VTimeLLM/flash-attention/flash_attn/cute/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'd need to install nvidia-cutlass-dsl==4.1.0.

# Supported features:
# - BF16 & FP16 dtype
# - noncausal & causal attention
# - MHA, GQA, MQA
# - hdim 64, 96, 128.
# - (hdim_qk, hdim_v) = (192, 128) for Blackwell (i.e. DeepSeek shape)
# - varlen
# - sliding window
# - bwd pass for Ampere (will also run on Hopper/Blackwell, but will be slow)

# Features not supported yet:
# - split (i.e. FlashDecoding)
# - tuned block sizes
# - paged KV
# - append KV to existing KV cache
# - FP8
# - bwd pass optimized for Hopper/Blackwell

import math
from typing import Optional, Tuple

import torch

import cuda.bindings.driver as cuda

import cutlass
import cutlass.cute as cute
from cutlass.cute.runtime import from_dlpack

from flash_attn.cute import utils
from flash_attn.cute.flash_fwd import FlashAttentionForwardSm80, FlashAttentionForwardSm90
from flash_attn.cute.flash_fwd_sm100 import FlashAttentionForwardSm100
from flash_attn.cute.flash_bwd_preprocess import FlashAttentionBackwardPreprocess
from flash_attn.cute.flash_bwd import FlashAttentionBackwardSm80
from flash_attn.cute.flash_bwd_postprocess import FlashAttentionBackwardPostprocess
from flash_attn.cute.flash_fwd_combine import FlashAttentionForwardCombine


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


torch2cute_dtype_map = {
    torch.float16: cutlass.Float16,
    torch.bfloat16: cutlass.BFloat16,
    torch.float32: cutlass.Float32,
}


def _flash_attn_fwd(
    q: torch.Tensor,
    k: torch.Tensor,
    v: torch.Tensor,
    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,
    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,
    # m_block_size: int = 128,
    # n_block_size: int = 64,
    # num_threads: int = 128,
    m_block_size: int = 128,
    n_block_size: int = 128,
    num_threads: int = 384,
    pack_gqa: Optional[bool] = None,
    _compute_capability: Optional[int] = None,
) -> Tuple[torch.Tensor, torch.Tensor]:
    q, k, v = [maybe_contiguous(t) for t in (q, k, v)]
    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], "inputs must be float16 or bfloat16"
    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"
    assert all(t is None or t.is_cuda for t in (q, k, v, cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k, page_table, learnable_sink)), "inputs must be on CUDA device"
    assert num_head % num_head_kv == 0, "num_head must be divisible by num_head_kv"
    assert head_dim <= 256, "head_dim must be less than or equal to 256"
    alignment = 16 // q.element_size()
    assert head_dim % alignment == 0, f"head_dim must be divisible by {alignment}"
    assert head_dim_v % alignment == 0, f"head_dim_v must be divisible by {alignment}"
    if softmax_scale is None:
        softmax_scale = 1.0 / math.sqrt(head_dim)
    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

    out_torch_dtype = q.dtype
    device = q.device
    q_batch_seqlen_shape = (batch_size, seqlen_q) if cu_seqlens_q is None else (total_q,)
    out = torch.empty(*q_batch_seqlen_shape, num_head, head_dim_v, dtype=out_torch_dtype, device=device)
    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
    lse = torch.empty(lse_shape, dtype=torch.float32, device=device) if requires_grad else None

    dtype = torch2cute_dtype_map[q.dtype]
    q_tensor, k_tensor, v_tensor, o_tensor = [
        from_dlpack(t.detach(), assumed_align=16).mark_layout_dynamic(leading_dim=t.ndim - 1)
        for t in (q, k, v, out)
    ]
    lse_tensor = from_dlpack(lse.detach(), assumed_align=4).mark_layout_dynamic(leading_dim=lse.ndim - 1) if lse is not None else None
    cu_seqlens_q_tensor, cu_seqlens_k_tensor, seqused_q_tensor, seqused_k_tensor, learnable_sink_tensor = [
        from_dlpack(t.detach(), assumed_align=4).mark_layout_dynamic(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 = from_dlpack(page_table.detach(), assumed_align=4).mark_layout_dynamic(leading_dim=1) if page_table is not None else None
    if causal:
        window_size_right = 0
    local = window_size_left is not None or window_size_right is not 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
        else:
            causal, local = False, True
    compute_capability = torch.cuda.get_device_capability()[0] if _compute_capability is None else _compute_capability
    assert compute_capability in [9, 10], "Unsupported compute capability. Supported: 9.x, 10.x"
    current_stream = cuda.CUstream(torch.cuda.current_stream().cuda_stream)

    if compute_capability == 9:  # TODO: tune block size according to hdim
        if head_dim == head_dim_v == 128 and not causal and not local:
            n_block_size = 192
    if compute_capability == 10:
        # TODO: fix the varlen case
        if pack_gqa and (128 % qhead_per_kvhead != 0) or (cu_seqlens_q is not None or seqused_q is not None):
            pack_gqa = False

    compile_key = (
        dtype, head_dim, head_dim_v, qhead_per_kvhead, causal, softcap is not None,
        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,
        m_block_size, n_block_size, num_threads, pack_gqa,
        compute_capability,
    )
    if compile_key not in _flash_attn_fwd.compile_cache:
        if compute_capability == 9:
            assert page_table is None, "paged KV not supported on SM 9.0"
            # fa_fwd = FlashAttentionForwardSm80(
            fa_fwd = FlashAttentionForwardSm90(
                dtype,
                head_dim,
                head_dim_v,
                qhead_per_kvhead,
                is_causal=causal,
                is_local=local,
                pack_gqa=pack_gqa,
                m_block_size=m_block_size,
                n_block_size=n_block_size,
                # num_stages=1,
                num_stages=2,
                num_threads=num_threads,
                Q_in_regs=False,
            )
        elif compute_capability == 10:
            assert page_size in [None, 128], "Only page_size=128 is supported for paged KV on SM 10.0"
            fa_fwd = FlashAttentionForwardSm100(
                head_dim,
                head_dim_v,
                qhead_per_kvhead=qhead_per_kvhead,
                is_causal=causal,
                is_local=local,
                pack_gqa=pack_gqa,
                is_persistent=not causal and not local and cu_seqlens_q is None and seqused_q is None,
            )
        else:
            raise ValueError(f"Unsupported compute capability: {compute_capability}. Supported: 9.x, 10.x")
        # TODO: check @can_implement
        _flash_attn_fwd.compile_cache[compile_key] = cute.compile(
            fa_fwd, q_tensor, k_tensor, v_tensor, o_tensor, lse_tensor, softmax_scale, current_stream,
            cu_seqlens_q_tensor, cu_seqlens_k_tensor, seqused_q_tensor, seqused_k_tensor,
            page_table_tensor,
            softcap, window_size_left, window_size_right, learnable_sink_tensor,
        )
    _flash_attn_fwd.compile_cache[compile_key](
        q_tensor, k_tensor, v_tensor, o_tensor, lse_tensor, softmax_scale, current_stream,
        cu_seqlens_q_tensor, cu_seqlens_k_tensor, seqused_q_tensor, seqused_k_tensor,
        page_table_tensor,
        softcap, window_size_left, window_size_right, learnable_sink_tensor,
    )
    return out, lse


_flash_attn_fwd.compile_cache = {}


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,
    m_block_size: int = 64,
    n_block_size: int = 128,
    num_threads: int = 256,
    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,
) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
    q, k, v, out, dout, lse = [maybe_contiguous(t) for t in (q, k, v, out, dout, lse)]
    batch_size, seqlen_q, num_head, head_dim = q.shape
    _, seqlen_k, num_head_kv, _ = k.shape
    _, _, _, head_dim_v = v.shape
    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)
    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"
    assert lse.dtype == torch.float32, "lse must be float32"
    assert all(t.is_cuda for t in (q, k, v, out, dout, lse)), "inputs must be on CUDA device"
    assert num_head % num_head_kv == 0, "num_head must be divisible by num_head_kv"
    assert head_dim <= 256, "head_dim must be less than or equal to 256"
    alignment = 16 // q.element_size()
    assert head_dim % alignment == 0, f"head_dim must be divisible by {alignment}"
    assert head_dim_v % alignment == 0, f"head_dim_v must be divisible by {alignment}"
    if softmax_scale is None:
        softmax_scale = 1.0 / math.sqrt(head_dim)
    qhead_per_kvhead = num_head // num_head_kv

    device = q.device
    # TODO: check if this is the right rounding
    seqlen_q_rounded = (seqlen_q + m_block_size - 1) // m_block_size * m_block_size
    head_dim_rounded = (head_dim + 32 - 1) // 32 * 32
    dq = torch.empty_like(q)
    dk = torch.empty_like(k)
    dv = torch.empty_like(v)
    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)
    if qhead_per_kvhead > 1:
        seqlen_k_rounded = (seqlen_k + n_block_size - 1) // n_block_size * n_block_size
        head_dim_v_rounded = (head_dim_v + 32 - 1) // 32 * 32
        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)

    dtype = torch2cute_dtype_map[q.dtype]
    q_tensor, k_tensor, v_tensor, o_tensor, do_tensor, dq_tensor, dk_tensor, dv_tensor = [
        from_dlpack(t.detach(), assumed_align=16).mark_layout_dynamic(leading_dim=t.ndim - 1)
        for t in (q, k, v, out, dout, dq, dk, dv)
    ]
    lse_tensor = from_dlpack(lse.detach(), assumed_align=4).mark_layout_dynamic(leading_dim=2)
    dq_accum_tensor, dpsum_tensor, lse_log2_tensor = [
        from_dlpack(t.detach(), assumed_align=16).mark_layout_dynamic(leading_dim=2)
        for t in (dq_accum, dpsum, lse_log2)
    ]
    if qhead_per_kvhead > 1:
        dk_accum_tensor, dv_accum_tensor = [
            from_dlpack(t.detach(), assumed_align=16).mark_layout_dynamic(leading_dim=2)
            for t in (dk_accum, dv_accum)
        ]
    current_stream = cuda.CUstream(torch.cuda.current_stream().cuda_stream)

    # Preprocess kernel: compute (o * dout).sum(dim=-1), lse * log2_e, and zero out dq_accum.
    compile_key_pre = (dtype, head_dim_v, m_block_size, num_threads)
    if compile_key_pre not in _flash_attn_bwd.compile_cache_pre:
        fa_bwd_pre = FlashAttentionBackwardPreprocess(
            dtype, head_dim_v, m_block_size, num_threads=num_threads,
        )
        # TODO: check @can_implement
        _flash_attn_bwd.compile_cache_pre[compile_key_pre] = cute.compile(
            fa_bwd_pre, o_tensor, do_tensor, dpsum_tensor, lse_tensor, lse_log2_tensor,
            dq_accum_tensor, current_stream
        )
    _flash_attn_bwd.compile_cache_pre[compile_key_pre](
        o_tensor, do_tensor, dpsum_tensor, lse_tensor, lse_log2_tensor, dq_accum_tensor, current_stream
    )

    # Backward kernel: compute dk, dv, dq_accum.
    compile_key = (
        dtype, head_dim, head_dim_v, qhead_per_kvhead, causal, softcap != 0.0, m_block_size,
        n_block_size, num_threads, num_stages_Q, num_stages_dO, SdP_swapAB, dKV_swapAB, dQ_swapAB,
        AtomLayoutMSdP, AtomLayoutNdKV, AtomLayoutMdQ, V_in_regs
    )
    if compile_key not in _flash_attn_bwd.compile_cache:
        fa_bwd_sm80 = FlashAttentionBackwardSm80(
            dtype,
            head_dim,
            head_dim_v,
            qhead_per_kvhead,
            m_block_size,
            n_block_size,
            num_stages_Q,
            num_stages_dO,
            num_threads,
            causal,
            SdP_swapAB,
            dKV_swapAB,
            dQ_swapAB,
            AtomLayoutMSdP,
            AtomLayoutNdKV,
            AtomLayoutMdQ,
            V_in_regs=V_in_regs,
        )
        # TODO: check @can_implement
        _flash_attn_bwd.compile_cache[compile_key] = cute.compile(
            fa_bwd_sm80, q_tensor, k_tensor, v_tensor, do_tensor, lse_log2_tensor, dpsum_tensor,
            dq_accum_tensor,
            dk_tensor if qhead_per_kvhead == 1 else dk_accum_tensor,
            dv_tensor if qhead_per_kvhead == 1 else dv_accum_tensor,
            softmax_scale, current_stream
        )
    _flash_attn_bwd.compile_cache[compile_key](
        q_tensor, k_tensor, v_tensor, do_tensor, lse_log2_tensor, dpsum_tensor,
        dq_accum_tensor,
        dk_tensor if qhead_per_kvhead == 1 else dk_accum_tensor,
        dv_tensor if qhead_per_kvhead == 1 else dv_accum_tensor,
        softmax_scale, current_stream
    )

    # Postprocess kernel: convert dq_accum from float32 to dq in bf16/fp16
    compile_key_post = (dtype, head_dim, m_block_size, num_threads, AtomLayoutMdQ, dQ_swapAB)
    if compile_key_post not in _flash_attn_bwd.compile_cache_post:
        fa_bwd_post = FlashAttentionBackwardPostprocess(
            dtype, head_dim, m_block_size, num_threads, AtomLayoutMdQ, dQ_swapAB
        )
        # TODO: check @can_implement
        _flash_attn_bwd.compile_cache_post[compile_key_post] = cute.compile(
            fa_bwd_post, dq_accum_tensor, dq_tensor, softmax_scale, current_stream
        )
    _flash_attn_bwd.compile_cache_post[compile_key_post](
        dq_accum_tensor, dq_tensor, softmax_scale, current_stream
    )

    if qhead_per_kvhead > 1:
        # Postprocess kernel: convert dk_accum & dv_accum from float32 to bf16/fp16
        compile_key_post = (dtype, head_dim, n_block_size, num_threads, AtomLayoutNdKV, dKV_swapAB)
        if compile_key_post not in _flash_attn_bwd.compile_cache_post:
            fa_bwd_post = FlashAttentionBackwardPostprocess(
                dtype, head_dim, n_block_size, num_threads, AtomLayoutNdKV, dKV_swapAB
            )
            # TODO: check @can_implement
            _flash_attn_bwd.compile_cache_post[compile_key_post] = cute.compile(
                fa_bwd_post, dk_accum_tensor, dk_tensor, softmax_scale, current_stream
            )
        _flash_attn_bwd.compile_cache_post[compile_key_post](
            dk_accum_tensor, dk_tensor, softmax_scale, current_stream
        )
        compile_key_post = (dtype, head_dim_v, n_block_size, num_threads, AtomLayoutNdKV, dKV_swapAB)
        if compile_key_post not in _flash_attn_bwd.compile_cache_post:
            fa_bwd_post = FlashAttentionBackwardPostprocess(
                dtype, head_dim_v, n_block_size, num_threads, AtomLayoutNdKV, dKV_swapAB
            )
            # TODO: check @can_implement
            _flash_attn_bwd.compile_cache_post[compile_key_post] = cute.compile(
                fa_bwd_post, dv_accum_tensor, dv_tensor, cutlass.Float32(1.0), current_stream
            )
        _flash_attn_bwd.compile_cache_post[compile_key_post](
            dv_accum_tensor, dv_tensor, cutlass.Float32(1.0), current_stream
        )

    return dq, dk, dv


_flash_attn_bwd.compile_cache_pre = {}
_flash_attn_bwd.compile_cache = {}
_flash_attn_bwd.compile_cache_post = {}


class FlashAttnFunc(torch.autograd.Function):

    @staticmethod
    def forward(
        ctx,
        q: torch.Tensor,
        k: torch.Tensor,
        v: torch.Tensor,
        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,
        pack_gqa: Optional[bool] = None,
    ):
        out, lse = _flash_attn_fwd(
            q,
            k,
            v,
            softmax_scale=softmax_scale,
            causal=causal,
            window_size_left=window_size[0],
            window_size_right=window_size[1],
            learnable_sink=learnable_sink,
            softcap=softcap,
            pack_gqa=pack_gqa,
        )
        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
        return out, lse

    @staticmethod
    def backward(ctx, dout, *args):
        q, k, v, out, lse = ctx.saved_tensors
        dq, dk, dv = _flash_attn_bwd(
            q,
            k,
            v,
            out,
            dout,
            lse,
            ctx.softmax_scale,
            ctx.causal,
            ctx.softcap,
        )
        return dq, dk, dv, *((None,) * 5)


class FlashAttnVarlenFunc(torch.autograd.Function):

    @staticmethod
    def forward(
        ctx,
        q: torch.Tensor,
        k: torch.Tensor,
        v: torch.Tensor,
        cu_seqlens_q: Optional[torch.Tensor],
        cu_seqlens_k: Optional[torch.Tensor],
        seqused_q: Optional[torch.Tensor] = None,
        seqused_k: 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,
        pack_gqa: Optional[bool] = None,
    ):
        out, lse = _flash_attn_fwd(
            q,
            k,
            v,
            cu_seqlens_q,
            cu_seqlens_k,
            seqused_q,
            seqused_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,
            pack_gqa=pack_gqa,
        )
        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
        return out, lse

    @staticmethod
    def backward(ctx, dout, *args):
        q, k, v, out, lse, cu_seqlens_q, cu_seqlens_k, seqused_q, seqused_k = ctx.saved_tensors
        raise NotImplementedError(
            "Backward pass for FlashAttention with variable length sequences is not implemented yet."
        )


def flash_attn_func(
    q: torch.Tensor,
    k: torch.Tensor,
    v: torch.Tensor,
    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,
    pack_gqa: Optional[bool] = None,
):
    return FlashAttnFunc.apply(
        q,
        k,
        v,
        softmax_scale,
        causal,
        window_size,
        learnable_sink,
        softcap,
        pack_gqa,
    )


def flash_attn_varlen_func(
    q: torch.Tensor,
    k: torch.Tensor,
    v: torch.Tensor,
    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,
    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,
    pack_gqa: Optional[bool] = None,
):
    return FlashAttnVarlenFunc.apply(
        q,
        k,
        v,
        cu_seqlens_q,
        cu_seqlens_k,
        seqused_q,
        seqused_k,
        page_table,
        softmax_scale,
        causal,
        window_size,
        learnable_sink,
        softcap,
        pack_gqa,
    )


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,
    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
    """
    # Input validation
    assert out_partial.dim() in [4, 5], "out_partial must have 4 or 5 dimensions"
    assert lse_partial.dim() in [3, 4], "lse_partial must have 3 or 4 dimensions"
    assert out_partial.dtype in [torch.float16, torch.bfloat16, torch.float32], "out_partial must be fp16, bf16, or fp32"
    assert lse_partial.dtype == torch.float32, "lse_partial must be fp32"
    assert out_partial.is_cuda and lse_partial.is_cuda, "tensors must be on CUDA device"
    assert out_partial.stride(-1) == 1, "out_partial must be contiguous in the last dimension"
    assert lse_partial.stride(-2) == 1, "lse_partial must be contiguous in the seqlen dimension"
    assert lse_partial.shape == out_partial.shape[:-1]

    # Determine if this is variable length based on dimensions
    is_varlen = out_partial.dim() == 4

    # Validate output tensor shapes and types
    assert out.shape == out_partial.shape[1:], "out shape mismatch"
    if lse is not None:
        assert lse.shape == lse_partial.shape[1:], "lse shape mismatch"
        assert lse.dtype == torch.float32, "lse must be fp32"

    # 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:
            assert t.dtype == torch.int32, f"{name} must be int32"
            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).
    m_block_size = 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 m_block_size == 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)

    # Convert to cute tensors (using kernel-formatted tensors)
    out_partial_tensor = from_dlpack(out_partial.detach(), assumed_align=16).mark_layout_dynamic(leading_dim=4)
    lse_partial_tensor = from_dlpack(lse_partial.detach(), assumed_align=4).mark_layout_dynamic(leading_dim=lse_partial.ndim - 2)
    out_tensor = from_dlpack(out.detach(), assumed_align=16).mark_layout_dynamic(leading_dim=3)
    lse_tensor = from_dlpack(lse.detach(), assumed_align=4).mark_layout_dynamic(leading_dim=lse.ndim - 2) if lse is not None else None

    optional_tensors = [
        from_dlpack(t.detach(), assumed_align=4).mark_layout_dynamic(leading_dim=0) if t is not None else None
        for t in (cu_seqlens, seqused, num_splits_dynamic_ptr, semaphore_to_reset)
    ]
    cu_seqlens_tensor, seqused_tensor, num_splits_dynamic_tensor, semaphore_tensor = optional_tensors

    current_stream = cuda.CUstream(torch.cuda.current_stream().cuda_stream)

    # 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, m_block_size, k_block_size,
        log_max_splits,
        cu_seqlens is not None, seqused is not None, lse is not None,
    )

    if compile_key not in _flash_attn_fwd_combine.compile_cache:
        fa_combine = FlashAttentionForwardCombine(
            dtype=dtype,
            dtype_partial=dtype_partial,
            head_dim=head_dim,
            m_block_size=m_block_size,
            k_block_size=k_block_size,
            log_max_splits=log_max_splits,
        )

        # Check if implementation is supported
        if not fa_combine.can_implement(
            dtype, dtype_partial, head_dim, m_block_size, k_block_size, log_max_splits, num_threads=256
        ):
            raise RuntimeError(f"FlashAttention combine kernel cannot be implemented with given parameters")

        _flash_attn_fwd_combine.compile_cache[compile_key] = cute.compile(
            fa_combine,
            out_partial_tensor,
            lse_partial_tensor,
            out_tensor,
            lse_tensor,
            cu_seqlens_tensor,
            seqused_tensor,
            num_splits_dynamic_tensor,
            semaphore_tensor,
            current_stream
        )

    _flash_attn_fwd_combine.compile_cache[compile_key](
        out_partial_tensor,
        lse_partial_tensor,
        out_tensor,
        lse_tensor,
        cu_seqlens_tensor,
        seqused_tensor,
        num_splits_dynamic_tensor,
        semaphore_tensor,
        current_stream
    )


_flash_attn_fwd_combine.compile_cache = {}


def flash_attn_combine(
    out_partial: torch.Tensor,
    lse_partial: torch.Tensor,
    out: Optional[torch.Tensor] = None,
    out_dtype: Optional[torch.dtype] = 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.
        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"
    assert lse_partial.dim() in [3, 4], "lse_partial must have 3 or 4 dimensions"
    assert out_partial.dtype == torch.float32, "out_partial must be fp32 (from accumulation)"
    assert lse_partial.dtype == torch.float32, "lse_partial must be fp32"

    # 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
        assert lse_partial.shape == (num_splits, total_q, num_heads), "lse_partial shape mismatch for varlen"
        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
        assert lse_partial.shape == (num_splits, batch_size, seqlen, num_heads), "lse_partial shape mismatch"

    # 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).transpose(0, 1)
        else:
            lse = torch.empty(batch_size, num_heads, seqlen, dtype=torch.float32, device=device).transpose(1, 2)
    else:
        lse = None

    _flash_attn_fwd_combine(out_partial, lse_partial, out, lse)
    return out, lse