example_gqa_fwd_bshd.py

import torch
import torch.nn.functional as F
import tilelang
from tilelang.autotuner import *
import tilelang.language as T
import itertools
import argparse
from functools import partial


class FlashAttentionTuneSpace:
    def __init__(
        self,
        block_sizes=(64, 128, 256),
        thread_options=(128, 256, 512),
        num_stages_range=(2, 3),
        max_shared_mem=100 * 1024,
        warp_alignment=16,
        dim=128,
        dtype_bytes=2,
    ):
        self.block_sizes = block_sizes
        self.thread_options = thread_options
        self.num_stages_range = num_stages_range
        self.max_shared_mem = max_shared_mem
        self.warp_alignment = warp_alignment
        self.dim = dim
        self.dtype_bytes = dtype_bytes


def get_configs(user_config=None):
    config = user_config or FlashAttentionTuneSpace()
    valid_configs = []

    for block_M, block_N in itertools.product(config.block_sizes, repeat=2):
        for threads in config.thread_options:
            assert threads % 32 == 0
            warp_count = threads // 32
            warp_M = block_M // warp_count
            warp_N = block_N // warp_count

            if warp_M % config.warp_alignment != 0 or warp_N % config.warp_alignment != 0:
                continue

            shared_mem = 2 * config.dtype_bytes * config.dim * (block_M + block_N)
            if shared_mem > config.max_shared_mem:
                continue

            for num_stages in config.num_stages_range:
                valid_configs.append(
                    {
                        "block_M": block_M,
                        "block_N": block_N,
                        "num_stages": num_stages,
                        "threads": threads,
                    }
                )
    return valid_configs


@autotune(configs=get_configs(), warmup=10, rep=10)
@tilelang.jit(
    out_idx=[3],
    pass_configs={
        tilelang.PassConfigKey.TL_ENABLE_FAST_MATH: True,
    },
)
def flashattn(batch, heads, seq_len, dim, is_causal, groups=1, block_M=64, block_N=64, num_stages=0, threads=128):
    scale = (1.0 / dim) ** 0.5 * 1.44269504  # log2(e)
    head_kv = heads // groups
    q_shape = [batch, seq_len, heads, dim]
    kv_shape = [batch, seq_len, head_kv, dim]
    dtype = T.float16
    accum_dtype = T.float32

    @T.macro
    def MMA0(
        K: T.Tensor(kv_shape, dtype),
        Q_shared: T.SharedBuffer([block_M, dim], dtype),
        K_shared: T.SharedBuffer([block_N, dim], dtype),
        acc_s: T.FragmentBuffer([block_M, block_N], accum_dtype),
        k: T.int32,
        bx: T.int32,
        by: T.int32,
        bz: T.int32,
    ):
        T.copy(K[bz, k * block_N : (k + 1) * block_N, by // groups, :], K_shared)
        if is_causal:
            for i, j in T.Parallel(block_M, block_N):
                acc_s[i, j] = T.if_then_else(bx * block_M + i >= k * block_N + j, 0, -T.infinity(acc_s.dtype))
        else:
            for i, j in T.Parallel(block_M, block_N):
                acc_s[i, j] = T.if_then_else(k * block_N + j >= seq_len, -T.infinity(acc_s.dtype), 0)
        T.gemm(Q_shared, K_shared, acc_s, transpose_B=True, policy=T.GemmWarpPolicy.FullRow)

    @T.macro
    def MMA1(
        V: T.Tensor(kv_shape, dtype),
        V_shared: T.SharedBuffer([block_N, dim], dtype),
        acc_s_cast: T.FragmentBuffer([block_M, block_N], dtype),
        acc_o: T.FragmentBuffer([block_M, dim], accum_dtype),
        k: T.int32,
        by: T.int32,
        bz: T.int32,
    ):
        T.copy(V[bz, k * block_N : (k + 1) * block_N, by // groups, :], V_shared)
        T.gemm(acc_s_cast, V_shared, acc_o, policy=T.GemmWarpPolicy.FullRow)

    @T.macro
    def Softmax(
        acc_s: T.FragmentBuffer([block_M, block_N], accum_dtype),
        acc_s_cast: T.FragmentBuffer([block_M, block_N], dtype),
        scores_max: T.FragmentBuffer([block_M], accum_dtype),
        scores_max_prev: T.FragmentBuffer([block_M], accum_dtype),
        scores_scale: T.FragmentBuffer([block_M], accum_dtype),
        scores_sum: T.FragmentBuffer([block_M], accum_dtype),
        logsum: T.FragmentBuffer([block_M], accum_dtype),
    ):
        T.copy(scores_max, scores_max_prev)
        T.fill(scores_max, -T.infinity(accum_dtype))
        T.reduce_max(acc_s, scores_max, dim=1, clear=False)
        for i in T.Parallel(block_M):
            scores_max[i] = T.max(scores_max[i], scores_max_prev[i])
        # To do causal softmax, we need to set the scores_max to 0 if it is -inf
        # This process is called Check_inf in FlashAttention3 code, and it only need to be done
        # in the first ceil_div(kBlockM, kBlockN) steps.
        # for i in T.Parallel(block_M):
        #     scores_max[i] = T.if_then_else(scores_max[i] == -T.infinity(accum_dtype), 0, scores_max[i])
        for i in T.Parallel(block_M):
            scores_scale[i] = T.exp2(scores_max_prev[i] * scale - scores_max[i] * scale)
        for i, j in T.Parallel(block_M, block_N):
            # Instead of computing exp(x - max), we compute exp2(x * log_2(e) -
            # max * log_2(e)) This allows the compiler to use the ffma
            # instruction instead of fadd and fmul separately.
            acc_s[i, j] = T.exp2(acc_s[i, j] * scale - scores_max[i] * scale)
        T.reduce_sum(acc_s, scores_sum, dim=1)
        for i in T.Parallel(block_M):
            logsum[i] = logsum[i] * scores_scale[i] + scores_sum[i]
        T.copy(acc_s, acc_s_cast)

    @T.macro
    def Rescale(
        acc_o: T.FragmentBuffer([block_M, dim], accum_dtype),
        scores_scale: T.FragmentBuffer([block_M], accum_dtype),
    ):
        for i, j in T.Parallel(block_M, dim):
            acc_o[i, j] *= scores_scale[i]

    @T.prim_func
    def main(
        Q: T.Tensor(q_shape, dtype),
        K: T.Tensor(kv_shape, dtype),
        V: T.Tensor(kv_shape, dtype),
        Output: T.Tensor(q_shape, dtype),
    ):
        with T.Kernel(T.ceildiv(seq_len, block_M), heads, batch, threads=threads) as (bx, by, bz):
            Q_shared = T.alloc_shared([block_M, dim], dtype)
            K_shared = T.alloc_shared([block_N, dim], dtype)
            V_shared = T.alloc_shared([block_N, dim], dtype)
            O_shared = T.alloc_shared([block_M, dim], dtype)
            acc_s = T.alloc_fragment([block_M, block_N], accum_dtype)
            acc_s_cast = T.alloc_fragment([block_M, block_N], dtype)
            acc_o = T.alloc_fragment([block_M, dim], accum_dtype)
            scores_max = T.alloc_fragment([block_M], accum_dtype)
            scores_max_prev = T.alloc_fragment([block_M], accum_dtype)
            scores_scale = T.alloc_fragment([block_M], accum_dtype)
            scores_sum = T.alloc_fragment([block_M], accum_dtype)
            logsum = T.alloc_fragment([block_M], accum_dtype)

            T.copy(Q[bz, bx * block_M : (bx + 1) * block_M, by, :], Q_shared)
            T.fill(acc_o, 0)
            T.fill(logsum, 0)
            T.fill(scores_max, -T.infinity(accum_dtype))

            loop_range = (
                T.min(T.ceildiv(seq_len, block_N), T.ceildiv((bx + 1) * block_M, block_N)) if is_causal else T.ceildiv(seq_len, block_N)
            )

            for k in T.Pipelined(loop_range, num_stages=num_stages):
                MMA0(K, Q_shared, K_shared, acc_s, k, bx, by, bz)
                Softmax(acc_s, acc_s_cast, scores_max, scores_max_prev, scores_scale, scores_sum, logsum)
                Rescale(acc_o, scores_scale)
                MMA1(V, V_shared, acc_s_cast, acc_o, k, by, bz)
            for i, j in T.Parallel(block_M, dim):
                acc_o[i, j] /= logsum[i]
            T.copy(acc_o, O_shared)
            T.copy(O_shared, Output[bz, bx * block_M : (bx + 1) * block_M, by, :])

    return main


def ref_program(Q, K, V, is_causal, groups=1):
    # Q: [B, T, HQ, D]
    # K: [B, T, HK, D]
    # V: [B, T, HV, D]
    # HQ = HKV * groups
    assert Q.size(2) == K.size(2) * groups, f"Q.size(2): {Q.size(2)}, K.size(2): {K.size(2)}, groups: {groups}"
    assert Q.size(2) == V.size(2) * groups, f"Q.size(2): {Q.size(2)}, V.size(2): {V.size(2)}, groups: {groups}"

    dim = Q.size(-1)
    K = K.repeat_interleave(groups, dim=2)
    V = V.repeat_interleave(groups, dim=2)
    scores = torch.einsum("bqhd,bkhd->bhqk", Q, K)
    scores = scores / torch.sqrt(torch.tensor(dim, dtype=scores.dtype))
    if is_causal:
        seq_len = Q.size(1)
        mask = torch.tril(torch.ones(seq_len, seq_len, device=scores.device))
        mask = mask.unsqueeze(0).unsqueeze(0)
        scores = scores.masked_fill(mask == 0, float("-inf"))
    attention_weights = F.softmax(scores, dim=-1)
    output = torch.einsum("bhqk,bkhd->bqhd", attention_weights, V)
    return output


def main(
    batch: int = 1, heads: int = 64, seq_len: int = 4096, dim: int = 128, is_causal: bool = False, groups: int = 16, tune: bool = False
):
    flops_per_matmul = 2.0 * batch * heads * seq_len * seq_len * dim
    total_flops = 2 * flops_per_matmul
    if is_causal:
        total_flops *= 0.5

    if not tune:
        kernel = flashattn(batch, heads, seq_len, dim, is_causal, groups=groups, block_M=64, block_N=64, num_stages=2, threads=128)
        ref_program_processed = partial(ref_program, is_causal=is_causal, groups=groups)
        profiler = kernel.get_profiler(tensor_supply_type=tilelang.TensorSupplyType.Normal)
        profiler.assert_allclose(ref_program_processed, rtol=0.01, atol=0.01)
        print("All checks pass.")
        latency = profiler.do_bench(ref_program_processed, warmup=500)
        print("Ref: {:.2f} ms".format(latency))
        print("Ref: {:.2f} TFlops".format(total_flops / latency * 1e-9))
        latency = profiler.do_bench(warmup=500)
        print("Tile-lang: {:.2f} ms".format(latency))
        print("Tile-lang: {:.2f} TFlops".format(total_flops / latency * 1e-9))
    else:
        kernel = flashattn(batch, heads, seq_len, dim, is_causal)
        best_latency = kernel.latency
        best_config = kernel.config
        ref_latency = kernel.ref_latency
        print(f"Best latency: {best_latency}")
        print(f"Best TFlops: {total_flops / best_latency * 1e-9}")
        print(f"Best config: {best_config}")
        print(f"Ref latency: {ref_latency}")


if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument("--batch", type=int, default=1, help="batch size")
    parser.add_argument("--heads", type=int, default=64, help="heads")
    parser.add_argument("--seq_len", type=int, default=4096, help="sequence length")
    parser.add_argument("--dim", type=int, default=128, help="dim")
    parser.add_argument("--is_causal", action="store_true", help="causal")
    parser.add_argument("--tune", action="store_true", help="tune configs")
    parser.add_argument("--groups", type=int, default=16, help="groups")
    args = parser.parse_args()
    main(args.batch, args.heads, args.seq_len, args.dim, args.is_causal, args.groups, args.tune)