example_amd_flash_attn_bwd.py

import torch
import torch.nn.functional as F
import tilelang
import tilelang.language as T
from tilelang.primitives.gemm.base import GemmWarpPolicy
import itertools
import argparse
from functools import partial
import numpy as np
import time


def ref_program(Q, K, V, is_causal, groups=1):
    assert Q.size(2) == K.size(2) * groups, f"Q heads {Q.size(2)} K heads {K.size(2)} groups {groups}"
    assert Q.size(2) == V.size(2) * groups, f"Q heads {Q.size(2)} V heads {V.size(2)} groups {groups}"
    dim = Q.size(-1)
    K_ref = K.repeat_interleave(groups, dim=2)
    V_ref = V.repeat_interleave(groups, dim=2)
    scores = torch.einsum("bqhd,bkhd->bhqk", Q, K_ref)
    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_ref)
    lse = torch.logsumexp(scores, dim=-1).float()
    return output, lse


def get_fwd_configs():
    block_M = [32, 64, 128, 256]
    block_N = [32, 64, 128, 256]
    threads = [128, 256, 512]
    num_split_q = [64, 128, 256]
    num_stages = [0, 1]
    enable_rasterization = [True]
    k_pack = [2]
    panel_size = [7, 8, 9, 10]
    qk_coalesced_width = [8]
    v_coalesced_width = [4]

    valid_configs = []

    for m, n, s, t, stages, r, k, p, qkw, vw in itertools.product(
        block_M, block_N, num_split_q, threads, num_stages, enable_rasterization, k_pack, panel_size, qk_coalesced_width, v_coalesced_width
    ):
        valid_configs.append(
            {
                "block_M": m,
                "block_N": n,
                "num_split_q": s,
                "threads": t,
                "num_stages": stages,
                "enable_rasterization": r,
                "k_pack": k,
                "panel_size": p,
                "qk_coalesced_width": qkw,
                "v_coalesced_width": vw,
            }
        )
    return valid_configs


@tilelang.autotune(configs=get_fwd_configs(), cache_input_tensors=True)
@tilelang.jit(out_idx=[3, 4])
def fast_flashattn(
    batch,
    heads,
    seq_len,
    dim,
    is_causal,
    groups,
    block_M: int,
    block_N: int,
    num_split_q: int,
    threads: int,
    num_stages: int,
    enable_rasterization: bool,
    k_pack: int,
    panel_size: int,
    qk_coalesced_width: int,
    v_coalesced_width: int,
):
    scale = (1.0 / dim) ** 0.5
    head_kv = heads // groups
    q_shape = [batch, seq_len, heads, dim]
    kv_shape = [batch, seq_len, head_kv, dim]
    dtype = "float16"
    accum_dtype = "float"

    vec_size = qk_coalesced_width
    v_vec_size = v_coalesced_width

    @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),
        LSE: T.Tensor([batch, heads, seq_len], accum_dtype),
    ):
        with T.Kernel(num_split_q, batch * heads, threads=threads) as (b_split, byz_combined):
            T.use_swizzle(panel_size, enable=enable_rasterization)

            bz = byz_combined // heads
            by = byz_combined % heads

            num_q_blocks = T.ceildiv(seq_len, block_M)

            bx_loop_var = T.alloc_var("int32")
            bx_loop_var = b_split

            with T.While(bx_loop_var < num_q_blocks):
                acc_o = T.alloc_fragment([block_M, dim], accum_dtype)
                m_i = T.alloc_fragment([block_M], accum_dtype)
                l_i = T.alloc_fragment([block_M], accum_dtype)

                T.fill(acc_o, 0)
                T.fill(m_i, -T.infinity(accum_dtype))
                T.fill(l_i, 0)

                current_bx = bx_loop_var
                q_block_offset = current_bx * block_M

                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)
                acc_s_cast = T.alloc_fragment([block_M, block_N], dtype)

                acc_s = T.alloc_fragment([block_M, block_N], accum_dtype)
                m_prev = T.alloc_fragment([block_M], accum_dtype)
                scale_factor = T.alloc_fragment([block_M], accum_dtype)

                T.copy(Q[bz, q_block_offset : q_block_offset + block_M, by, :], Q_shared, coalesced_width=vec_size)

                loop_end_k = T.ceildiv(q_block_offset + block_M, block_N) if is_causal else T.ceildiv(seq_len, block_N)

                row_sum = T.alloc_fragment([block_M], accum_dtype)

                for k in T.Pipelined(loop_end_k, num_stages=num_stages):
                    kv_idx = k * block_N

                    T.copy(K[bz, kv_idx : kv_idx + block_N, by // groups, :], K_shared, coalesced_width=vec_size)
                    T.copy(V[bz, kv_idx : kv_idx + block_N, by // groups, :], V_shared, coalesced_width=v_vec_size)

                    if is_causal:
                        for i, j in T.Parallel(block_M, block_N):
                            acc_s[i, j] = T.if_then_else(q_block_offset + i >= kv_idx + j, 0, -T.infinity(acc_s.dtype))
                    else:
                        T.clear(acc_s)
                    T.gemm(
                        Q_shared,
                        K_shared,
                        acc_s,
                        transpose_B=True,
                        k_pack=k_pack,
                        policy=GemmWarpPolicy.FullRow,
                    )

                    for i, j in T.Parallel(block_M, block_N):
                        acc_s[i, j] = acc_s[i, j] * scale

                    T.copy(m_i, m_prev)
                    T.reduce_max(acc_s, m_i, dim=1, clear=False)
                    for i in T.Parallel(block_M):
                        m_i[i] = T.max(m_i[i], m_prev[i])

                    for i in T.Parallel(block_M):
                        if m_prev[i] == -T.infinity(accum_dtype):
                            scale_factor[i] = 0.0
                        else:
                            scale_factor[i] = T.exp(m_prev[i] - m_i[i])

                        l_i[i] *= scale_factor[i]

                    for i, j in T.Parallel(block_M, dim):
                        acc_o[i, j] *= scale_factor[i]

                    for i, j in T.Parallel(block_M, block_N):
                        if acc_s[i, j] == -T.infinity(acc_s.dtype):
                            acc_s[i, j] = 0.0
                        else:
                            acc_s[i, j] = T.exp(acc_s[i, j] - m_i[i])

                    T.reduce_sum(acc_s, row_sum, dim=1)
                    for i in T.Parallel(block_M):
                        l_i[i] += row_sum[i]

                    T.copy(acc_s, acc_s_cast)

                    T.gemm(acc_s_cast, V_shared, acc_o, policy=GemmWarpPolicy.FullRow)

                l_inv = T.alloc_fragment([block_M], accum_dtype)
                for i in T.Parallel(block_M):
                    safe_l = T.if_then_else(l_i[i] > 1e-6, l_i[i], 1.0)
                    l_inv[i] = 1.0 / safe_l

                for i, j in T.Parallel(block_M, dim):
                    Output[bz, q_block_offset + i, by, j] = acc_o[i, j] * l_inv[i]

                for i in T.Parallel(block_M):
                    if q_block_offset + i < seq_len:
                        lse_val = T.if_then_else(l_i[i] > 0, T.log(l_i[i]) + m_i[i], -T.infinity(accum_dtype))
                        LSE[bz, by, q_block_offset + i] = lse_val

                bx_loop_var = current_bx + num_split_q

    return main


def get_bwd_configs():
    block_M = [16, 32, 64, 128, 256]
    block_N = [16, 32, 64, 128, 256]
    threads = [64, 128, 256, 512, 1024]
    num_stages = [0, 1, 2]
    enable_rasterization = [True]
    panel_size = [7, 8, 9, 10]

    configs = []
    for m, n, stages, t, r, p in itertools.product(block_M, block_N, num_stages, threads, enable_rasterization, panel_size):
        configs.append(
            {
                "block_M": m,
                "block_N": n,
                "num_stages": stages,
                "threads": t,
                "enable_rasterization": r,
                "panel_size": p,
            }
        )

    return configs


@tilelang.jit(out_idx=[2])
def flashattn_bwd_preprocess(batch, heads, seq_len, dim):
    dtype = "float16"
    accum_dtype = "float"
    shape = [batch, seq_len, heads, dim]
    blk = 32

    @T.prim_func
    def flash_bwd_prep(O: T.Tensor(shape, dtype), dO: T.Tensor(shape, dtype), Delta: T.Tensor([batch, heads, seq_len], accum_dtype)):
        with T.Kernel(batch, heads, T.ceildiv(seq_len, blk)) as (bz, bx, by):
            o = T.alloc_fragment([blk, blk], dtype)
            do = T.alloc_fragment([blk, blk], dtype)
            acc = T.alloc_fragment([blk, blk], accum_dtype)
            delta = T.alloc_fragment([blk], accum_dtype)
            T.clear(acc)
            for k in range(T.ceildiv(dim, blk)):
                T.copy(O[bz, by * blk : (by + 1) * blk, bx, k * blk : (k + 1) * blk], o)
                T.copy(dO[bz, by * blk : (by + 1) * blk, bx, k * blk : (k + 1) * blk], do)
                for i, j in T.Parallel(blk, blk):
                    acc[i, j] += o[i, j] * do[i, j]
            T.reduce_sum(acc, delta, 1)
            T.copy(delta, Delta[bz, bx, by * blk : (by + 1) * blk])

    return flash_bwd_prep


@tilelang.autotune(configs=get_bwd_configs(), cache_input_tensors=True)
@tilelang.jit
def flashattn_bwd(
    batch,
    heads,
    seq_len,
    dim,
    is_causal,
    groups,
    block_M: int,
    block_N: int,
    num_stages: int,
    threads: int,
    enable_rasterization: bool,
    panel_size: int,
):
    sm_scale = (1.0 / dim) ** 0.5
    head_kv = heads // groups
    q_shape = [batch, seq_len, heads, dim]
    kv_shape = [batch, seq_len, head_kv, dim]
    dtype = "float16"
    accum_dtype = "float"

    @T.prim_func
    def flash_bwd_kernel(
        Q: T.Tensor(q_shape, dtype),
        K: T.Tensor(kv_shape, dtype),
        V: T.Tensor(kv_shape, dtype),
        dO: T.Tensor(q_shape, dtype),
        lse: T.Tensor([batch, heads, seq_len], accum_dtype),
        Delta: T.Tensor([batch, heads, seq_len], accum_dtype),
        dQ: T.Tensor(q_shape, accum_dtype),
        dK: T.Tensor(kv_shape, accum_dtype),
        dV: T.Tensor(kv_shape, accum_dtype),
    ):
        with T.Kernel(heads, T.ceildiv(seq_len, block_M), batch, threads=threads) as (bx, by, bz):
            T.use_swizzle(panel_size, enable=enable_rasterization)

            K_shared = T.alloc_shared([block_M, dim], dtype)
            V_shared = T.alloc_shared([block_M, dim], dtype)
            q_shared = T.alloc_shared([block_N, dim], dtype)
            do_shared = T.alloc_shared([block_N, dim], dtype)
            lse_shared = T.alloc_shared([block_N], accum_dtype)
            delta_shared = T.alloc_shared([block_N], accum_dtype)
            ds_shared = T.alloc_shared([block_M, block_N], dtype)

            p_cast = T.alloc_fragment([block_M, block_N], dtype)
            qkT = T.alloc_fragment([block_M, block_N], accum_dtype)
            P_acc = T.alloc_fragment([block_M, block_N], accum_dtype)
            dP = T.alloc_fragment([block_M, block_N], accum_dtype)

            dv = T.alloc_fragment([block_M, dim], accum_dtype)
            dk = T.alloc_fragment([block_M, dim], accum_dtype)
            dq = T.alloc_fragment([block_N, dim], accum_dtype)

            T.copy(K[bz, by * block_M : (by + 1) * block_M, bx // groups, :], K_shared)
            T.copy(V[bz, by * block_M : (by + 1) * block_M, bx // groups, :], V_shared)
            T.clear(dv)
            T.clear(dk)

            loop_st = T.floordiv(by * block_M, block_N) if is_causal else 0
            loop_ed = T.ceildiv(seq_len, block_N)

            for k in T.Pipelined(loop_st, loop_ed, num_stages=num_stages):
                T.copy(Q[bz, k * block_N : (k + 1) * block_N, bx, :], q_shared)
                T.clear(qkT)

                T.gemm(K_shared, q_shared, qkT, transpose_B=True, policy=T.GemmWarpPolicy.FullRow)

                T.copy(lse[bz, bx, k * block_N : (k + 1) * block_N], lse_shared)

                for i, j in T.Parallel(block_M, block_N):
                    P_acc[i, j] = T.exp(qkT[i, j] * sm_scale - lse_shared[j])

                if is_causal:
                    for i, j in T.Parallel(block_M, block_N):
                        P_acc[i, j] = T.if_then_else(by * block_M + i <= k * block_N + j, P_acc[i, j], 0.0)

                T.copy(dO[bz, k * block_N : (k + 1) * block_N, bx, :], do_shared)
                T.clear(dP)

                T.gemm(V_shared, do_shared, dP, transpose_B=True, policy=T.GemmWarpPolicy.FullRow)

                T.copy(P_acc, p_cast)
                T.gemm(p_cast, do_shared, dv, policy=T.GemmWarpPolicy.FullRow)

                T.copy(Delta[bz, bx, k * block_N : (k + 1) * block_N], delta_shared)

                for i, j in T.Parallel(block_M, block_N):
                    p_cast[i, j] = P_acc[i, j] * (dP[i, j] - delta_shared[j]) * sm_scale

                T.gemm(p_cast, q_shared, dk, policy=T.GemmWarpPolicy.FullRow)

                T.copy(p_cast, ds_shared)
                T.clear(dq)
                T.gemm(ds_shared, K_shared, dq, transpose_A=True)
                for i, j in T.Parallel(block_N, dim):
                    T.atomic_add(dQ[bz, k * block_N + i, bx, j], dq[i, j])

            for i, j in T.Parallel(block_M, dim):
                T.atomic_add(dV[bz, by * block_M + i, bx // groups, j], dv[i, j])
                T.atomic_add(dK[bz, by * block_M + i, bx // groups, j], dk[i, j])

    return flash_bwd_kernel


@tilelang.jit(out_idx=[1])
def flashattn_bwd_postprocess(batch, heads, seq_len, dim):
    dtype = "float16"
    accum_dtype = "float"
    shape = [batch, seq_len, heads, dim]
    blk = 64

    @T.prim_func
    def flash_bwd_post(dQ_in: T.Tensor(shape, accum_dtype), dQ_out: T.Tensor(shape, dtype)):
        with T.Kernel(T.ceildiv(seq_len, blk), heads, batch, threads=128) as (bx, by, bz):
            T.copy(
                dQ_in[bz, bx * blk : (bx + 1) * blk, by, :],
                dQ_out[bz, bx * blk : (bx + 1) * blk, by, :],
            )

    return flash_bwd_post


def debug_tensor_comparison(tensor1, tensor2, name, rtol=1e-3, atol=1e-3):
    print(f"\n=== {name} Comparison ===")
    print(f"Shape: {tensor1.shape} vs {tensor2.shape}")
    print(f"Data type: {tensor1.dtype} vs {tensor2.dtype}")
    print(f"Device: {tensor1.device} vs {tensor2.device}")

    diff = torch.abs(tensor1 - tensor2)
    max_diff = diff.max().item()
    mean_diff = diff.mean().item()
    std_diff = diff.std().item()

    print(f"Max difference: {max_diff:.6f}")
    print(f"Mean difference: {mean_diff:.6f}")
    print(f"Difference std: {std_diff:.6f}")

    if max_diff > atol:
        max_idx = torch.argmax(diff)
        max_idx = np.unravel_index(max_idx.cpu().numpy(), tensor1.shape)
        print(f"Max difference position: {max_idx}")
        print(f"Value1: {tensor1[max_idx].item():.6f}, Value2: {tensor2[max_idx].item():.6f}")

    nan_count1 = torch.isnan(tensor1).sum().item()
    nan_count2 = torch.isnan(tensor2).sum().item()
    inf_count1 = torch.isinf(tensor1).sum().item()
    inf_count2 = torch.isinf(tensor2).sum().item()

    print(f"NaN count: {nan_count1} vs {nan_count2}")
    print(f"Inf count: {inf_count1} vs {inf_count2}")

    relative_diff = diff / (torch.abs(tensor2) + 1e-8)
    max_relative_diff = relative_diff.max().item()
    mean_relative_diff = relative_diff.mean().item()

    print(f"Max relative difference: {max_relative_diff:.6f}")
    print(f"Mean relative difference: {mean_relative_diff:.6f}")

    close = torch.allclose(tensor1, tensor2, rtol=rtol, atol=atol)
    print(f"Within tolerance (rtol={rtol}, atol={atol}): {close}")

    return close, max_diff, mean_diff


def benchmark_function(func, *args, warmup=10, repeat=100):
    for _ in range(warmup):
        func(*args)

    if torch.cuda.is_available():
        torch.cuda.synchronize()

    times = []
    for _ in range(repeat):
        start = time.time()
        func(*args)
        if torch.cuda.is_available():
            torch.cuda.synchronize()
        end = time.time()
        times.append((end - start) * 1000)

    return np.median(times)


def main(batch: int = 1, heads: int = 8, seq_len: int = 4096, dim: int = 128, is_causal: bool = False, groups: int = 1):
    device = "cuda"
    dtype = torch.float16

    torch.manual_seed(42)
    torch.cuda.manual_seed(42)

    print(f"Test configuration: batch={batch}, heads={heads}, seq_len={seq_len}, dim={dim}, is_causal={is_causal}, groups={groups}")

    flops_per_gemm = 2.0 * batch * heads * seq_len * seq_len * dim
    total_flops = 5 * flops_per_gemm

    print(f"Total FLOPs: {total_flops / 1e12:.2f} TFlops")

    q = torch.randn(batch, seq_len, heads, dim, device=device, dtype=dtype)
    k = torch.randn(batch, seq_len, heads // groups, dim, device=device, dtype=dtype)
    v = torch.randn(batch, seq_len, heads // groups, dim, device=device, dtype=dtype)
    dO = torch.randn_like(q)

    print("Starting autotuning for Fast FlashAttention-V2 Forward Pass...")
    fwd_kernel = fast_flashattn(batch, heads, seq_len, dim, is_causal, groups)
    if fwd_kernel is None or fwd_kernel.config is None:
        print("Forward pass auto-tuning failed.")
        return
    print(f"Autotuning finished. Best Forward Configuration: {fwd_kernel.config}")

    ref_program_processed = partial(ref_program, is_causal=is_causal, groups=groups)

    profiler = fwd_kernel.get_profiler(tensor_supply_type=tilelang.TensorSupplyType.Normal)

    print("Verifying correctness...")
    profiler.assert_allclose(ref_program_processed, rtol=0.01, atol=0.01)
    print("Forward pass is correct.")

    o_tl, lse_tl = fwd_kernel(q, k, v)

    bwd_prep = flashattn_bwd_preprocess(batch, heads, seq_len, dim)
    delta_tl = bwd_prep(o_tl, dO)

    print("\nStarting FlashAttention-V2 backward pass autotuning...")
    bwd_kernel = flashattn_bwd(batch, heads, seq_len, dim, is_causal, groups)
    if bwd_kernel is None or bwd_kernel.config is None:
        print("Backward pass autotuning failed.")
        return
    print(f"Autotuning completed. Best backward pass configuration: {bwd_kernel.config}")

    dQ_accum = torch.zeros_like(q, dtype=torch.float32)
    dK_tl = torch.zeros_like(k, dtype=torch.float32)
    dV_tl = torch.zeros_like(v, dtype=torch.float32)

    bwd_kernel(q, k, v, dO, lse_tl, delta_tl, dQ_accum, dK_tl, dV_tl)

    post_kernel = flashattn_bwd_postprocess(batch, heads, seq_len, dim)
    dQ_tl = post_kernel(dQ_accum)

    q_ref = q.clone().detach().requires_grad_()
    k_ref = k.clone().detach().requires_grad_()
    v_ref = v.clone().detach().requires_grad_()

    o_ref, _ = ref_program(q_ref, k_ref, v_ref, is_causal, groups)
    o_ref.backward(dO)

    print("Verifying backward pass correctness...")
    dq_close, dq_max_diff, dq_mean_diff = debug_tensor_comparison(dQ_tl, q_ref.grad, "dQ", rtol=0.05, atol=0.05)
    if dq_close:
        print("dQ is correct.")
    else:
        print("dQ mismatch detected.")

    dk_close, dk_max_diff, dk_mean_diff = debug_tensor_comparison(dK_tl.to(torch.float16), k_ref.grad, "dK", rtol=0.05, atol=0.05)
    if dk_close:
        print("dK is correct.")
    else:
        print("dK mismatch detected.")

    dv_close, dv_max_diff, dv_mean_diff = debug_tensor_comparison(dV_tl.to(torch.float16), v_ref.grad, "dV", rtol=0.05, atol=0.05)
    if dv_close:
        print("dV is correct.")
    else:
        print("dV mismatch detected.")

    print("\n=== Performance Benchmarking ===")

    def run_reference_fwd_bwd():
        q_ref_bench = q.clone().detach().requires_grad_()
        k_ref_bench = k.clone().detach().requires_grad_()
        v_ref_bench = v.clone().detach().requires_grad_()

        o_ref_bench, _ = ref_program(q_ref_bench, k_ref_bench, v_ref_bench, is_causal, groups)

        o_ref_bench.backward(dO)

        if torch.cuda.is_available():
            torch.cuda.synchronize()

    ref_latency = benchmark_function(run_reference_fwd_bwd, warmup=10, repeat=100)
    print(f"Reference PyTorch Forward+Backward: {ref_latency:.2f} ms | {total_flops / ref_latency * 1e-9:.2f} TFlops")

    def run_complete_fwd_bwd():
        o_tl_bench, lse_tl_bench = fwd_kernel(q, k, v)

        delta_tl_bench = bwd_prep(o_tl_bench, dO)

        dQ_bench = torch.zeros_like(q, dtype=torch.float32)
        dK_bench = torch.zeros_like(k, dtype=torch.float32)
        dV_bench = torch.zeros_like(v, dtype=torch.float32)
        bwd_kernel(q, k, v, dO, lse_tl_bench, delta_tl_bench, dQ_bench, dK_bench, dV_bench)

        post_kernel(dQ_bench)

        if torch.cuda.is_available():
            torch.cuda.synchronize()

    tile_latency = benchmark_function(run_complete_fwd_bwd, warmup=10, repeat=100)
    print(
        f"Complete Flash Attention V2 Forward+Backward (Tile-lang): {tile_latency:.2f} ms | {total_flops / tile_latency * 1e-9:.2f} TFlops"
    )

    speedup = ref_latency / tile_latency
    print(f"Speedup: {speedup:.2f}x")

    print("Forward output: Passed")
    print(f"dQ: {'Passed' if dq_close else 'Failed'} (Max diff: {dq_max_diff:.6f})")
    print(f"dK: {'Passed' if dk_close else 'Failed'} (Max diff: {dk_max_diff:.6f})")
    print(f"dV: {'Passed' if dv_close else 'Failed'} (Max diff: {dv_max_diff:.6f})")

    if all([dq_close, dk_close, dv_close]):
        print("All checks passed!")
    else:
        print("Some checks failed, may need further debugging.")


if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument("--batch", type=int, default=1, help="batch size")
    parser.add_argument("--heads", type=int, default=8, help="heads")
    parser.add_argument("--seq_len", type=int, default=1024, help="sequence length")
    parser.add_argument("--dim", type=int, default=64, help="dim")
    parser.add_argument("--is_causal", action="store_true", help="causal")
    parser.add_argument("--groups", type=int, default=1, help="groups")
    args = parser.parse_args()

    main(args.batch, args.heads, args.seq_len, args.dim, args.is_causal, args.groups)