"""Roofline: the ceiling, and whether you are compute- or memory-bound.

Measures the device rather than trusting a datasheet -- achieved bandwidth from a large stream-copy,
achieved dense throughput from a big GEMM. Both are what you can actually reach, which is the number
that matters when deciding where the headroom is.

    python3 roofline.py --flops 3.4e11 --bytes 2.5e9 --dtype bf16
"""
import argparse
import torch


def measured_bandwidth(mb=512):
    n = mb * 1024 * 1024 // 2
    a = torch.empty(n, device="cuda", dtype=torch.bfloat16)
    b = torch.empty_like(a)
    for _ in range(3):
        b.copy_(a)
    torch.cuda.synchronize()
    s, e = torch.cuda.Event(enable_timing=True), torch.cuda.Event(enable_timing=True)
    s.record()
    for _ in range(10):
        b.copy_(a)
    e.record(); torch.cuda.synchronize()
    return 10 * 2 * a.numel() * 2 / (s.elapsed_time(e) / 1e3)      # read+write


def measured_flops(dtype=torch.bfloat16, n=8192):
    a = torch.randn(n, n, device="cuda", dtype=dtype)
    b = torch.randn(n, n, device="cuda", dtype=dtype)
    for _ in range(3):
        a @ b
    torch.cuda.synchronize()
    s, e = torch.cuda.Event(enable_timing=True), torch.cuda.Event(enable_timing=True)
    s.record()
    for _ in range(10):
        a @ b
    e.record(); torch.cuda.synchronize()
    return 10 * 2 * n ** 3 / (s.elapsed_time(e) / 1e3)


def main():
    ap = argparse.ArgumentParser()
    ap.add_argument("--flops", type=float, default=0.0)
    ap.add_argument("--bytes", type=float, default=0.0)
    ap.add_argument("--dtype", default="bf16")
    a = ap.parse_args()

    p = torch.cuda.get_device_properties(0)
    bw, fl = measured_bandwidth(), measured_flops()
    print(f"device            {p.name}  sm_{p.major}{p.minor}  SMs={p.multi_processor_count}  "
          f"smem_optin={p.shared_memory_per_block_optin//1024}KB")
    print(f"measured bandwidth {bw/1e12:.3f} TB/s   measured dense bf16 {fl/1e12:.1f} TFLOP/s")
    if not (a.flops or a.bytes):
        return
    t_mem = a.bytes / bw if a.bytes else 0.0
    t_cmp = a.flops / fl if a.flops else 0.0
    bound = "memory" if t_mem >= t_cmp else "compute"
    print(f"\nwork              {a.flops:.3g} FLOP   {a.bytes:.3g} B   "
          f"intensity {(a.flops/a.bytes if a.bytes else float('inf')):.1f} FLOP/B")
    print(f"floor (memory)    {t_mem*1e6:9.1f} us")
    print(f"floor (compute)   {t_cmp*1e6:9.1f} us")
    print(f"-> {bound}-bound, floor {max(t_mem,t_cmp)*1e6:.1f} us")
    print("\nNote: this floor comes from an ATTRIBUTION formula, not a physical wall. A kernel that "
          "moves fewer bytes than counted -- cache reuse, recompute, a better layout -- can beat it. "
          "Orientation, not a stopping rule.")


if __name__ == "__main__":
    main()