lse, cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k = [
        maybe_contiguous(t)
        for t in (q, k, v, out, dout, lse, cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k)
    ]
    if cu_seqlens_q is None:
        batch_size, seqlen_q = q.shape[:2]
        total_q = batch_size * seqlen_q
    else:
        batch_size = cu_seqlens_q.shape[0] - 1
        total_q = q.shape[0]
        seqlen_q = max_seqlen_q if max_seqlen_q is not None else total_q

    if cu_seqlens_k is None:
        batch_size, seqlen_k = k.shape[:2]
        total_k = batch_size * seqlen_k
    else:
        batch_size = cu_seqlens_k.shape[0] - 1
        total_k = k.shape[0]
        seqlen_k = max_seqlen_k if max_seqlen_k is not None else total_k

    num_head_kv = k.shape[-2]

    use_block_sparsity = block_sparse_tensors is not None
    subtile_factor = sparse_q // m_block_size if sparse_q is not None else 2
    seqlen_q_rounded = (seqlen_q + m_block_size - 1) // m_block_size * m_block_size
    seqlen_k_rounded = (seqlen_k + n_block_size - 1) // n_block_size * n_block_size
    num_n_blocks = seqlen_k_rounded // n_block_size
    if cluster_size == 2 and num_n_blocks % cluster_size != 0:
        seqlen_k_rounded = seqlen_k_rounded + n_block_size

    if cu_seqlens_k is None:
        assert k.shape == (batch_size, seqlen_k, num_head_kv, head_dim)
        assert v.shape == (batch_size, seqlen_k, num_head_kv, head_dim_v)
    else:
        assert k.shape == (total_k, num_head_kv, head_dim)
        assert v.shape == (total_k, num_head_kv, head_dim_v)
        assert cu_seqlens_k.shape == (batch_size + 1,), (
            "cu_seqlens_k must have shape (batch_size + 1,)"
        )

    if cu_seqlens_q is not None:
        assert cu_seqlens_q.shape == (batch_size + 1,), (
            "cu_seqlens_q must have shape (batch_size + 1,)"
        )

        assert out.shape == (total_q, num_head, head_dim_v)
        assert dout.shape == (total_q, num_head, head_dim_v)
        assert lse.shape == (num_head, total_q), "lse must have shape (num_head, total_q)"
    else:
        assert out.shape == (batch_size, seqlen_q, num_head, head_dim_v)
        assert dout.shape == (batch_size, seqlen_q, num_head, head_dim_v)
        assert lse.shape == (batch_size, num_head, seqlen_q), (
            "lse must have shape (batch_size, num_head, seqlen_q)"
        )

    assert q.dtype in [torch.float16, torch.bfloat16], "inputs must be float16 or bfloat16"
    assert q.dtype == k.dtype == v.dtype == out.dtype == dout.dtype, (
        "inputs must have the same dtype"
    )
    for t in [cu_seqlens_q, cu_seqlens_k]:
        if t is not None:
            assert t.dtype == torch.int32, "cu_seqlens_q, cu_seqlens_k must be int32"
    assert lse.dtype == torch.float32, "lse must be float32"
    if dlse is not None:
        dlse = maybe_contiguous(dlse)
    if not is_fake_mode():
        assert all(
            t is None or t.is_cuda for t in (q, k, v, out, dout, lse, cu_seqlens_q, cu_seqlens_k)
        ), "inputs must be on CUDA device"
    assert num_head % num_head_kv == 0, "num_head must be divisible by num_head_kv"
    alignment = 16 // q.element_size()
    if arch // 10 != 12:
        _validate_head_dims(head_dim, head_dim_v, arch // 10, alignment)
    if softmax_scale is None:
        softmax_scale = 1.0 / math.sqrt(head_dim)
    qhead_per_kvhead = num_head // num_head_kv
    if pack_gqa is None:
        pack_gqa = qhead_per_kvhead > 1
    # pack_gqa backward not yet supported in bwd
    pack_gqa = False
    
    if softcap != 0.0:
        assert score_mod is None and score_mod_bwd is None, (
            "softcap and score_mod/score_mod_bwd cannot be used together"
        )
        score_mod = utils.create_softcap_scoremod(softcap)
        score_mod_bwd = utils.create_softcap_scoremod_bwd(softcap)
    elif score_mod is not None:
        assert score_mod_bwd is not None, "score_mod_bwd is required when score_mod is provided"
        assert cu_seqlens_q is None and cu_seqlens_k is None, (
            "varlen + score_mod not supported in bwd yet"
        )
        if arch // 10 == 8:
            raise NotImplementedError("Custom user-provided score_mod is not supported on SM8x architectures.")

    device = q.device
    out_torch_dtype = q.dtype

    if dq is None:
        dq = torch.empty_like(q)
    else:
        _validate_tensor(dq, "dq", q.shape, out_torch_dtype, device)

    if dk is None:
        dk = torch.empty_like(k)
    else:
        _validate_tensor(dk, "dk", k.shape, out_torch_dtype, device)

    if dv is None:
        dv = torch.empty_like(v)
    else:
        _validate_tensor(dv, "dv", v.shape, out_torch_dtype, device)

    head_dim_rounded = (head_dim + 32 - 1) // 32 * 32

    if cu_seqlens_q is None:
        dq_accum = torch.empty(
            batch_size,
            num_head,
            seqlen_q_rounded * head_dim_rounded,
            dtype=torch.float32,
            device=device,
        )
        dpsum = torch.empty(
            batch_size, num_head, seqlen_q_rounded, dtype=torch.float32, device=device
        )
        lse_log2 = torch.empty(
            batch_size, num_head, seqlen_q_rounded, dtype=torch.float32, device=device
        )
    else:
        total_q_rounded_padded = (
            (total_q + cu_seqlens_q.shape[0] * m_block_size - 1) // m_block_size * m_block_size
        )
        dq_accum = torch.empty(
            num_head, total_q_rounded_padded * head_dim_rounded, dtype=torch.float32, device=device
        )
        dpsum = torch.empty(num_head, total_q_rounded_padded, dtype=torch.float32, device=device)
        lse_log2 = torch.empty(num_head, total_q_rounded_padded, dtype=torch.float32, device=device)

    # GQA (qhead_per_kvhead > 1) needs dK/dV accum+postprocess since multiple Q heads
    # accumulate into the same dK/dV. SM90 varlen_k with qhead_per_kvhead==1 now uses
    # ragged TMA tensors for direct store, so no longer needs accum+postprocess.
    dKV_postprocess = qhead_per_kvhead > 1
    if dKV_postprocess:
        head_dim_v_rounded = (head_dim_v + 32 - 1) // 32 * 32
        if cu_seqlens_k is None:
            dk_accum = torch.zeros(
                batch_size,
                num_head_kv,
                seqlen_k_rounded * head_dim_rounded,
                dtype=torch.float32,
                device=device,
            )
            dv_accum = torch.zeros(
                batch_size,
                num_head_kv,
                seqlen_k_rounded * head_dim_v_rounded,
                dtype=torch.float32,
                device=device,
            )
        else:
            cluster_tile_n = cluster_size * n_block_size
            total_k_rounded_padded = (
                (total_k + cu_seqlens_k.shape[0] * cluster_tile_n - 1) // cluster_tile_n * cluster_tile_n
            )
            dk_accum = torch.zeros(
                num_head_kv,
                total_k_rounded_padded * head_dim_rounded,
                dtype=torch.float32,
                device=device,
            )
            dv_accum = torch.zeros(
                num_head_kv,
                total_k_rounded_padded * head_dim_v_rounded,
                dtype=torch.float32,
                device=device,
            )

    dtype = torch2cute_dtype_map[q.dtype]
    current_stream = cute.runtime.make_fake_stream(use_tvm_ffi_env_stream=True)

    if deterministic:
        dQ_semaphore = torch.zeros(batch_size, num_head, seqlen_q_rounded // m_block_size, cluster_size, dtype=torch.int32, device=device)
    else:
        dQ_semaphore = None

    if deterministic and qhead_per_kvhead > 1:
        dK_semaphore = torch.zeros(batch_size, num_head_kv, seqlen_k_rounded // n_block_size, 2, dtype=torch.int32, device=device)
        dV_semaphore = torch.zeros(batch_size, num_head_kv, seqlen_k_rounded // n_block_size, 2, dtype=torch.int32, device=device)
    else:
        dK_semaphore = None
        dV_semaphore = None

    # Preprocess kernel: compute (o * dout).sum(dim=-1) - dLSE, lse * log2_e, and zero out dq_accum.
    _bwd_preprocess(
        out, dout, dpsum, lse, lse_log2, dq_accum,
        cu_seqlens_q, seqused_q, dlse,
        dtype, head_dim, head_dim_v, m_block_size,
    )
    # num_threads: SM90 derives from BwdConfig.num_wg, SM120 is set to 128 above,
    # SM100/SM110 uses default from function signature (384).
    if arch // 10 not in [9, 12]:
        num_threads = 384

    # Backward kernel: compute dk, dv, dq_accum.
    score_mod_hash = utils.hash_callable(score_mod) if score_mod else False
    score_mod_bwd_hash = utils.hash_callable(score_mod_bwd) if score_mod_bwd else False
    mask_mod_hash = utils.hash_callable(mask_mod) if mask_mod else False
    num_aux_tensors = len(aux_tensors) if aux_tensors else 0
    cute_aux_tensors = None
    if aux_tensors is not None:
        cute_aux_tensors = [to_cute_tensor(buf, assumed_align=None, fully_dynamic=True) for buf in aux_tensors]

    block_sparse_broadcast_pattern = None
    normalized_block_sparse_tensors = None
    if block_sparse_tensors is not None:
        (
            normalized_block_sparse_tensors,
            block_sparse_broadcast_pattern,
        ) = normalize_block_sparse_config_bwd(
            block_sparse_tensors,
            batch_size=batch_size,
            num_head=num_head,
            seqlen_q=seqlen_q,
            seqlen_k=seqlen_k,
            block_size=(m_block_size, n_block_size),
            subtile_factor=subtile_factor,
        )

    if arch // 10 in [8, 9, 12]:
        compile_key = (
            arch,
            dtype,
            head_dim,
            head_dim_v,
            qhead_per_kvhead,
            causal,
            window_size_left is not None,
            window_size_right is not None,
            m_block_size,
            n_block_size,
            num_threads,
            pack_gqa,
            num_stages_Q,
            num_stages_dO,
            SdP_swapAB,
            dKV_swapAB,
            dQ_swapAB,
            AtomLayoutMSdP,
            AtomLayoutNdKV,
            AtomLayoutMdQ,
            V_in_regs,
            dQ_single_wg,
            deterministic,
            cu_seqlens_q is None,
            cu_seqlens_k is None,
            seqused_q is None,
            seqused_k is None,
            score_mod_hash,
            score_mod_bwd_hash,
            mask_mod_hash,
            num_aux_tensors,
            use_block_sparsity,
            block_sparse_broadcast_pattern,
            get_broadcast_dims(q),
            get_broadcast_dims(k),
            get_broadcast_dims(v),
            get_broadcast_dims(dout),
            # Prevent TVM stride poisoning when only one block is present.
            (seqlen_q_rounded // m_block_size == 1),
            (seqlen_k_rounded // n_block_size == 1),
            dropout_p,
        )
    else:
        compile_key = (
            arch,
            dtype,
            head_dim,
            head_dim_v,
            qhead_per_kvhead,
            causal,
            window_size_left is not None,
            window_size_right is not None,
            m_block_size,
            n_block_size,
            num_threads,
            pack_gqa,
            cluster_size,
            use_2cta_instrs,
            deterministic,
            score_mod_hash,
            score_mod_bwd_hash,
            mask_mod_hash,
            num_aux_tensors,
            use_block_sparsity,
            block_sparse_broadcast_pattern,
            cu_seqlens_q is None,
            cu_seqlens_k is None,
            seqused_q is None,
            seqused_k is None,
            get_broadcast_dims(q),
            get_broadcast_dims(k),
            get_broadcast_dims(v),
            get_broadcast_dims(dout),
            # Prevent TVM stride poisoning when only one block is present.
            (seqlen_q_rounded // m_block_size == 1),
            (seqlen_k_rounded // n_block_size == 1),
            dropout_p,
        )
    if compile_key not in _flash_attn_bwd.compile_cache:
        q_tensor, k_tensor, v_tensor, do_tensor, dq_tensor, dk_tensor, dv_tensor = [
            to_cute_tensor(t) for t in (q, k, v, dout, dq, dk, dv)
        ]
        dq_accum_tensor, dpsum_tensor, lse_log2_tensor = [
            to_cute_tensor(t) for t in (dq_accum, dpsum, lse_log2)
        ]
        if dKV_postprocess:
            dk_accum_tensor, dv_accum_tensor = [
                to_cute_tensor(t) for t in (dk_accum, dv_accum)
            ]
        cu_seqlens_q_tensor, cu_seqlens_k_tensor, seqused_q_tensor, seqused_k_tensor = [
            to_cute_tensor(t, assumed_align=4) if t is not None else None
            for t in (cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k)
        ]
        dQ_semaphore_tensor, dK_semaphore_tensor, dV_semaphore_tensor = [
            utils.convert_from_dlpack_leading_static(t.detach(), leading_dim=3, alignment=4, stride_order=t.dim_order())
            if t is not None else None
            for t in (dQ_semaphore, dK_semaphore, dV_semaphore)
        ]
        if arch // 10 in [8, 12]:
            flash_bwd_obj_cls = FlashAttentionBackwardSm120 if arch // 10 == 12 else FlashAttentionBackwardSm80
            fa_bwd_obj = flash_bwd_obj_cls(
                dtype,
                head_dim,
                head_dim_v,
                qhead_per_kvhead,
                m_block_size,
                n_block_size,
                num_stages_Q,
                num_stages_dO,
                num_threads,
                pack_gqa,
                causal,
                SdP_swapAB,
                dKV_swapAB,
                dQ_swapAB,
                AtomLayoutMSdP,
                AtomLayoutNdKV,
                AtomLayoutMdQ,
                V_in_regs=V_in_regs,
                score_mod=score_mod,
                score_mod_bwd=score_mod_bwd,
                p_dropout=dropout_p,
            )
        elif arch // 10 == 9:
            fa_bwd_obj = FlashAttentionBackwardSm90(
                dtype,
                head_dim,
                head_dim_v,
                qhead_per_kvhead,
                causal,
                is_local=local,
                deterministic=deterministic,
                tile_m=m_block_size,
                tile_n=n_block_size,
                Q_stage=num_stages_Q,
                dO_stage=num_stages_dO,
                PdS_stage=num_stages_PdS,
                SdP_swapAB=SdP_swapAB,
                dKV_swapAB=dKV_swapAB,
                dQ_swapAB=dQ_swapAB,
                AtomLayoutMSdP=AtomLayoutMSdP,
                AtomLayoutNdKV=AtomLayoutNdKV,
                AtomLayoutMdQ=AtomLayoutMdQ,
                num_threads=num_threads,
                V_in_regs=V_in_regs,
                score_mod=score_mod,
                score_mod_bwd=score_mod_bwd,
                mask_mod=mask_mod,
                has_aux_tensors=aux_tensors is not None,
                subtile_factor=subtile_factor,
                dQ_single_wg=dQ_single_wg,
                p_dropout=dropout_p,
            )
        else:
            fa_bwd_obj = FlashAttentionBackwardSm100(
                head_dim,
                head_dim_v,
                is_causal=causal,
                is_local=local,
                qhead_per_kvhead=qhead_per_kvhead,
                tile_m=m_block_size,
                tile_n=n_block_size,
                cluster_size=cluster_size,
                use_2cta_instrs=use_2cta_instrs,
                deterministic=deterministic,
                score_mod=score_mod,
                score_mod_bwd=score_mod_bwd,
                mask_mod=mask_mod,
                has_aux_tensors=aux_tensors is not None,
                subtile_factor=subtile_factor,
                p_dropout=dropout_p,
            )

        # Block sparse tensors for backward use Q-direction indexing (transposed from forward).
        sparse_tensors_compile = None
        if normalized_block_sparse_tensors is not None:
            sparse_tensors_compile = to_cute_block_sparse_tensors(normalized_block_sparse_tensors)

        # TODO: check @can_implement
        _flash_attn_bwd.compile_cache[compile_key] = cute.compile(
            fa_bwd_obj,
            q_tensor,
            k_tensor,
            v_tensor,
            do_tensor,
            lse_log2_tensor,
            dpsum_tensor,
            dq_accum_tensor,
            dk_tensor if not dKV_postprocess else dk_accum_tensor,
            dv_tensor if not dKV_postprocess else dv_accum_tensor,
            softmax_scale,
            cu_seqlens_q_tensor,
            cu_seqlens_k_tensor,
            seqused_q_tensor,
            seqused_k_tensor,
            window_size_left,
            window_size_right,
            dQ_semaphore_tensor,
            dK_semaphore_tensor,
            dV_semaphore_tensor,
            cute_aux_tensors,
            sparse_tensors_compile,
            int(dropout_seed & 0xFFFFFFFF) if dropout_seed is not None and dropout_p > 0.0 else None,
            int((dropout_seed >> 32) & 0xFFFFFFFF) if dropout_seed is not None and dropout_p > 0.0 else None,
            current_stream,
            options="--enable-tvm-ffi",
        )
    if not is_fake_mode():
        _flash_attn_bwd.compile_cache[compile_key](
            q.detach(),
            k.detach(),
            v.detach(),
            dout,
            lse_log2,
            dpsum,
            dq_accum,
            dk if not dKV_postprocess else dk_accum,
            dv if not dKV_postprocess else dv_accum,
            softmax_scale,
            cu_seqlens_q,
            cu_seqlens_k,
            seqused_q,
            seqused_k,
            window_size_left,
            window_size_right,
            dQ_semaphore,
            dK_semaphore,
            dV_semaphore,
            aux_tensors,
            normalized_block_sparse_tensors[:4] if normalized_block_sparse_tensors is not None else None,
            int(dropout_seed & 0xFFFFFFFF) if dropout_seed is not None and dropout_p > 0.0 else None,
            int((dropout_seed >> 32) & 0xFFFFFFFF) if dropout_seed is not None and dropout_p > 0.0 else None,
        )

    if arch // 10 == 9:
        # dQ postprocess: match main kernel's MMA WG count, unless dQ_single_wg
        num_threads_post_dQ = 128 if dQ_single_wg else cfg.num_wg * 128
        num_threads_post_dKV = cfg.num_wg * 128
    else:
        num_threads_post_dQ = 128
        num_threads_post_dKV = 128

    # Postprocess: convert dq_accum from float32 to dq in bf16/fp16
    _bwd_postprocess_convert(
        dq_accum, dq, softmax_scale,
        cu_seqlens_q, seqused_q,
        arch, dtype, head_dim, m_block_size, num_threads_post_dQ,
        AtomLayoutMdQ, dQ_swapAB,
        use_2cta_instrs=use_2cta_instrs, cluster_size=1,
    )

    if dKV_postprocess:
        # Postprocess: convert dk_accum from float32 to dk in bf16/fp16
        _bwd_postprocess_convert(
            dk_accum, dk, softmax_scale,
            cu_seqlens_k, seqused_k,
            arch, dtype, head_dim, n_block_size, num_threads_post_dKV,
            AtomLayoutNdKV, dKV_swapAB,
            cluster_size=cluster_size,
        )
        # Postprocess: convert dv_accum from float32 to dv in bf16/fp16
        _bwd_postprocess_convert(
            dv_accum, dv, 1.0,
            cu_seqlens_k, seqused_k,
            arch, dtype, head_dim_v, n_block_size, num_threads_post_dKV,
            AtomLayoutNdKV, dKV_swapAB,
            cluster_size=cluster_size,
        )

    return dq, dk, dv


_flash_attn_bwd.compile_cache = get_jit_cache("bwd")


class FlashAttnFunc(torch.autograd.Function):
    @staticmethod
    def forward(
        ctx,
        q: torch.Tensor,
        k: torch.Tensor,
        v: torch.Tensor,
        qv: Optional[torch.Tensor] = None,
        gather_kv_indices: Optional[torch.Tensor] = None,
        softmax_scale: Optional[float] = None,
        causal: bool = False,
        window_size: Tuple[Optional[int], Optional[int]] = (None, None),
        learnable_sink: Optional[torch.Tensor] = None,
        softcap: float = 0.0,
        num_splits: int = 1,
        pack_gqa: Optional[bool] = None,
        deterministic: bool = False,
        mask_mod: Optional[Callable] = None,
        full_block_cnt: Optional[torch.Tensor] = None,
        full_block_idx: Optional[torch.Tensor] = None,
        mask_block_cnt: Optional[torch.Tensor] = None,
        mask_block_idx: Optional[torch.Tensor] = None,
        block_size: Optional[Tuple[int, int]] = None,
        return_lse: bool = False,
        dropout_p: float = 0.0,
        dropout_seed: Optional[int] = None,
    ):
        if dropout_p > 0.0 and dropout_seed is None:
            dropout_seed = torch.randint(0, 2**63, (1,), dtype=torch.int64).item()
        # Only create block sparse tensors if at least one block sparse parameter is provided
        block_sparse_tensors = None
        if any(t is not None for t in [full_block_cnt, full_block_idx, mask_block_cnt, mask_block_idx]):
            block_sparse_tensors = BlockSparseTensorsTorch(
                full_block_cnt=full_block_cnt,
                full_block_idx=full_block_idx,
                mask_block_cnt=mask_block_cnt,
                mask_block_idx=mask_block_idx,
                block_size=block_size,
            )
        out, lse = _flash_attn_fwd(
            q,
            k,
            v,
            qv=qv,
            softmax_scale=softmax_scale,
            causal=causal,
            window_size_left=window_size[0],
            window_size_right=window_size[1],
            learnable_sink=learnable_sink,
            softcap=softcap,
            num_splits=num_splits,
            pack_gqa=pack_gqa,
            mask_mod=mask_mod,
            block_sparse_tensors=block_sparse_tensors,
            return_lse=return_lse,
            gather_kv_indices=gather_kv_indices,
            dropout_p=dropout_p,
            dropout_seed=dropout_seed,
        )
        ctx.save_for_backward(q, k, v, out, lse)
        ctx.softmax_scale = softmax_scale
        ctx.causal = causal
        ctx.window_size = window_size
        ctx.softcap = softcap
        ctx.deterministic = deterministic
        ctx.return_lse = return_lse
        ctx.dropout_p = dropout_p
        ctx.dropout_seed = dropout_seed
        ctx.set_materialize_grads(False)
        return out, lse

    @staticmethod
    def backward(ctx, dout, dlse):
        q, k, v, out, lse = ctx.saved_tensors
        if not ctx.return_lse:
            dlse = None
        if dout is None:
            dout = torch.zeros_like(out)
        dq, dk, dv = _flash_attn_bwd(
            q,
            k,
            v,
            out,
            dout,
            lse,
            ctx.softmax_scale,
            ctx.causal,
            ctx.softcap,
            window_size_left=ctx.window_size[0],
            window_size_right=ctx.window_size[1],
            deterministic=ctx.deterministic,
            dlse=dlse,
            dropout_p=ctx.dropout_p,
            dropout_seed=ctx.dropout_seed,
        )
        return dq, dk, dv, *((None,) * 30)  # Extra Nones is fine


class FlashAttnVarlenFunc(torch.autograd.Function):
    @staticmethod
    def forward(
        ctx,
        q: torch.Tensor,
        k: torch.Tensor,
        v: torch.Tensor,
        qv: Optional[torch.Tensor] = None,
        cu_seqlens_q: Optional[torch.Tensor] = None,
        cu_seqlens_k: Optional[torch.Tensor] = None,
        seqused_q: Optional[torch.Tensor] = None,
        seqused_k: Optional[torch.Tensor] = None,
        max_seqlen_q: Optional[int] = None,
        max_seqlen_k: Optional[int] = None,
        min_seqlen_k: Optional[int] = None,
        gather_kv_indices: Optional[torch.Tensor] = None,
        page_table: Optional[torch.Tensor] = None,
        softmax_scale: Optional[float] = None,
        causal: bool = False,
        window_size: Tuple[Optional[int], Optional[int]] = (None, None),
        learnable_sink: Optional[torch.Tensor] = None,
        softcap: float = 0.0,
        num_splits: int = 1,
        pack_gqa: Optional[bool] = None,
        deterministic: bool = False,
        score_mod: Optional[Callable] = None,
        aux_tensors: Optional[list] = None,
        return_lse: bool = False,
        dropout_p: float = 0.0,
        dropout_seed: Optional[int] = None,
    ):
        if dropout_p > 0.0 and dropout_seed is None:
            dropout_seed = torch.randint(0, 2**63, (1,), dtype=torch.int64).item()
        out, lse = _flash_attn_fwd(
            q,
            k,
            v,
            qv=qv,
            cu_seqlens_q=cu_seqlens_q,
            cu_seqlens_k=cu_seqlens_k,
            seqused_q=seqused_q,
            seqused_k=seqused_k,
            max_seqlen_q=max_seqlen_q,
            max_seqlen_k=max_seqlen_k,
            min_seqlen_k=min_seqlen_k,
            page_table=page_table,
            softmax_scale=softmax_scale,
            causal=causal,
            window_size_left=window_size[0],
            window_size_right=window_size[1],
            learnable_sink=learnable_sink,
            softcap=softcap,
            num_splits=num_splits,
            pack_gqa=pack_gqa,
            score_mod=score_mod,
            aux_tensors=aux_tensors,
            return_lse=return_lse,
            gather_kv_indices=gather_kv_indices,
            dropout_p=dropout_p,
            dropout_seed=dropout_seed,
        )
        ctx.save_for_backward(q, k, v, out, lse, cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k)
        ctx.softmax_scale = softmax_scale
        ctx.causal = causal
        ctx.window_size = window_size
        ctx.softcap = softcap
        ctx.deterministic = deterministic
        ctx.max_seqlen_q = max_seqlen_q
        ctx.max_seqlen_k = max_seqlen_k
        ctx.return_lse = return_lse
        ctx.dropout_p = dropout_p
        ctx.dropout_seed = dropout_seed
        ctx.set_materialize_grads(False)
        return out, lse

    @staticmethod
    def backward(ctx, dout, dlse):
        q, k, v, out, lse, cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k = ctx.saved_tensors
        if not ctx.return_lse:
            dlse = None
        if dout is None:
            dout = torch.zeros_like(out)
        dq, dk, dv = _flash_attn_bwd(
            q,
            k,
            v,
            out,
            dout,
            lse,
            ctx.softmax_scale,
            ctx.causal,
            ctx.softcap,
            window_size_left=ctx.window_size[0],
            window_size_right=ctx.window_size[1],
            cu_seqlens_q=cu_seqlens_q,
            cu_seqlens_k=cu_seqlens_k,
            seqused_q=seqused_q,
            seqused_k=seqused_k,
            max_seqlen_q=ctx.max_seqlen_q,
            max_seqlen_k=ctx.max_seqlen_k,
            deterministic=ctx.deterministic,
            dlse=dlse,
            dropout_p=ctx.dropout_p,
            dropout_seed=ctx.dropout_seed,
        )

        return dq, dk, dv, *((None,) * 30)


def flash_attn_func(
    q: torch.Tensor,
    k: torch.Tensor,
    v: torch.Tensor,
    qv: Optional[torch.Tensor] = None,
    gather_kv_indices: Optional[torch.Tensor] = None,
    softmax_scale: Optional[float] = None,
    causal: bool = False,
    window_size: Tuple[Optional[int], Optional[int]] = (None, None),
    learnable_sink: Optional[torch.Tensor] = None,
    softcap: float = 0.0,
    num_splits: int = 1,
    pack_gqa: Optional[bool] = None,
    deterministic: bool = False,
    mask_mod: Optional[Callable] = None,
    full_block_cnt: Optional[torch.Tensor] = None,
    full_block_idx: Optional[torch.Tensor] = None,
    mask_block_cnt: Optional[torch.Tensor] = None,
    mask_block_idx: Optional[torch.Tensor] = None,
    block_size: Optional[Tuple[int, int]] = None,
    return_lse: bool = False,
    dropout_p: float = 0.0,
    dropout_seed: Optional[int] = None,
):
    return FlashAttnFunc.apply(
        q,
        k,
        v,
        qv,
        gather_kv_indices,
        softmax_scale,
        causal,
        window_size,
        learnable_sink,
        softcap,
        num_splits,
        pack_gqa,
        deterministic,
        mask_mod,
        full_block_cnt,
        full_block_idx,
        mask_block_cnt,
        mask_block_idx,
        block_size,
        return_lse,
        dropout_p,
        dropout_seed,
    )


def flash_attn_varlen_func(
    q: torch.Tensor,
    k: torch.Tensor,
    v: torch.Tensor,
    qv: Optional[torch.Tensor] = None,
    cu_seqlens_q: Optional[torch.Tensor] = None,
    cu_seqlens_k: Optional[torch.Tensor] = None,
    max_seqlen_q: Optional[int] = None,
    max_seqlen_k: Optional[int] = None,
    min_seqlen_k: Optional[int] = None,
    seqused_q: Optional[torch.Tensor] = None,
    seqused_k: Optional[torch.Tensor] = None,
    gather_kv_indices: Optional[torch.Tensor] = None,
    page_table: Optional[torch.Tensor] = None,
    softmax_scale: Optional[float] = None,
    causal: bool = False,
    window_size: Tuple[Optional[int], Optional[int]] = (None, None),
    learnable_sink: Optional[torch.Tensor] = None,
    softcap: float = 0.0,
    num_splits: int = 1,
    pack_gqa: Optional[bool] = None,
    deterministic: bool = False,
    score_mod: Optional[Callable] = None,
    aux_tensors: Optional[list] = None,
    return_lse: bool = False,
    dropout_p: float = 0.0,
    dropout_seed: Optional[int] = None,
):
    """
    Explanation of some optional arguments:

    qv: we write the MLA weight absorbed formula as
        O = softmax(scale * (Q @ K.T + Qv @ V.T)) @ V
        where Q = q_pe, Qv = q_nope, K = pe_cache, V = kv_cache.

    gather_kv_indices: a tensor of shape (batch, seqlen_q, gather_kv_length) or
        (total_q, gather_kv_length) if there is cu_seqlens_q.
        Currently, only used for topk sparsity with MLA absorption kernel.
    
    min_seqlen_k: for varlen, specifies the minimum kv sequence length for any batch.
        Used with gather_kv_indices to determine if we need oob masking.
    """
    return FlashAttnVarlenFunc.apply(
        q,
        k,
        v,
        qv,
        cu_seqlens_q,
        cu_seqlens_k,
        seqused_q,
        seqused_k,
        max_seqlen_q,
        max_seqlen_k,
        min_seqlen_k,
        gather_kv_indices,
        page_table,
        softmax_scale,
        causal,
        window_size,
        learnable_sink,
        softcap,
        num_splits,
        pack_gqa,
        deterministic,
        score_mod,
        aux_tensors,
        return_lse,
        dropout_p,
        dropout_seed,
    )


def _compile_fwd_combine(
    dtype, dtype_partial, head_dim, tile_m, k_block_size, log_max_splits,
    has_cu_seqlens, has_seqused, has_lse, has_varlen_batch_idx,
):
    """Compile fwd combine kernel using cute fake tensors (no real GPU tensors needed)."""
    sym = cute.sym_int
    div = 128 // dtype_partial.width  # 16-byte alignment in elements

    fa_combine = FlashAttentionForwardCombine(
        dtype=dtype,
        dtype_partial=dtype_partial,
        head_dim=head_dim,
        tile_m=tile_m,
        k_block_size=k_block_size,
        log_max_splits=log_max_splits,
    )
    if not fa_combine.can_implement(
        dtype, dtype_partial, head_dim, tile_m, k_block_size, log_max_splits,
        num_threads=256,
    ):
        raise RuntimeError(
            "FlashAttention combine kernel cannot be implemented with given parameters"
        )

    if has_cu_seqlens:
        # Varlen: (num_splits, total_q, nheads, headdim)
        num_splits, total_q, nheads = sym(), sym(), sym()
        mO_partial = fake_tensor(dtype_partial, (num_splits, total_q, nheads, head_dim), divisibility=div)
        mLSE_partial = fake_tensor(Float32, (num_splits, total_q, nheads), divisibility=1, leading_dim=1)
        mO = fake_tensor(dtype, (total_q, nheads, head_dim), divisibility=div)
        mLSE = fake_tensor(Float32, (total_q, nheads), divisibility=1, leading_dim=0) if has_lse else None
    else:
        # Batched: (num_splits, batch, seqlen, nheads, headdim)
        num_splits, batch, seqlen, nheads = sym(), sym(), sym(), sym()
        mO_partial = fake_tensor(dtype_partial, (num_splits, batch, seqlen, nheads, head_dim), divisibility=div)
        mLSE_partial = fake_tensor(Float32, (num_splits, batch, seqlen, nheads), divisibility=1, leading_dim=2)
        mO = fake_tensor(dtype, (batch, seqlen, nheads, head_dim), divisibility=div)
        mLSE = fake_tensor(Float32, (batch, seqlen, nheads), divisibility=1, leading_dim=1) if has_lse else None
        batch = mO_partial.shape[1]

    batch_for_1d = batch if not has_cu_seqlens else sym()
    batchp1 = sym()
    mCuSeqlens = fake_tensor(Int32, (batchp1,), divisibility=1) if has_cu_seqlens else None
    mSeqused = fake_tensor(Int32, (batch_for_1d,), divisibility=1) if has_seqused else None
    mNumSplitsDynamic = None  # Not parametrized in compile_key
    mVarlenBatchIdx = fake_tensor(Int32, (batch_for_1d,), divisibility=1) if has_varlen_batch_idx else None
    mSemaphore = None  # Not parametrized in compile_key

    return cute.compile(
        fa_combine,
        mO_partial, mLSE_partial, mO, mLSE,
        mCuSeqlens, mSeqused, mNumSplitsDynamic, mVarlenBatchIdx, mSemaphore,
        cute.runtime.make_fake_stream(use_tvm_ffi_env_stream=True),
        options="--enable-tvm-ffi",
    )


def _flash_attn_fwd_combine(
    out_partial: torch.Tensor,
    lse_partial: torch.Tensor,
    out: torch.Tensor,
    lse: Optional[torch.Tensor] = None,
    cu_seqlens: Optional[torch.Tensor] = None,
    seqused: Optional[torch.Tensor] = None,
    num_splits_dynamic_ptr: Optional[torch.Tensor] = None,
    varlen_batch_idx: Optional[torch.Tensor] = None,
    semaphore_to_reset: Optional[torch.Tensor] = None,
) -> None:
    """Forward combine kernel for split attention computation.

    Combines partial outputs and log-sum-exp values from multiple splits
    of attention computation into final outputs.

    Args:
        out_partial: Partial outputs tensor (num_splits, batch, seqlen, nheads, headdim) or
                                            (num_splits, total_q, nheads, headdim) if there's cu_seqlens
        lse_partial: Partial LSE tensor (num_splits, batch, seqlen, nheads) or
                                       (num_splits, total_q, nheads) if there's cu_seqlens
        out: Output tensor (batch, seqlen, nheads, headdim) or (total_q, nheads, headdim) if there's cu_seqlens
        lse: Output LSE tensor (batch, seqlen, nheads) or (total_q, nheads) if there's cu_seqlens.
        cu_seqlens: Cumulative sequence lengths for variable length sequences
        seqused: Used sequence lengths for each batch
        num_splits_dynamic_ptr: Dynamic number of splits per batch
        semaphore_to_reset: Semaphore for synchronization
        k_block_size: Block size for head dimension

    Returns:
        None
    """
    assert out_partial.dtype in [torch.float16, torch.bfloat16, torch.float32], (
        "out_partial must be fp16, bf16, or fp32"
    )
    if not is_fake_mode():
        assert out_partial.is_cuda and lse_partial.is_cuda, "tensors must be on CUDA device"
    # Determine if this is variable length based on dimensions
    is_varlen = out_partial.dim() == 4
    # Validate optional tensors
    for t, name in [
        (cu_seqlens, "cu_seqlens"),
        (seqused, "seqused"),
        (num_splits_dynamic_ptr, "num_splits_dynamic_ptr"),
    ]:
        if t is not None:
            if not is_fake_mode():
                assert t.is_cuda, f"{name} must be on CUDA device"
            assert t.is_contiguous(), f"{name} must be contiguous"
    head_dim = out_partial.shape[-1]
    num_splits = out_partial.shape[0]
    assert num_splits <= 256
    # If hdim is 96 or 192, it's faster to round them to 128 or 256 respectively
    # so that kBlockM is smaller and we have more parallelism.
    k_block_size = 64 if head_dim <= 64 else 128
    # We want kBlockM to be as small as possible to maximize parallelism.
    # E.g., if hdim is 64, we want kBlockM to be 16 so that we can use 256 threads, each reading 4 elements (floats).
    tile_m = 8 if k_block_size % 128 == 0 else (16 if k_block_size % 64 == 0 else 32)
    log_max_splits = max(math.ceil(math.log2(num_splits)), 4)
    if tile_m == 8:
        # If kBlockM == 8 then the minimum number of splits is 32.
        # TODO: we can deal w this by using 128 threads instead
        log_max_splits = max(log_max_splits, 5)

    # Create combine kernel configuration
    dtype = torch2cute_dtype_map[out.dtype]
    dtype_partial = torch2cute_dtype_map[out_partial.dtype]
    compile_key = (
        dtype,
        dtype_partial,
        head_dim,
        tile_m,
        k_block_size,
        log_max_splits,
        cu_seqlens is not None,
        seqused is not None,
        lse is not None,
        varlen_batch_idx is not None,
    )
    if compile_key not in _flash_attn_fwd_combine.compile_cache:
        _flash_attn_fwd_combine.compile_cache[compile_key] = _compile_fwd_combine(
            *compile_key
        )
    if not is_fake_mode():
        _flash_attn_fwd_combine.compile_cache[compile_key](
            out_partial, lse_partial, out, lse,
            cu_seqlens, seqused, num_splits_dynamic_ptr, varlen_batch_idx,
            semaphore_to_reset,
        )


_flash_attn_fwd_combine.compile_cache = get_jit_cache("fwd_combine")


def flash_attn_combine(
    out_partial: torch.Tensor,
    lse_partial: torch.Tensor,
    out: Optional[torch.Tensor] = None,
    out_dtype: Optional[torch.dtype] = None,
    cu_seqlens: Optional[torch.Tensor] = None,
    seqused: Optional[torch.Tensor] = None,
    varlen_batch_idx: Optional[torch.Tensor] = None,
    return_lse: bool = True,
) -> Tuple[torch.Tensor, Optional[torch.Tensor]]:
    """Flash Attention combine function for split attention computation.

    Combines partial outputs and log-sum-exp values from multiple splits
    of attention computation into final outputs. This is the main user-facing
    interface for the combine kernel.

    Args:
        out_partial: Partial outputs tensor with shape:
            - (num_splits, batch_size, seqlen, num_heads, head_size) for regular batched input
            - (num_splits, total_q, num_heads, head_size) for variable length input
        lse_partial: Partial LSE tensor with shape:
            - (num_splits, batch_size, seqlen, num_heads) for regular batched input
            - (num_splits, total_q, num_heads) for variable length input
        out: Optional output tensor. If None, will be created automatically.
        out_dtype: Optional output dtype. If None, will use fp16/bf16 based on input.
        cu_seqlens: Cumulative sequence lengths for variable length sequences
        seqused: Used sequence lengths for each batch
        varlen_batch_idx: Optional mapping from virtual batch index to real batch index
            (int32 tensor of shape (batch_size,)). Used by persistent tile schedulers
            that reorder batch processing for load balancing.
        return_lse: Whether to return the combined LSE tensor. Default is True.

    Returns:
        Tuple of (out, lse) where:
        - out: Combined output tensor with shape (batch_size, seqlen, num_heads, head_size)
              or (total_q, num_heads, head_size) for varlen
        - lse: Combined log-sum-exp tensor with shape (batch_size, seqlen, num_heads)
              or (total_q, num_heads) for varlen. None if return_lse=False

    Note:
        This function expects the input tensors to be in the format produced by
        split attention computation, where the first dimension is num_splits.
        The permuting from user format to kernel format is now done inside the kernel.
    """
    # Input validation
    assert out_partial.dim() in [4, 5], "out_partial must have 4 or 5 dimensions"
    # Determine if this is variable length based on dimensions
    is_varlen = out_partial.dim() == 4
    if is_varlen:
        # Variable length: (num_splits, total_q, num_heads, head_size)
        num_splits, total_q, num_heads, head_size = out_partial.shape
        batch_size = 1  # Treat as single batch for varlen
        seqlen = total_q
    else:
        # Regular batched: (num_splits, batch_size, seqlen, num_heads, head_size)
        num_splits, batch_size, seqlen, num_heads, head_size = out_partial.shape
    # Determine output dtype
    if out_dtype is None:
        out_dtype = out_partial.dtype
    # Create output if not provided
    device = out_partial.device
    if out is None:
        if is_varlen:
            out = torch.empty(total_q, num_heads, head_size, dtype=out_dtype, device=device)
        else:
            out = torch.empty(
                batch_size, seqlen, num_heads, head_size, dtype=out_dtype, device=device
            )
    # Create lse output only if requested
    if return_lse:
        if is_varlen:
            lse = torch.empty(num_heads, total_q, dtype=torch.float32, device=device)
        else:
            lse = torch.empty(batch_size, num_heads, seqlen, dtype=torch.float32, device=device)
        lse = lse.transpose(-1, -2)
    else:
        lse = None
    _flash_attn_fwd_combine(
        out_partial,
        lse_partial,
        out,
        lse,
        cu_seqlens,
        seqused,
        varlen_batch_idx=varlen_batch_idx,
    )
    return out, lse