LoopTiling.cpp

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//===- LoopTiling.cpp --- Loop tiling pass ------------------------------*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements a pass to tile loop nests.
//
//===----------------------------------------------------------------------===//

#include "PassDetail.h"
#include "mlir/Dialect/Affine/Analysis/AffineAnalysis.h"
#include "mlir/Dialect/Affine/Analysis/AffineStructures.h"
#include "mlir/Dialect/Affine/Analysis/LoopAnalysis.h"
#include "mlir/Dialect/Affine/Analysis/Utils.h"
#include "mlir/Dialect/Affine/IR/AffineOps.h"
#include "mlir/Dialect/Affine/IR/AffineValueMap.h"
#include "mlir/Dialect/Affine/LoopUtils.h"
#include "mlir/Dialect/Affine/Passes.h"
#include "mlir/Dialect/Affine/Utils.h"
#include "mlir/IR/BlockAndValueMapping.h"
#include "mlir/IR/Builders.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
using namespace mlir;

#define DEBUG_TYPE "affine-loop-tile"

namespace {

/// A pass to perform loop tiling on all suitable loop nests of a Function.
struct LoopTiling : public AffineLoopTilingBase<LoopTiling> {
  LoopTiling() = default;
  explicit LoopTiling(uint64_t cacheSizeBytes, bool avoidMaxMinBounds = true)
      : avoidMaxMinBounds(avoidMaxMinBounds) {
    this->cacheSizeInKiB = cacheSizeBytes / 1024;
  }

  void runOnOperation() override;
  void getTileSizes(ArrayRef<AffineForOp> band,
                    SmallVectorImpl<unsigned> *tileSizes);

  // Default tile size if nothing is provided.
  constexpr static unsigned kDefaultTileSize = 4;

  // If true, tile sizes are set to avoid max/min in bounds if possible.
  bool avoidMaxMinBounds = true;
};

} // namespace

/// Creates a pass to perform loop tiling on all suitable loop nests of a
/// Function.
std::unique_ptr<OperationPass<func::FuncOp>>
mlir::createLoopTilingPass(uint64_t cacheSizeBytes) {
  return std::make_unique<LoopTiling>(cacheSizeBytes);
}
std::unique_ptr<OperationPass<func::FuncOp>> mlir::createLoopTilingPass() {
  return std::make_unique<LoopTiling>();
}

/// Reduces each tile size to the largest divisor of the corresponding trip
/// count (if the trip count is known).
static void adjustToDivisorsOfTripCounts(ArrayRef<AffineForOp> band,
                                         SmallVectorImpl<unsigned> *tileSizes) {
  assert(band.size() == tileSizes->size() && "invalid tile size count");
  for (unsigned i = 0, e = band.size(); i < e; i++) {
    unsigned &tSizeAdjusted = (*tileSizes)[i];
    Optional<uint64_t> mayConst = getConstantTripCount(band[i]);
    if (!mayConst)
      continue;
    // Adjust the tile size to largest factor of the trip count less than
    // tSize.
    uint64_t constTripCount = mayConst.getValue();
    if (constTripCount > 1 && tSizeAdjusted > constTripCount / 2)
      tSizeAdjusted = constTripCount / 2;
    while (constTripCount % tSizeAdjusted != 0)
      tSizeAdjusted--;
  }
}

// Returns tile sizes to use. Checks CL options; if none are specified, sets it
// based on a simple model that looks at the memory footprint and determines
// tile sizes assuming identity accesses / 1:1 tile size proportional footprint
// along each of the dimensions being tiled.
// TODO: evolve this model. Tile size determination is a large area
// to play with in general.
void LoopTiling::getTileSizes(ArrayRef<AffineForOp> band,
                              SmallVectorImpl<unsigned> *tileSizes) {
  if (band.empty())
    return;

  // Use command-line tileSize for all loops if specified.
  if (tileSize) {
    tileSizes->assign(band.size(), tileSize);
    return;
  }

  // Use tileSizes and fill them with default tile size if it's short.
  if (!this->tileSizes.empty()) {
    tileSizes->assign(this->tileSizes.begin(), this->tileSizes.end());
    tileSizes->resize(band.size(), kDefaultTileSize);
    return;
  }
  tileSizes->resize(band.size());

  // The first loop in the band.
  AffineForOp rootForOp = band[0];
  (void)rootForOp;

  // Obtain memory footprint and set tile sizes so that a tile fits in
  // the cache size. This is an approximation with the assumption that the
  // footprint increases with the tile size linearly in that dimension (i.e.,
  // assumes one-to-one access function).
  Optional<int64_t> fp = getMemoryFootprintBytes(band[0], 0);
  if (!fp) {
    // Fill with default tile sizes if footprint is unknown.
    std::fill(tileSizes->begin(), tileSizes->end(),
              LoopTiling::kDefaultTileSize);
    if (avoidMaxMinBounds)
      adjustToDivisorsOfTripCounts(band, tileSizes);
    LLVM_DEBUG(
        rootForOp.emitWarning("memory footprint unknown: using default tile "
                              "sizes adjusted to trip count divisors"));
    return;
  }

  // Check how many times larger the cache size is when compared to footprint.
  uint64_t cacheSizeBytes = cacheSizeInKiB * 1024;
  uint64_t excessFactor = llvm::divideCeil(fp.getValue(), cacheSizeBytes);
  if (excessFactor <= 1) {
    // No need of any tiling - set tile size to 1.
    std::fill(tileSizes->begin(), tileSizes->end(), 1);
    return;
  }

  // Divide all loops equally in an attempt to reduce footprint.
  // TODO: this is approximate. Ideally, obtain reuse factor /
  // profitability along each dimension and weight tile sizes based on that as
  // one possible approach. Or compute a polynomial in tile sizes and solve for
  // it.

  // For an n-d tileable band, compute the n^th root of the excess.
  unsigned tSize =
      static_cast<unsigned>(floorl(std::pow(excessFactor, 1.0 / band.size())));
  // We'll keep a running product to determine the last tile size better.
  unsigned cumulProductOfTileSizes = 1;
  for (unsigned i = 0, e = band.size(); i < e; i++) {
    if (i < e - 1)
      (*tileSizes)[i] = tSize;
    else
      // Set last tile size to cover the balance.
      (*tileSizes)[i] = std::max(
          1U, static_cast<unsigned>(excessFactor / cumulProductOfTileSizes));
    cumulProductOfTileSizes *= (*tileSizes)[i];
  }
  if (avoidMaxMinBounds)
    adjustToDivisorsOfTripCounts(band, tileSizes);
}

void LoopTiling::runOnOperation() {
  // Bands of loops to tile.
  std::vector<SmallVector<AffineForOp, 6>> bands;
  getTileableBands(getOperation(), &bands);

  // Tile each band.
  for (auto &band : bands) {
    // Set up tile sizes; fill missing tile sizes at the end with default tile
    // size or tileSize if one was provided.
    SmallVector<unsigned, 6> tileSizes;
    getTileSizes(band, &tileSizes);
    if (llvm::DebugFlag) {
      auto diag = band[0].emitRemark("using tile sizes [");
      for (unsigned tSize : tileSizes)
        diag << tSize << ' ';
      diag << "]\n";
    }
    SmallVector<AffineForOp, 6> tiledNest;
    if (failed(tilePerfectlyNested(band, tileSizes, &tiledNest))) {
      // An empty band always succeeds.
      assert(!band.empty() && "guaranteed to succeed on empty bands");
      LLVM_DEBUG(band.front()->emitRemark("loop tiling failed!\n"));
      continue;
    }

    // Separate full and partial tiles.
    if (separate) {
      auto intraTileLoops =
          MutableArrayRef<AffineForOp>(tiledNest).drop_front(band.size());
      if (failed(separateFullTiles(intraTileLoops))) {
        assert(!intraTileLoops.empty() &&
               "guaranteed to succeed on empty bands");
        LLVM_DEBUG(intraTileLoops.front()->emitRemark(
            "separation post tiling failed!\n"));
      }
    }
  }
}

constexpr unsigned LoopTiling::kDefaultTileSize;