loader.h

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// Copyright © 2024 Apple Inc.
 
#pragma once
 
#include "mlx/backend/metal/kernels/steel/utils.h"
 
// Loading helper
 
namespace mlx {
namespace steel {
 
template <
    typename T,
    short BROWS,
    short BCOLS,
    short dst_ld,
    short reduction_dim,
    short tgp_size,
    short alignment = 1,
    short n_reads = (BCOLS * BROWS) / (tgp_size),
    short TCOLS = BCOLS / n_reads,
    short TROWS = tgp_size / TCOLS>
struct BlockLoader {
  STEEL_CONST short n_rows = (BROWS + TROWS - 1) / TROWS;
  STEEL_CONST short vec_size = n_reads;
 
  // Leading dimension for src
  const int src_ld;
  const int tile_stride;
 
  // Thread location indices
  const short thread_idx;
  const short bi;
  const short bj;
 
  // threadgroup and device memory
  threadgroup T* dst;
  const device T* src;
 
  struct alignas(alignment * sizeof(T)) ReadVector {
    uint8_t v[sizeof(T) * vec_size];
  };
 
  /* Constructor */
  METAL_FUNC BlockLoader(
      const device T* src_,
      const int src_ld_,
      threadgroup T* dst_,
      ushort simd_group_id [[simdgroup_index_in_threadgroup]],
      ushort simd_lane_id [[thread_index_in_simdgroup]])
      : src_ld(src_ld_),
        tile_stride(reduction_dim ? BCOLS : BROWS * src_ld),
        thread_idx(simd_group_id * 32 + simd_lane_id),
        bi(thread_idx / TCOLS),
        bj(vec_size * (thread_idx % TCOLS)),
        dst(dst_ + bi * dst_ld + bj),
        src(src_ + bi * src_ld + bj) {}
 
  /* Load from device memory into threadgroup memory - without bound checking */
  METAL_FUNC void load_unsafe() const {
    STEEL_PRAGMA_UNROLL
    for (short i = 0; i < BROWS; i += TROWS) {
      *((threadgroup ReadVector*)(&dst[i * dst_ld])) =
          *((const device ReadVector*)(&src[i * src_ld]));
    }
  }
 
  /* Load from device memory into threadgroup memory - with bound checking */
  METAL_FUNC void load_safe(short2 src_tile_dim) const {
    src_tile_dim = src_tile_dim - short2(bj, bi);
 
    // Skip loading if thread has no valid reads
    if (src_tile_dim.x <= 0 || src_tile_dim.y <= 0) {
      STEEL_PRAGMA_UNROLL
      for (short i = 0; i < BROWS; i += TROWS) {
        STEEL_PRAGMA_UNROLL
        for (short j = 0; j < vec_size; j++) {
          dst[i * dst_ld + j] = T(0);
        }
      }
      return;
    }
 
    // Use fast thread memory for bound checks
    bool tmp_idx[vec_size];
    T tmp_val[vec_size];
 
    STEEL_PRAGMA_UNROLL
    for (short i = 0; i < BROWS; i += TROWS) {
      // Make sure tmp_idx only contains valid indices
      STEEL_PRAGMA_UNROLL
      for (short j = 0; j < vec_size; j++) {
        tmp_idx[j] = (i < src_tile_dim.y) && (j < src_tile_dim.x);
      }
 
      // Read valid indices into tmp_val
      STEEL_PRAGMA_UNROLL
      for (short j = 0; j < vec_size; j++) {
        tmp_val[j] = src[(tmp_idx[j] ? i * src_ld + j : 0)];
      }
 
      // Zero out uneeded values
      STEEL_PRAGMA_UNROLL
      for (short j = 0; j < vec_size; j++) {
        tmp_val[j] = tmp_idx[j] ? tmp_val[j] : T(0);
      }
 
      // Copy values to threadgroup memory
      STEEL_PRAGMA_UNROLL
      for (short j = 0; j < vec_size; j++) {
        dst[i * dst_ld + j] = tmp_val[j];
      }
    }
  }
 
  /* Iteration helper */
  METAL_FUNC void next() {
    src += tile_stride;
  }
};
 
} // namespace steel
} // namespace mlx