aphrodite-engine/kernels/flash_attn/kernel_traits.h

/******************************************************************************
 * Copyright (c) 2024, Tri Dao.
 ******************************************************************************/

#pragma once

#include "cute/tensor.hpp"

#include "cutlass/cutlass.h"
#include "cutlass/layout/layout.h"
#include <cutlass/numeric_types.h>

using namespace cute;

template<int kHeadDim_, int kBlockM_, int kBlockN_, int kNWarps_, typename elem_type=cutlass::half_t>
struct Flash_kernel_traits {

#if defined(__CUDA_ARCH__) &&  __CUDA_ARCH__ >= 800
    using Element = elem_type;
    static constexpr bool Has_cp_async = true;
#else
    using Element = cutlass::half_t;
    static constexpr bool Has_cp_async = false;
#endif

    using ElementAccum = float;
    using index_t = int64_t;

#if defined(__CUDA_ARCH__) &&  __CUDA_ARCH__ >= 800
    using MMA_Atom_Arch = std::conditional_t<
        std::is_same_v<elem_type, cutlass::half_t>,
        MMA_Atom<SM80_16x8x16_F32F16F16F32_TN>,
        MMA_Atom<SM80_16x8x16_F32BF16BF16F32_TN>
    >;
#else
    using MMA_Atom_Arch = MMA_Atom<SM75_16x8x8_F32F16F16F32_TN>;
#endif

#if defined(__CUDA_ARCH__) &&  __CUDA_ARCH__ >= 750
    using SmemCopyAtom = Copy_Atom<SM75_U32x4_LDSM_N, elem_type>;
    using SmemCopyAtomTransposed = Copy_Atom<SM75_U16x8_LDSM_T, elem_type>;
#else
    using SmemCopyAtom = Copy_Atom<DefaultCopy, elem_type>;
    using SmemCopyAtomTransposed = Copy_Atom<DefaultCopy, elem_type>;
#endif
};

// If Share_Q_K_smem is true, that forces Is_Q_in_regs to be true
template<int kHeadDim_, int kBlockM_, int kBlockN_, int kNWarps_, bool Is_Q_in_regs_=false, bool Share_Q_K_smem_=false, typename elem_type=cutlass::half_t,
         typename Base=Flash_kernel_traits<kHeadDim_, kBlockM_, kBlockN_, kNWarps_, elem_type> >
struct Flash_fwd_kernel_traits : public Base {
    using Element = typename Base::Element;
    using ElementAccum = typename Base::ElementAccum;
    using index_t = typename Base::index_t;
    static constexpr bool Has_cp_async = Base::Has_cp_async;
    using SmemCopyAtom = typename Base::SmemCopyAtom;
    using SmemCopyAtomTransposed = typename Base::SmemCopyAtomTransposed;

    static constexpr bool Share_Q_K_smem = Share_Q_K_smem_;
    static constexpr bool Is_Q_in_regs = Is_Q_in_regs_ || Share_Q_K_smem;

    // The number of threads.
    static constexpr int kNWarps = kNWarps_;
    static constexpr int kNThreads = kNWarps * 32;

    static constexpr int kBlockM = kBlockM_;
    static constexpr int kBlockN = kBlockN_;
    static constexpr int kHeadDim = kHeadDim_;
    static_assert(kHeadDim % 32 == 0);
    static constexpr int kBlockKSmem = kHeadDim % 64 == 0 ? 64 : 32;
    static constexpr int kBlockKGmem = kHeadDim % 128 == 0 ? 128 : (kHeadDim % 64 == 0 ? 64 : 32);
    static constexpr int kSwizzle = kBlockKSmem == 32 ? 2 : 3;

    using TiledMma = TiledMMA<
        typename Base::MMA_Atom_Arch,
        Layout<Shape<Int<kNWarps>,_1,_1>>,  // 4x1x1 or 8x1x1 thread group
        Tile<Int<16 * kNWarps>, _16, _16>>;

    using SmemLayoutAtomQ = decltype(
        composition(Swizzle<kSwizzle, 3, 3>{},
                    // This has to be kBlockKSmem, using kHeadDim gives wrong results for d=128
                    Layout<Shape<_8, Int<kBlockKSmem>>,
                           Stride<Int<kBlockKSmem>, _1>>{}));
    using SmemLayoutQ = decltype(tile_to_shape(
        SmemLayoutAtomQ{},
        Shape<Int<kBlockM>, Int<kHeadDim>>{}));

    using SmemLayoutKV = decltype(tile_to_shape(
        SmemLayoutAtomQ{},
        Shape<Int<kBlockN>, Int<kHeadDim>>{}));

    // https://github.com/ColfaxResearch/cutlass-kernels/blob/a222587e6d59b93ba704853d3946fb686d8b8892/src/fmha/fmha_forward.cu#L434
    using SmemLayoutVtransposed = decltype(
        composition(SmemLayoutKV{}, make_layout(Shape<Int<kHeadDim>, Int<kBlockN>>{}, GenRowMajor{})));
    using SmemLayoutVtransposedNoSwizzle = decltype(get_nonswizzle_portion(SmemLayoutVtransposed{}));

    using SmemLayoutAtomO = decltype(
        composition(Swizzle<kSwizzle, 3, 3>{},
                    Layout<Shape<Int<8>, Int<kBlockKSmem>>,
                           Stride<Int<kBlockKSmem>, _1>>{}));
    using SmemLayoutO = decltype(tile_to_shape(
        SmemLayoutAtomO{},
        Shape<Int<kBlockM>, Int<kHeadDim>>{}));
    using SmemCopyAtomO = Copy_Atom<DefaultCopy, Element>;
    using SmemCopyAtomOaccum = Copy_Atom<DefaultCopy, ElementAccum>;

    static constexpr int kSmemQSize = size(SmemLayoutQ{}) * sizeof(Element);
    static constexpr int kSmemKVSize = size(SmemLayoutKV{}) * 2 * sizeof(Element);
    static constexpr int kSmemSize = Share_Q_K_smem ? std::max(kSmemQSize, kSmemKVSize) : kSmemQSize + kSmemKVSize;

    static constexpr int kGmemElemsPerLoad = sizeof(cute::uint128_t) / sizeof(Element);
    static_assert(kHeadDim % kGmemElemsPerLoad == 0, "kHeadDim must be a multiple of kGmemElemsPerLoad");
    // Using kBlockKSmem here is 6-10% faster than kBlockKGmem for d=128 because of bank conflicts.
    // For example, for d=128, smem is split into 2 "pages", each page takes care of columns
    // 0-63 and 64-127. If we have 16 threads per row for gmem read, when we write to smem,
    // thread 0 - 7 will write to the first page and thread 8 - 15 will write to the second page,
    // to the same banks.
    static constexpr int kGmemThreadsPerRow = kBlockKSmem / kGmemElemsPerLoad;
    static_assert(kNThreads % kGmemThreadsPerRow == 0, "kNThreads must be a multiple of kGmemThreadsPerRow");
    using GmemLayoutAtom = Layout<Shape <Int<kNThreads / kGmemThreadsPerRow>, Int<kGmemThreadsPerRow>>,
                                  Stride<Int<kGmemThreadsPerRow>, _1>>;

    // We use CACHEGLOBAL instead of CACHEALWAYS for both Q and K/V, since we won't be reading
    // from the same address by the same threadblock. This is slightly faster.
    using Gmem_copy_struct = std::conditional_t<
        Has_cp_async,
        SM80_CP_ASYNC_CACHEGLOBAL<cute::uint128_t>,
        DefaultCopy
    >;
    using GmemTiledCopyQKV = decltype(
        make_tiled_copy(Copy_Atom<Gmem_copy_struct, Element>{},
                        GmemLayoutAtom{},
                        Layout<Shape<_1, _8>>{}));  // Val layout, 8 vals per read

    // from how many rows does each thread have to fetch
    static constexpr int kGmemRowsPerThread = kBlockN / (kNThreads / kGmemThreadsPerRow);
    // Here we assign a contiguous tile to each thread, rather than a 1x8 row every 
    // (kNThreads / kGmemThreadsPerRow) rows, ensuring that the elements assigned to each thread
    // do not cross a page boundary. This way, each thread need only fetch 1 page index per
    // mainloop iteration. R>udimentary testing shows no slowdown.
    using GmemTiledCopyQKVPaged = decltype(
        make_tiled_copy(Copy_Atom<Gmem_copy_struct, Element>{},
                        GmemLayoutAtom{},
                        Layout<Shape<Int<kGmemRowsPerThread>, _8>, Stride<_8, _1>>{}));
    using GmemTiledCopyO = decltype(
        make_tiled_copy(Copy_Atom<DefaultCopy, Element>{},
                        GmemLayoutAtom{},
                        Layout<Shape<_1, _8>>{}));  // Val layout, 8 vals per store

    using GmemLayoutAtomOaccum = std::conditional_t<
        kBlockKSmem == 32,
        Layout<Shape <_16, _8>,  // Thread layout, 8 threads per row
               Stride< _8, _1>>,
        Layout<Shape <_8, _16>,  // Thread layout, 16 threads per row
               Stride< _16, _1>>
    >;
    using GmemTiledCopyOaccum = decltype(
        make_tiled_copy(Copy_Atom<DefaultCopy, ElementAccum>{},
                        GmemLayoutAtomOaccum{},
                        Layout<Shape < _1, _4>>{}));  // Val layout, 4 vals per store
    using GmemLayoutAtomRotcossin = GmemLayoutAtom;
    using GmemTiledCopyRotcossin = decltype(
        make_tiled_copy(Copy_Atom<UniversalCopy<uint64_t>, Element>{},
                        GmemLayoutAtomRotcossin{},
                        Layout<Shape < _1, _4>>{}));  // Val layout, 4 vals per load
    using GmemTiledCopyRotcossinCont = decltype(
        make_tiled_copy(Copy_Atom<DefaultCopy, Element>{},
                        GmemLayoutAtomRotcossin{},
                        Layout<Shape < _1, _8>>{}));  // Val layout, 8 vals per load
    using GmemTiledCopyRotcossinPaged = decltype(
        make_tiled_copy(Copy_Atom<UniversalCopy<uint64_t>, Element>{},
                        GmemLayoutAtomRotcossin{},
                        Layout<Shape<Int<kGmemRowsPerThread>, _4>, Stride<_4, _1>>{}));  // Val layout, 4 vals per load
    using GmemTiledCopyRotcossinContPaged = decltype(
        make_tiled_copy(Copy_Atom<DefaultCopy, Element>{},
                        GmemLayoutAtomRotcossin{},
                        Layout<Shape<Int<kGmemRowsPerThread>, _8>, Stride<_8, _1>>{}));  // Val layout, 8 vals per load
};

////////////////////////////////////////////////////////////////////////////////////////////////////