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candle/candle-flash-attn/kernels/flash_fwd_kernel.h

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/******************************************************************************
* Copyright (c) 2023, Tri Dao.
******************************************************************************/
#pragma once
#include <cmath>
#include <cute/algorithm/copy.hpp>
#include <cute/algorithm/gemm.hpp>
#include <cutlass/cutlass.h>
#include <cutlass/array.h>
#include <cutlass/numeric_types.h>
#include <cutlass/numeric_conversion.h>
#include "block_info.h"
#include "kernel_traits.h"
#include "utils.h"
#include "softmax.h"
#include "philox.cuh"
namespace flash {
using namespace cute;
////////////////////////////////////////////////////////////////////////////////////////////////////
template <int MMA_M,
class... Args,
class TiledMMA>
CUTE_HOST_DEVICE
auto
make_tiled_copy_A_warpcontiguousM(Copy_Atom<Args...> const& copy_atom,
TiledMMA const& tiled_mma) {
using TileShape_MNK = typename TiledMMA::TiledShape_MNK;
using AtomShape_MNK = typename TiledMMA::AtomShape_MNK;
constexpr int AtomShape_M = decltype(size<0>(AtomShape_MNK{}))::value;
constexpr int kNWarps = decltype(size<0>(TileShape_MNK{}))::value / AtomShape_M;
constexpr int MMAStride_M = MMA_M * AtomShape_M;
auto t = make_tile(Layout<Shape<Int<AtomShape_M>, Int<kNWarps>>,
Stride<_1, Int<MMAStride_M>> >{},
make_layout(size<2>(TileShape_MNK{})));
// if (cute::thread0()) {printf("make_tiled_copy_A_warpcontiguousM "); print(t); printf("\n"); }
return make_tiled_copy_impl(copy_atom, tiled_mma.get_layoutA_TV(), t);
}
////////////////////////////////////////////////////////////////////////////////////////////////////
template <int MMA_M,
class... Args,
class TiledMMA>
CUTE_HOST_DEVICE
auto
make_tiled_copy_C_warpcontiguousM(Copy_Atom<Args...> const& copy_atom,
TiledMMA const& tiled_mma) {
using TileShape_MNK = typename TiledMMA::TiledShape_MNK;
using AtomShape_MNK = typename TiledMMA::AtomShape_MNK;
constexpr int AtomShape_M = decltype(size<0>(AtomShape_MNK{}))::value;
constexpr int kNWarps = decltype(size<0>(TileShape_MNK{}))::value / AtomShape_M;
constexpr int MMAStride_M = MMA_M * AtomShape_M;
auto t = make_tile(Layout<Shape<Int<AtomShape_M>, Int<kNWarps>>,
Stride<_1, Int<MMAStride_M>> >{},
// TODO: Shouldn't this be size<1>?
make_layout(size<2>(TileShape_MNK{})));
// if (cute::thread0()) {printf("make_tiled_copy_C_warpcontiguousM "); print(t); printf("\n"); }
return make_tiled_copy_impl(copy_atom, tiled_mma.get_layoutC_TV(), t);
}
////////////////////////////////////////////////////////////////////////////////////////////////////
template<bool Is_first, bool Check_inf=false, typename Tensor0, typename Tensor1, typename Tensor2>
inline __device__ void softmax_rescale_o(Tensor0 &scores, Tensor1 &scores_max, Tensor1 &scores_sum,
Tensor2 &acc_o, float softmax_scale_log2) {
if (Is_first) {
flash::template reduce_max</*zero_init=*/true>(scores, scores_max);
flash::scale_apply_exp2(scores, scores_max, softmax_scale_log2);
flash::reduce_sum(scores, scores_sum);
} else {
Tensor scores_max_prev = make_fragment_like(scores_max);
copy(scores_max, scores_max_prev);
flash::template reduce_max</*zero_init=*/false>(scores, scores_max);
// Reshape acc_o from (MMA=4, MMA_M, MMA_K) to (nrow=(2, MMA_M), ncol=(2, MMA_K))
Tensor acc_o_rowcol = make_tensor(acc_o.data(), flash::convert_layout_acc_rowcol(acc_o.layout()));
#pragma unroll
for (int mi = 0; mi < size(scores_max); ++mi) {
float scores_max_cur = !Check_inf
? scores_max(mi)
: (scores_max(mi) == -INFINITY ? 0.0f : scores_max(mi));
float scores_scale = exp2f((scores_max_prev(mi) - scores_max_cur) * softmax_scale_log2);
scores_sum(mi) *= scores_scale;
#pragma unroll
for (int ni = 0; ni < size<1>(acc_o_rowcol); ++ni) { acc_o_rowcol(mi, ni) *= scores_scale; }
}
flash::scale_apply_exp2(scores, scores_max, softmax_scale_log2);
Tensor scores_sum_cur = make_fragment_like(scores_sum);
flash::reduce_sum(scores, scores_sum_cur);
#pragma unroll
for (int mi = 0; mi < size(scores_sum); ++mi) { scores_sum(mi) += scores_sum_cur(mi); }
}
};
////////////////////////////////////////////////////////////////////////////////////////////////////
template<typename Engine0, typename Layout0, typename Engine1, typename Layout1, typename TiledCopy>
inline __device__ void write_softmax_to_gmem(
Tensor<Engine0, Layout0> const &tOrP, Tensor<Engine1, Layout1> &tPgP, TiledCopy gmem_thr_copy_P
) {
// Reshape tOrP from (8, MMA_M, MMA_N) to (8, MMA_M * MMA_N)
Layout l = tOrP.layout();
Tensor tPrP = make_tensor(tOrP.data(), make_layout(get<0>(l), make_layout(get<1>(l), get<2>(l))));
CUTE_STATIC_ASSERT_V(size<2>(tPgP) == _1{});
// TODO(laurent): reactivate the following
// CUTE_STATIC_ASSERT_V(size<1>(tPrP) == size<1>(tPgP));
#pragma unroll
for (int mi = 0; mi < size<1>(tPrP); ++mi) {
copy(gmem_thr_copy_P, tPrP(_, mi), tPgP(_, mi, 0));
}
};
////////////////////////////////////////////////////////////////////////////////////////////////////
template<typename Kernel_traits, bool Is_dropout, bool Is_causal, bool Is_even_N, bool Is_even_K, bool Return_softmax, typename Params>
inline __device__ void compute_attn_1rowblock(const Params &params, const int bidb, const int bidh, const int m_block) {
using Element = typename Kernel_traits::Element;
using ElementAccum = typename Kernel_traits::ElementAccum;
using index_t = typename Kernel_traits::index_t;
// Shared memory.
extern __shared__ char smem_[];
// The thread index.
const int tidx = threadIdx.x;
constexpr int kBlockM = Kernel_traits::kBlockM;
constexpr int kBlockN = Kernel_traits::kBlockN;
constexpr int kHeadDim = Kernel_traits::kHeadDim;
constexpr int kNWarps = Kernel_traits::kNWarps;
constexpr int MMA_M = kBlockM / decltype(size<0>(typename Kernel_traits::TiledMma::TiledShape_MNK{}))::value;
const BlockInfo</*Varlen=*/!Is_even_N> binfo(params, bidb);
if (m_block * kBlockM >= binfo.actual_seqlen_q || binfo.actual_seqlen_k == 0) return;
int n_block_max = cute::ceil_div(binfo.actual_seqlen_k, kBlockN);
if (Is_causal) {
n_block_max = std::min(n_block_max, cute::ceil_div((m_block + 1) * kBlockM, kBlockN));
// if (threadIdx.x == 0 && blockIdx.y == 0 && blockIdx.z == 0) {
// printf("m_block = %d, n_block_max = %d\n", m_block, n_block_max);
// }
}
// We iterate over the blocks in reverse order. This is because the last block is the only one
// that needs masking when we read K and V from global memory. Moreover, iterating in reverse
// might save us 1 register (we just need n_block instead of both n_block and n_block_max).
const index_t row_offset_q = binfo.q_offset(params.q_batch_stride, params.q_row_stride, bidb)
+ m_block * kBlockM * params.q_row_stride + bidh * params.q_head_stride;
// We move K and V to the last block.
const index_t row_offset_k = binfo.k_offset(params.k_batch_stride, params.k_row_stride, bidb)
+ (n_block_max - 1) * kBlockN * params.k_row_stride + (bidh / params.h_h_k_ratio) * params.k_head_stride;
const index_t row_offset_v = binfo.k_offset(params.v_batch_stride, params.v_row_stride, bidb)
+ (n_block_max - 1) * kBlockN * params.v_row_stride + (bidh / params.h_h_k_ratio) * params.v_head_stride;
const index_t row_offset_p = ((bidb * params.h + bidh) * params.seqlen_q_rounded
+ m_block * kBlockM) * params.seqlen_k_rounded + (n_block_max - 1) * kBlockN;
Tensor gQ = make_tensor(make_gmem_ptr(reinterpret_cast<Element *>(params.q_ptr) + row_offset_q),
Shape<Int<kBlockM>, Int<kHeadDim>>{},
make_stride(params.q_row_stride, _1{}));
Tensor gK = make_tensor(make_gmem_ptr(reinterpret_cast<Element *>(params.k_ptr) + row_offset_k),
Shape<Int<kBlockN>, Int<kHeadDim>>{},
make_stride(params.k_row_stride, _1{}));
Tensor gV = make_tensor(make_gmem_ptr(reinterpret_cast<Element *>(params.v_ptr) + row_offset_v),
Shape<Int<kBlockN>, Int<kHeadDim>>{},
make_stride(params.v_row_stride, _1{}));
Tensor gP = make_tensor(make_gmem_ptr(reinterpret_cast<Element *>(params.p_ptr) + row_offset_p),
Shape<Int<kBlockM>, Int<kBlockN>>{},
make_stride(params.seqlen_k_rounded, _1{}));
Tensor sQ = make_tensor(make_smem_ptr(reinterpret_cast<Element *>(smem_)),
typename Kernel_traits::SmemLayoutQ{});
// Careful we're using the same smem for sQ and sK | sV if Share_Q_K_smem;
Tensor sK = make_tensor(sQ.data() + (Kernel_traits::Share_Q_K_smem ? 0 : size(sQ)),
typename Kernel_traits::SmemLayoutKV{});
Tensor sV = make_tensor(sK.data() + size(sK), typename Kernel_traits::SmemLayoutKV{});
Tensor sVt = make_tensor(sV.data(), typename Kernel_traits::SmemLayoutVtransposed{});
Tensor sVtNoSwizzle = make_tensor(sV.data(), typename Kernel_traits::SmemLayoutVtransposedNoSwizzle{});
auto gmem_thr_copy_QKV = typename Kernel_traits::GmemTiledCopyQKV{}.get_thread_slice(tidx);
auto gmem_thr_copy_P = typename Kernel_traits::GmemTiledCopyP{}.get_thread_slice(tidx);
Tensor tQgQ = gmem_thr_copy_QKV.partition_S(gQ);
Tensor tQsQ = gmem_thr_copy_QKV.partition_D(sQ);
Tensor tKgK = gmem_thr_copy_QKV.partition_S(gK); // (KCPY, KCPY_N, KCPY_K)
Tensor tKsK = gmem_thr_copy_QKV.partition_D(sK);
Tensor tVgV = gmem_thr_copy_QKV.partition_S(gV); // (VCPY, VCPY_N, VCPY_K)
Tensor tVsV = gmem_thr_copy_QKV.partition_D(sV);
Tensor tPgP = gmem_thr_copy_P.partition_D(gP);
typename Kernel_traits::TiledMma tiled_mma;
auto thr_mma = tiled_mma.get_thread_slice(tidx);
Tensor tSrQ = thr_mma.partition_fragment_A(sQ); // (MMA,MMA_M,MMA_K)
Tensor tSrK = thr_mma.partition_fragment_B(sK); // (MMA,MMA_N,MMA_K)
Tensor tOrVt = thr_mma.partition_fragment_B(sVtNoSwizzle); // (MMA, MMA_K,MMA_N)
Tensor acc_o = partition_fragment_C(tiled_mma, Shape<Int<kBlockM>, Int<kHeadDim>>{}); // MMA, MMA_M, MMA_K
//
// Copy Atom retiling
//
auto smem_thr_copy_Q = make_tiled_copy_A(typename Kernel_traits::SmemCopyAtom{}, tiled_mma).get_thread_slice(tidx);
// auto smem_thr_copy_Q = make_tiled_copy_A_warpcontiguousM<MMA_M>(typename Kernel_traits::SmemCopyAtom{}, tiled_mma).get_thread_slice(tidx);
// if (cute::thread0()) {smem_thr_copy_Q.print_all();}
Tensor tSsQ = smem_thr_copy_Q.partition_S(sQ);
// if (cute::thread0()) {print(tSsQ.layout()); printf("\n");}
auto smem_thr_copy_K = make_tiled_copy_B(typename Kernel_traits::SmemCopyAtom{}, tiled_mma).get_thread_slice(tidx);
Tensor tSsK = smem_thr_copy_K.partition_S(sK);
auto smem_thr_copy_V = make_tiled_copy_B(typename Kernel_traits::SmemCopyAtomTransposed{}, tiled_mma).get_thread_slice(tidx);
Tensor tOsVt = smem_thr_copy_V.partition_S(sVt);
// TODO: this might need to change if we change the mma instruction in SM70
Tensor scores_max = make_tensor<ElementAccum>(Shape<Int<2 * size<1>(acc_o)>>{});
Tensor scores_sum = make_fragment_like(scores_max);
//
// PREDICATES
//
// // Allocate predicate tensors for m and n
// Tensor tQpQ = make_tensor<bool>(make_shape(size<1>(tQsQ), size<2>(tQsQ)), Stride<_1,_0>{});
// Tensor tKVpKV = make_tensor<bool>(make_shape(size<1>(tKsK), size<2>(tKsK)), Stride<_1,_0>{});
// Construct identity layout for sQ and sK
Tensor cQ = make_identity_tensor(make_shape(size<0>(sQ), size<1>(sQ))); // (BLK_M,BLK_K) -> (blk_m,blk_k)
Tensor cKV = make_identity_tensor(make_shape(size<0>(sK), size<1>(sK))); // (BLK_N,BLK_K) -> (blk_n,blk_k)
// Tensor tScQ = thr_mma.partition_A(cQ); // (MMA,MMA_M,MMA_K)
// if (cute::thread0()) {
// print(tScQ.layout()); printf("\n");
// for (int i = 0; i < size(tScQ); ++i) {
// printf("%d ", get<0>(tScQ(i)));
// }
// printf("\n");
// for (int i = 0; i < size(tScQ); ++i) {
// printf("%d ", get<1>(tScQ(i)));
// }
// printf("\n");
// }
// Repeat the partitioning with identity layouts
Tensor tQcQ = gmem_thr_copy_QKV.partition_S(cQ); // (ACPY,ACPY_M,ACPY_K) -> (blk_m,blk_k)
Tensor tKVcKV = gmem_thr_copy_QKV.partition_S(cKV); // (BCPY,BCPY_N,BCPY_K) -> (blk_n,blk_k)
// Allocate predicate tensors for k
Tensor tQpQ = make_tensor<bool>(make_shape(size<2>(tQsQ)));
Tensor tKVpKV = make_tensor<bool>(make_shape(size<2>(tKsK)));
// Set predicates for k bounds
if (!Is_even_K) {
#pragma unroll
for (int k = 0; k < size(tQpQ); ++k) { tQpQ(k) = get<1>(tQcQ(0, 0, k)) < params.d; }
#pragma unroll
for (int k = 0; k < size(tKVpKV); ++k) { tKVpKV(k) = get<1>(tKVcKV(0, 0, k)) < params.d; }
}
// Prologue
Tensor tQrQ = make_fragment_like(tQgQ);
// We don't need to clear the sQ smem tiles since we'll only write out the valid outputs
flash::copy</*Is_even_MN=*/false, Is_even_K>(gmem_thr_copy_QKV, tQgQ, tQsQ, tQcQ, tQpQ,
binfo.actual_seqlen_q - m_block * kBlockM);
if (Kernel_traits::Is_Q_in_regs) { cute::cp_async_fence(); }
// // Copy rmem to smem
// // copy(tQrQ, tQsQ);
// flash::cp_async_wait<0>();
// __syncthreads();
// // if (cute::thread(1, 0)) { print(tQsQ); }
// // Tensor sQNoSwizzle = make_tensor(make_smem_ptr(reinterpret_cast<Element *>(smem_)), typename Kernel_traits::SmemLayoutQNoSwizzle{});
// // if (cute::thread0()) { print(sQNoSwizzle); }
if (Kernel_traits::Share_Q_K_smem) {
flash::cp_async_wait<0>();
__syncthreads();
Tensor tSrQ_copy_view = smem_thr_copy_Q.retile_D(tSrQ);
CUTE_STATIC_ASSERT_V(size<1>(tSsQ) == size<1>(tSrQ_copy_view)); // M
copy(smem_thr_copy_Q, tSsQ, tSrQ_copy_view);
__syncthreads();
}
int n_block = n_block_max - 1;
// We don't need to clear the sK smem tiles since we'll mask out the scores anyway.
flash::copy<Is_even_N, Is_even_K>(gmem_thr_copy_QKV, tKgK, tKsK, tKVcKV, tKVpKV,
binfo.actual_seqlen_k - n_block * kBlockN);
cute::cp_async_fence();
// if (threadIdx.x == 0 && blockIdx.y == 0 && blockIdx.z < 2) { print(tKgK); }
// __syncthreads();
if (Kernel_traits::Is_Q_in_regs && !Kernel_traits::Share_Q_K_smem) {
flash::cp_async_wait<1>();
__syncthreads();
Tensor tSrQ_copy_view = smem_thr_copy_Q.retile_D(tSrQ);
CUTE_STATIC_ASSERT_V(size<1>(tSsQ) == size<1>(tSrQ_copy_view)); // M
copy(smem_thr_copy_Q, tSsQ, tSrQ_copy_view);
}
// auto seeds = at::cuda::philox::unpack(params.philox_args);
// unsigned long long seed = std::get<0>(seeds);
// unsigned long long offset = std::get<1>(seeds) + (bidb * params.h + bidh) * 32 + tidx % 32;
unsigned long long seed = 0;
unsigned long long offset = 0;
clear(acc_o);
// For performance reason, we separate out two kinds of iterations:
// those that need masking on S, and those that don't.
// We need masking on S for the very last block when K and V has length not multiple of kBlockN.
// We also need masking on S if it's causal, for the last ceil_div(kBlockM, kBlockN) blocks.
// We will have at least 1 "masking" iteration.
constexpr int n_masking_steps = Is_causal ? cute::ceil_div(kBlockM, kBlockN) : 1;
#pragma unroll
for (int masking_step = 0; masking_step < n_masking_steps; ++masking_step, --n_block) {
Tensor acc_s = partition_fragment_C(tiled_mma, Shape<Int<kBlockM>, Int<kBlockN>>{}); // (MMA=4, MMA_M, MMA_N)
clear(acc_s);
flash::cp_async_wait<0>();
__syncthreads();
// Advance gV
if (masking_step > 0) {
tVgV.data() = tVgV.data() + (-int(kBlockN * params.v_row_stride));
flash::copy</*Is_even_MN=*/true, Is_even_K>(gmem_thr_copy_QKV, tVgV, tVsV, tKVcKV, tKVpKV);
} else {
// Clear the smem tiles to account for predicated off loads
flash::copy<Is_even_N, Is_even_K, /*Clear_OOB_MN=*/true>(
gmem_thr_copy_QKV, tVgV, tVsV, tKVcKV, tKVpKV, binfo.actual_seqlen_k - n_block * kBlockN
);
}
cute::cp_async_fence();
flash::gemm</*A_in_regs=*/Kernel_traits::Is_Q_in_regs>(
acc_s, tSrQ, tSrK, tSsQ, tSsK, tiled_mma, smem_thr_copy_Q, smem_thr_copy_K
);
// if (cute::thread0()) { print(acc_s); }
// Reshape acc_s from (MMA=4, MMA_M, MMA_N) to (nrow=(2, MMA_M), ncol=(2, MMA_N))
Tensor scores = make_tensor(acc_s.data(), flash::convert_layout_acc_rowcol(acc_s.layout()));
// if (cute::thread0()) { print(scores); }
// We don't put the masking before the matmul S = Q K^T because we don't clear sK
// for rows outside actual_seqlen_k. So those rows could have Inf / NaN, and the matmul
// can produce Inf / NaN.
if (!Is_causal) {
if (!Is_even_N) { flash::apply_mask(scores, binfo.actual_seqlen_k - n_block * kBlockN); }
} else {
// Tensor caccS = make_identity_tensor(Shape<Int<kBlockM>, Int<kBlockN>>{}); // (BLK_M,BLK_N) -> (blk_m,blk_n)
// Tensor taccScS = thr_mma.partition_C(caccS); // (MMA,MMA_M,MMA_N)
// static_assert(decltype(size<0>(taccScS))::value == 4);
// // Convert to ((2, 2), MMA_M, MMA_N) then take only the row indices.
// Tensor idx_row = logical_divide(taccScS, Shape<_2>{})(make_coord(0, _), _, 0);
// Tensor idx_rowcol = make_tensor(taccScS.data(), flash::convert_layout_acc_rowcol(taccScS.layout()));
// flash::apply_mask_causal_w_idx(scores, idx_rowcol, n_block * kBlockN, binfo.actual_seqlen_k,
// m_block * kBlockM);
// Idk why it's get<1> and not get<0> of the stride.
// if (cute::thread0()) { print(idx_row.layout()); print(stride<1>(idx_row)); printf("stride = %d \n", get<1>(stride<1>(idx_row))); }
// I can't get the stride from idx_row
flash::apply_mask_causal(scores, n_block * kBlockN, binfo.actual_seqlen_k,
// m_block * kBlockM + get<0>(idx_row(0)),
m_block * kBlockM + (tidx / 32) * 16 + (tidx % 32) / 4,
kNWarps * 16);
// m_block * kBlockM + (tidx / 32) * 16, kNWarps * 16);
// m_block * kBlockM + (tidx / 32) * (kBlockM / kNWarps), 16);
}
flash::cp_async_wait<0>();
__syncthreads();
if (n_block > 0) {
// Advance gK
tKgK.data() = tKgK.data() + (-int(kBlockN * params.k_row_stride));
flash::copy</*Is_even_MN=*/true, Is_even_K>(gmem_thr_copy_QKV, tKgK, tKsK, tKVcKV, tKVpKV);
// This cp_async_fence needs to be in the if block, otherwise the synchronization
// isn't right and we get race conditions.
cute::cp_async_fence();
}
// TODO: when we have key_padding_mask we'll need to Check_inf
masking_step == 0
? softmax_rescale_o</*Is_first=*/true, /*Check_inf=*/Is_causal>(scores, scores_max, scores_sum, acc_o, params.scale_softmax_log2)
: softmax_rescale_o</*Is_first=*/false, /*Check_inf=*/Is_causal>(scores, scores_max, scores_sum, acc_o, params.scale_softmax_log2);
// Convert scores from fp32 to fp16/bf16
Tensor rP = flash::convert_type<Element>(scores);
// Reshape rP from (nrow=(2, MMA_M), ncol=(2, MMA_N)) to ((2, 2, 2), MMA_M, MMA_N / 2)
// if using m16n8k16 or ((2, 2, 1), MMA_M, MMA_N) if using m16n8k8.
Tensor tOrP = make_tensor(rP.data(), flash::convert_layout_rowcol_Aregs<Kernel_traits::TiledMma>(rP.layout()));
uint32_t block_row_idx = m_block * (kBlockM / 16) + tidx / 32;
uint32_t block_col_idx = n_block * (kBlockN / 32);
if (Return_softmax) {
Tensor tOrP_copy = make_fragment_like(tOrP);
copy(tOrP, tOrP_copy);
flash::apply_dropout</*encode_dropout_in_sign_bit=*/true>(
tOrP_copy, params.p_dropout_in_uint8_t, seed, offset,
block_row_idx, block_col_idx, kNWarps
);
flash::write_softmax_to_gmem(tOrP_copy, tPgP, gmem_thr_copy_P);
tPgP.data() = tPgP.data() + (-kBlockN);
}
if (Is_dropout) {
flash::apply_dropout(tOrP, params.p_dropout_in_uint8_t, seed, offset,
block_row_idx, block_col_idx, kNWarps);
}
// if (cute::thread0()) { print(tOrP); }
flash::gemm_A_in_regs(acc_o, tOrP, tOrVt, tOsVt, tiled_mma, smem_thr_copy_V);
// if (cute::thread0()) { print(scores); }
// This check is at the end of the loop since we always have at least 1 iteration
if (n_masking_steps > 1 && n_block <= 0) {
--n_block;
break;
}
}
// These are the iterations where we don't need masking on S
for (; n_block >= 0; --n_block) {
Tensor acc_s = partition_fragment_C(tiled_mma, Shape<Int<kBlockM>, Int<kBlockN>>{}); // (MMA=4, MMA_M, MMA_N)
clear(acc_s);
flash::cp_async_wait<0>();
__syncthreads();
// Advance gV
tVgV.data() = tVgV.data() + (-int(kBlockN * params.v_row_stride));
flash::copy</*Is_even_MN=*/true, Is_even_K>(gmem_thr_copy_QKV, tVgV, tVsV, tKVcKV, tKVpKV);
cute::cp_async_fence();
flash::gemm</*A_in_regs=*/Kernel_traits::Is_Q_in_regs>(
acc_s, tSrQ, tSrK, tSsQ, tSsK, tiled_mma, smem_thr_copy_Q, smem_thr_copy_K
);
flash::cp_async_wait<0>();
__syncthreads();
if (n_block > 0) {
// Advance gK
tKgK.data() = tKgK.data() + (-int(kBlockN * params.k_row_stride));
flash::copy</*Is_even_MN=*/true, Is_even_K>(gmem_thr_copy_QKV, tKgK, tKsK, tKVcKV, tKVpKV);
// This cp_async_fence needs to be in the if block, otherwise the synchronization
// isn't right and we get race conditions.
cute::cp_async_fence();
}
// Reshape acc_s from (MMA=4, MMA_M, MMA_N) to (nrow=(2, MMA_M), ncol=(2, MMA_N))
Tensor scores = make_tensor(acc_s.data(), flash::convert_layout_acc_rowcol(acc_s.layout()));
softmax_rescale_o</*Is_first=*/false>(scores, scores_max, scores_sum, acc_o, params.scale_softmax_log2);
Tensor rP = flash::convert_type<Element>(scores);
// Reshape rP from (nrow=(2, MMA_M), ncol=(2, MMA_N)) to ((2, 2, 2), MMA_M, MMA_N / 2)
// if using m16n8k16 or ((2, 2, 1), MMA_M, MMA_N) if using m16n8k8.
Tensor tOrP = make_tensor(rP.data(), flash::convert_layout_rowcol_Aregs<Kernel_traits::TiledMma>(rP.layout()));
uint32_t block_row_idx = m_block * (kBlockM / 16) + tidx / 32;
uint32_t block_col_idx = n_block * (kBlockN / 32);
if (Return_softmax) {
Tensor tOrP_copy = make_fragment_like(tOrP);
copy(tOrP, tOrP_copy);
flash::apply_dropout</*encode_dropout_in_sign_bit=*/true>(
tOrP_copy, params.p_dropout_in_uint8_t, seed, offset,
block_row_idx, block_col_idx, kNWarps
);
flash::write_softmax_to_gmem(tOrP_copy, tPgP, gmem_thr_copy_P);
tPgP.data() = tPgP.data() + (-kBlockN);
}
if (Is_dropout) {
flash::apply_dropout(tOrP, params.p_dropout_in_uint8_t, seed, offset,
block_row_idx, block_col_idx, kNWarps);
}
flash::gemm_A_in_regs(acc_o, tOrP, tOrVt, tOsVt, tiled_mma, smem_thr_copy_V);
}
// Epilogue
// Reshape acc_o from (MMA=4, MMA_M, MMA_K) to (nrow=(2, MMA_M), ncol=(2, MMA_K))
Tensor acc_o_rowcol = make_tensor(acc_o.data(), flash::convert_layout_acc_rowcol(acc_o.layout()));
Tensor lse = make_fragment_like(scores_sum);
#pragma unroll
for (int mi = 0; mi < size<0>(acc_o_rowcol); ++mi) {
float sum = scores_sum(mi);
float inv_sum = (sum == 0.f || sum != sum) ? 1.f : 1.f / sum;
lse(mi) = (sum == 0.f || sum != sum) ? INFINITY : scores_max(mi) * params.scale_softmax + __logf(sum);
float scale = !Is_dropout ? inv_sum : inv_sum * params.rp_dropout;
#pragma unroll
for (int ni = 0; ni < size<1>(acc_o_rowcol); ++ni) { acc_o_rowcol(mi, ni) *= scale; }
}
// if (cute::thread0()) { print(acc_o_rowcol); }
// Convert acc_o from fp32 to fp16/bf16
Tensor rO = flash::convert_type<Element>(acc_o);
Tensor sO = make_tensor(sQ.data(), typename Kernel_traits::SmemLayoutO{}); // (SMEM_M,SMEM_N)
// Partition sO to match the accumulator partitioning
auto smem_thr_copy_O = make_tiled_copy_C(typename Kernel_traits::SmemCopyAtomO{}, tiled_mma).get_thread_slice(tidx);
// auto smem_thr_copy_O = make_tiled_copy_C_warpcontiguousM<MMA_M>(typename Kernel_traits::SmemCopyAtomO{}, tiled_mma).get_thread_slice(tidx);
Tensor taccOrO = smem_thr_copy_O.retile_S(rO); // ((Atom,AtomNum), MMA_M, MMA_N)
Tensor taccOsO = smem_thr_copy_O.partition_D(sO); // ((Atom,AtomNum),PIPE_M,PIPE_N)
// sO has the same size as sQ, so we don't need to sync here.
if (Kernel_traits::Share_Q_K_smem) { __syncthreads(); }
copy(smem_thr_copy_O, taccOrO, taccOsO);
const index_t row_offset_o = binfo.q_offset(params.o_batch_stride, params.o_row_stride, bidb)
+ m_block * kBlockM * params.o_row_stride + bidh * params.o_head_stride;
const index_t row_offset_lse = (bidb * params.h + bidh) * params.seqlen_q + m_block * kBlockM;
Tensor gO = make_tensor(make_gmem_ptr(reinterpret_cast<Element *>(params.o_ptr) + row_offset_o),
Shape<Int<kBlockM>, Int<kHeadDim>>{},
make_stride(params.o_row_stride, _1{}));
Tensor gLSE = make_tensor(make_gmem_ptr(reinterpret_cast<ElementAccum *>(params.softmax_lse_ptr) + row_offset_lse),
Shape<Int<kBlockM>>{}, Stride<_1>{});
auto gmem_thr_copy_O = typename Kernel_traits::GmemTiledCopyO{}.get_thread_slice(tidx);
Tensor tOsO = gmem_thr_copy_O.partition_S(sO); // ((Atom,AtomNum),ATOM_M,ATOM_N)
Tensor tOgO = gmem_thr_copy_O.partition_D(gO);
__syncthreads();
Tensor tOrO = make_tensor<Element>(shape(tOgO));
copy(gmem_thr_copy_O, tOsO, tOrO);
Tensor caccO = make_identity_tensor(Shape<Int<kBlockM>, Int<kHeadDim>>{}); // (BLK_M,BLK_K) -> (blk_m,blk_k)
Tensor taccOcO = thr_mma.partition_C(caccO); // (MMA,MMA_M,MMA_K)
static_assert(decltype(size<0>(taccOcO))::value == 4);
// Convert to ((2, 2), MMA_M, MMA_K) then take only the row indices.
Tensor taccOcO_row = logical_divide(taccOcO, Shape<_2>{})(make_coord(0, _), _, 0);
CUTE_STATIC_ASSERT_V(size(lse) == size(taccOcO_row)); // MMA_M
if (get<1>(taccOcO_row(0)) == 0) {
#pragma unroll
for (int mi = 0; mi < size(lse); ++mi) {
const int row = get<0>(taccOcO_row(mi));
if (row < binfo.actual_seqlen_q - m_block * kBlockM) { gLSE(row) = lse(mi); }
}
}
// Construct identity layout for sO
Tensor cO = make_identity_tensor(make_shape(size<0>(sO), size<1>(sO))); // (BLK_M,BLK_K) -> (blk_m,blk_k)
// Repeat the partitioning with identity layouts
Tensor tOcO = gmem_thr_copy_O.partition_D(cO); // (ACPY,ACPY_M,ACPY_K) -> (blk_m,blk_k)
Tensor tOpO = make_tensor<bool>(make_shape(size<2>(tOgO)));
if (!Is_even_K) {
#pragma unroll
for (int k = 0; k < size(tOpO); ++k) { tOpO(k) = get<1>(tOcO(0, 0, k)) < params.d; }
}
// Clear_OOB_K must be false since we don't want to write zeros to gmem
flash::copy</*Is_even_MN=*/false, Is_even_K, /*Clear_OOB_MN=*/false, /*Clear_OOB_K=*/false>(
gmem_thr_copy_O, tOrO, tOgO, tOcO, tOpO, binfo.actual_seqlen_q - m_block * kBlockM
);
}
////////////////////////////////////////////////////////////////////////////////////////////////////
template<typename Kernel_traits, bool Is_dropout, bool Is_causal, bool Is_even_N, bool Is_even_K, bool Return_softmax, typename Params>
inline __device__ void compute_attn(const Params &params) {
const int m_block = blockIdx.x;
// The block index for the batch.
const int bidb = blockIdx.y;
// The block index for the head.
const int bidh = blockIdx.z;
// We want the fwd and bwd to generate the same dropout pattern (RNG), without restricting
// them to have the same number of threads or have to traverse the attention matrix
// in the same order.
// In the Philox RNG, we use the offset to store the batch, head, and the lane id
// (within a warp). We use the subsequence to store the location of the 16 x 32 blocks within
// the attention matrix. This way, as long as we have the batch, head, and the location of
// the 16 x 32 block within the attention matrix, we can generate the exact same dropout pattern.
flash::compute_attn_1rowblock<Kernel_traits, Is_dropout, Is_causal, Is_even_N, Is_even_K, Return_softmax>(params, bidb, bidh, m_block);
}
////////////////////////////////////////////////////////////////////////////////////////////////////
} // namespace flash
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