submission 80638

# k: 16384; l: 1; m: 7168; seed: 1111
#  ⏱ 40.0 ± 0.03 µs
#  ⚡ 39.8 µs 🐌 41.3 µs

# k: 7168; l: 8; m: 4096; seed: 1111
#  ⏱ 59.5 ± 0.07 µs
#  ⚡ 58.2 µs 🐌 62.5 µs

# k: 2048; l: 4; m: 7168; seed: 1111
#  ⏱ 29.6 ± 0.03 µs
#  ⚡ 27.6 µs 🐌 29.8 µs
import torch
from task import input_t, output_t

import cutlass
import cutlass.cute as cute
from cutlass.cute.runtime import make_ptr
import cutlass.utils.blockscaled_layout as blockscaled_utils
from cutlass import Float32
from cutlass.cutlass_dsl import T, dsl_user_op
from cutlass._mlir.dialects import nvvm, llvm
from cutlass.utils import SmemAllocator

# Kernel configuration parameters
threads_per_m = 128
# Make sure threads_per_m is divisible by 1024
threads_per_k  = 1024 // threads_per_m
mma_tiler_mnk = (threads_per_m, 1, 64)
ab_dtype = cutlass.Float4E2M1FN  # FP4 data type for A and B
sf_dtype = cutlass.Float8E4M3FN  # FP8 data type for scale factors
c_dtype = cutlass.Float16  # FP16 output type
sf_vec_size = 16  # Scale factor block size (16 elements share one scale)

# Helper function for ceiling division
def ceil_div(a, b):
    return (a + b - 1) // b

# https://github.com/Dao-AILab/flash-attention/blob/5d2cd3bcbaeff6fe1bfc5d0ff489451b0d4827a6/flash_attn/cute/utils.py#L403
@dsl_user_op
def atomic_add_fp32(a: float | Float32, gmem_ptr: cute.Pointer, *, loc=None, ip=None) -> None:
    nvvm.atomicrmw(
        res=T.f32(), op=nvvm.AtomicOpKind.FADD, ptr=gmem_ptr.llvm_ptr, a=Float32(a).ir_value()
    )

# https://github.com/Dao-AILab/flash-attention/blob/5d2cd3bcbaeff6fe1bfc5d0ff489451b0d4827a6/flash_attn/cute/utils.py#L424C1-L426C70
@dsl_user_op
def elem_pointer(x: cute.Tensor, coord: cute.Coord, *, loc=None, ip=None) -> cute.Pointer:
    return x.iterator + cute.crd2idx(coord, x.layout, loc=loc, ip=ip)

# https://github.com/Dao-AILab/flash-attention/blob/5d2cd3bcbaeff6fe1bfc5d0ff489451b0d4827a6/flash_attn/cute/utils.py#L778C1-L783C22
@cute.jit
def scalar_to_ssa(a: cute.Numeric, dtype) -> cute.TensorSSA:
    """ Convert a scalar to a cute TensorSSA of shape (1,) and given dtype """
    vec = cute.make_fragment(1, dtype)
    vec[0] = a
    return vec.load()

# The CuTe reference implementation for NVFP4 block-scaled GEMV
@cute.kernel
def kernel(
    mA_mkl: cute.Tensor,
    mB_nkl: cute.Tensor,
    mSFA_mkl: cute.Tensor,
    mSFB_nkl: cute.Tensor,
    mC_mnl: cute.Tensor,
):
    # Get CUDA block and thread indices
    bidx, bidy, bidz = cute.arch.block_idx()
    tidx, tidy, _ = cute.arch.thread_idx()

    # Extract the local tile for input matrix A (shape: [block_M, block_K, rest_M, rest_K, rest_L])
    gA_mkl = cute.local_tile(
        mA_mkl, cute.slice_(mma_tiler_mnk, (None, 0, None)), (None, None, None)
    )
    # Extract the local tile for scale factor tensor for A (same shape as gA_mkl)
    # Here, block_M = (32, 4); block_K = (16, 4)
    gSFA_mkl = cute.local_tile(
        mSFA_mkl, cute.slice_(mma_tiler_mnk, (None, 0, None)), (None, None, None)
    )
    # Extract the local tile for input matrix B (shape: [block_N, block_K, rest_N, rest_K, rest_L])
    gB_nkl = cute.local_tile(
        mB_nkl, cute.slice_(mma_tiler_mnk, (0, None, None)), (None, None, None)
    )
    # Extract the local tile for scale factor tensor for B (same shape as gB_nkl)
    gSFB_nkl = cute.local_tile(
        mSFB_nkl, cute.slice_(mma_tiler_mnk, (0, None, None)), (None, None, None)
    )
    # Extract the local tile for output matrix C (shape: [block_M, block_N, rest_M, rest_N, rest_L])
    gC_mnl = cute.local_tile(
        mC_mnl, cute.slice_(mma_tiler_mnk, (None, None, 0)), (None, None, None)
    )

    # Select output element corresponding to this thread and block indices
    tCgC = gC_mnl[tidx, None, bidx, bidy, bidz]
    tCgC = cute.make_tensor(tCgC.iterator, 1)
    res = cute.zeros_like(tCgC, cutlass.Float32)

    allocator = SmemAllocator()
    # Allocate a buffer for row sum accumulation in shared memory
    row_sum_buffer = allocator.allocate_tensor(element_type=cutlass.Float32, layout=cute.make_layout(mma_tiler_mnk[0]))

    if tidy == 0:
        # Set the row sum buffer to 0 using the first thread in each row
        row_sum_buffer[tidx] = 0.0
    cute.arch.sync_threads()

    k_tile_cnt = gA_mkl.layout[3].shape
    for k_tile in range(tidy, k_tile_cnt, threads_per_k):
        tAgA = gA_mkl[tidx, None, bidx, k_tile, bidz]
        tBgB = gB_nkl[0, None, bidy, k_tile, bidz]
        tAgSFA = gSFA_mkl[tidx, (0, None), bidx, k_tile, bidz]
        tBgSFB = gSFB_nkl[0, (0, None), bidy, k_tile, bidz]

        tArA = cute.make_rmem_tensor_like(tAgA, cutlass.Float16)
        tBrB = cute.make_rmem_tensor_like(tBgB, cutlass.Float16)
        tArSFA = cute.make_rmem_tensor_like(tAgSFA, cutlass.Float32)
        tBrSFB = cute.make_rmem_tensor_like(tBgSFB, cutlass.Float32)
        

        # Load NVFP4 or FP8 values from global memory
        a_val_nvfp4 = tAgA.load()
        b_val_nvfp4 = tBgB.load()
        sfa_val_fp8 = tAgSFA.load()
        sfb_val_fp8 = tBgSFB.load()

        # Store the converted values to RMEM CuTe tensors
        tArA.store(a_val_nvfp4.to(cutlass.Float16))
        tBrB.store(b_val_nvfp4.to(cutlass.Float16))
        tArSFA.store(sfa_val_fp8.to(cutlass.Float32))
        tBrSFB.store(sfb_val_fp8.to(cutlass.Float32))

        # Iterate over SF vector tiles and compute the scale&matmul accumulation
        for sf_block in cutlass.range_constexpr(mma_tiler_mnk[2] // sf_vec_size):
            tmp = cute.zeros_like(tCgC, cutlass.Float32)
            base = sf_block * sf_vec_size

            for offset in cutlass.range_constexpr(sf_vec_size):
                tmp += tArA[base + offset] * tBrB[base + offset]
            res += tArSFA[sf_block] * tBrSFB[sf_block] * tmp

    atomic_add_fp32(res[0], elem_pointer(row_sum_buffer, tidx))
    cute.arch.sync_threads()
    if tidy == 0:
        out = scalar_to_ssa(row_sum_buffer[tidx], cutlass.Float32)
        tCgC.store(out.to(cutlass.Float16))
    return

@cute.jit
def my_kernel(
    a_ptr: cute.Pointer,
    b_ptr: cute.Pointer,
    sfa_ptr: cute.Pointer,
    sfb_ptr: cute.Pointer,
    c_ptr: cute.Pointer,
    problem_size: tuple,
):
    """
    Host-side JIT function to prepare tensors and launch GPU kernel.
    """
    m, _, k, l = problem_size
    # Create CuTe Tensor via pointer and problem size.
    a_tensor = cute.make_tensor(
        a_ptr,
        cute.make_layout(
            (m, cute.assume(k, 32), l),
            stride=(cute.assume(k, 32), 1, cute.assume(m * k, 32)),
        ),
    )
    # We use n=128 to create the torch tensor to do fp4 computation via torch._scaled_mm
    # then copy torch tensor to cute tensor for cute customize kernel computation
    # therefore we need to ensure b_tensor has the right stride with this 128 padded size on n.
    n_padded_128 = 128
    b_tensor = cute.make_tensor(
        b_ptr,
        cute.make_layout(
            (n_padded_128, cute.assume(k, 32), l),
            stride=(cute.assume(k, 32), 1, cute.assume(n_padded_128 * k, 32)),
        ),
    )
    c_tensor = cute.make_tensor(
        c_ptr, cute.make_layout((cute.assume(m, 32), 1, l), stride=(1, 1, m))
    )
    # Convert scale factor tensors to MMA layout
    # The layout matches Tensor Core requirements: (((32, 4), REST_M), ((SF_K, 4), REST_K), (1, REST_L))
    sfa_layout = blockscaled_utils.tile_atom_to_shape_SF(a_tensor.shape, sf_vec_size)
    sfa_tensor = cute.make_tensor(sfa_ptr, sfa_layout)

    sfb_layout = blockscaled_utils.tile_atom_to_shape_SF(b_tensor.shape, sf_vec_size)
    sfb_tensor = cute.make_tensor(sfb_ptr, sfb_layout)

    # Compute grid dimensions
    # Grid is (M_blocks, 1, L) where:
    # - M_blocks = ceil(M / 128) to cover all output rows
    # - L = batch size
    grid = (
        cute.ceil_div(c_tensor.shape[0], threads_per_m),
        1,
        c_tensor.shape[2],
    )

    # Launch the CUDA kernel
    kernel(a_tensor, b_tensor, sfa_tensor, sfb_tensor, c_tensor).launch(
        grid=grid,
        block=[threads_per_m, threads_per_k, 1],
        cluster=(1, 1, 1),
    )
    return


# Global cache for compiled kernel
_compiled_kernel_cache = None


# This function is used to compile the kernel once and cache it and then allow users to
# run the kernel multiple times to get more accurate timing results.
def compile_kernel():
    """
    Compile the kernel once and cache it.
    This should be called before any timing measurements.

    Returns:
        The compiled kernel function
    """
    global _compiled_kernel_cache

    if _compiled_kernel_cache is not None:
        return _compiled_kernel_cache

    # Create CuTe pointers for A/B/C/SFA/SFB via torch tensor data pointer
    a_ptr = make_ptr(ab_dtype, 0, cute.AddressSpace.gmem, assumed_align=16)
    b_ptr = make_ptr(ab_dtype, 0, cute.AddressSpace.gmem, assumed_align=16)
    c_ptr = make_ptr(c_dtype, 0, cute.AddressSpace.gmem, assumed_align=16)
    sfa_ptr = make_ptr(sf_dtype, 0, cute.AddressSpace.gmem, assumed_align=32)
    sfb_ptr = make_ptr(sf_dtype, 0, cute.AddressSpace.gmem, assumed_align=32)

    # Compile the kernel
    _compiled_kernel_cache = cute.compile(
        my_kernel, a_ptr, b_ptr, sfa_ptr, sfb_ptr, c_ptr, (0, 0, 0, 0)
    )

    return _compiled_kernel_cache


def custom_kernel(data: input_t) -> output_t:
    """
    Execute the block-scaled GEMV kernel.

    This is the main entry point called by the evaluation framework.
    It converts PyTorch tensors to CuTe tensors, launches the kernel,
    and returns the result.

    Args:
        data: Tuple of (a, b, sfa_cpu, sfb_cpu, c) PyTorch tensors
            a: [m, k, l] - Input matrix in float4e2m1fn
            b: [1, k, l] - Input vector in float4e2m1fn
            sfa_cpu: [m, k, l] - Scale factors in float8_e4m3fn
            sfb_cpu: [1, k, l] - Scale factors in float8_e4m3fn
            sfa_permuted: [32, 4, rest_m, 4, rest_k, l] - Scale factors in float8_e4m3fn
            sfb_permuted: [32, 4, rest_n, 4, rest_k, l] - Scale factors in float8_e4m3fn
            c: [m, 1, l] - Output vector in float16

    Returns:
        Output tensor c with computed GEMV results
    """
    a, b, _, _, sfa_permuted, sfb_permuted, c = data

    # Ensure kernel is compiled (will use cached version if available)
    # To avoid the compilation overhead, we compile the kernel once and cache it.
    compiled_func = compile_kernel()

    # Get dimensions from MxKxL layout
    m, k, l = a.shape
    # Torch use e2m1_x2 data type, thus k is halved
    k = k * 2
    # GEMV N dimension is always 1
    n = 1

    # Create CuTe pointers for A/B/C/SFA/SFB via torch tensor data pointer
    a_ptr = make_ptr(ab_dtype, a.data_ptr(), cute.AddressSpace.gmem, assumed_align=16)
    b_ptr = make_ptr(ab_dtype, b.data_ptr(), cute.AddressSpace.gmem, assumed_align=16)
    c_ptr = make_ptr(c_dtype, c.data_ptr(), cute.AddressSpace.gmem, assumed_align=16)

    sfa_ptr = make_ptr(
        sf_dtype, sfa_permuted.data_ptr(), cute.AddressSpace.gmem, assumed_align=32
    )
    sfb_ptr = make_ptr(
        sf_dtype, sfb_permuted.data_ptr(), cute.AddressSpace.gmem, assumed_align=32
    )

    # Execute the compiled kernel
    compiled_func(a_ptr, b_ptr, sfa_ptr, sfb_ptr, c_ptr, (m, n, k, l))

    return c