MLIR  17.0.0git
LoopTiling.cpp
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1 //===- LoopTiling.cpp --- Loop tiling pass ------------------------------*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements a pass to tile loop nests.
10 //
11 //===----------------------------------------------------------------------===//
12 
14 
24 #include "mlir/IR/Builders.h"
25 #include "mlir/IR/IRMapping.h"
26 #include "llvm/Support/CommandLine.h"
27 #include "llvm/Support/Debug.h"
28 #include <optional>
29 
30 namespace mlir {
31 #define GEN_PASS_DEF_AFFINELOOPTILING
32 #include "mlir/Dialect/Affine/Passes.h.inc"
33 } // namespace mlir
34 
35 using namespace mlir;
36 
37 #define DEBUG_TYPE "affine-loop-tile"
38 
39 namespace {
40 
41 /// A pass to perform loop tiling on all suitable loop nests of a Function.
42 struct LoopTiling : public impl::AffineLoopTilingBase<LoopTiling> {
43  LoopTiling() = default;
44  explicit LoopTiling(uint64_t cacheSizeBytes, bool avoidMaxMinBounds = true)
45  : avoidMaxMinBounds(avoidMaxMinBounds) {
46  this->cacheSizeInKiB = cacheSizeBytes / 1024;
47  }
48 
49  void runOnOperation() override;
50  void getTileSizes(ArrayRef<AffineForOp> band,
51  SmallVectorImpl<unsigned> *tileSizes);
52 
53  // Default tile size if nothing is provided.
54  constexpr static unsigned kDefaultTileSize = 4;
55 
56  // If true, tile sizes are set to avoid max/min in bounds if possible.
57  bool avoidMaxMinBounds = true;
58 };
59 
60 } // namespace
61 
62 /// Creates a pass to perform loop tiling on all suitable loop nests of a
63 /// Function.
64 std::unique_ptr<OperationPass<func::FuncOp>>
65 mlir::createLoopTilingPass(uint64_t cacheSizeBytes) {
66  return std::make_unique<LoopTiling>(cacheSizeBytes);
67 }
68 std::unique_ptr<OperationPass<func::FuncOp>> mlir::createLoopTilingPass() {
69  return std::make_unique<LoopTiling>();
70 }
71 
72 /// Reduces each tile size to the largest divisor of the corresponding trip
73 /// count (if the trip count is known).
75  SmallVectorImpl<unsigned> *tileSizes) {
76  assert(band.size() == tileSizes->size() && "invalid tile size count");
77  for (unsigned i = 0, e = band.size(); i < e; i++) {
78  unsigned &tSizeAdjusted = (*tileSizes)[i];
79  std::optional<uint64_t> mayConst = getConstantTripCount(band[i]);
80  if (!mayConst)
81  continue;
82  // Adjust the tile size to largest factor of the trip count less than
83  // tSize.
84  uint64_t constTripCount = *mayConst;
85  if (constTripCount > 1 && tSizeAdjusted > constTripCount / 2)
86  tSizeAdjusted = constTripCount / 2;
87  while (constTripCount % tSizeAdjusted != 0)
88  tSizeAdjusted--;
89  }
90 }
91 
92 // Returns tile sizes to use. Checks CL options; if none are specified, sets it
93 // based on a simple model that looks at the memory footprint and determines
94 // tile sizes assuming identity accesses / 1:1 tile size proportional footprint
95 // along each of the dimensions being tiled.
96 // TODO: evolve this model. Tile size determination is a large area
97 // to play with in general.
98 void LoopTiling::getTileSizes(ArrayRef<AffineForOp> band,
99  SmallVectorImpl<unsigned> *tileSizes) {
100  if (band.empty())
101  return;
102 
103  // Use command-line tileSize for all loops if specified.
104  if (tileSize) {
105  tileSizes->assign(band.size(), tileSize);
106  return;
107  }
108 
109  // Use tileSizes and fill them with default tile size if it's short.
110  if (!this->tileSizes.empty()) {
111  tileSizes->assign(this->tileSizes.begin(), this->tileSizes.end());
112  tileSizes->resize(band.size(), kDefaultTileSize);
113  return;
114  }
115  tileSizes->resize(band.size());
116 
117  // The first loop in the band.
118  AffineForOp rootForOp = band[0];
119  (void)rootForOp;
120 
121  // Obtain memory footprint and set tile sizes so that a tile fits in
122  // the cache size. This is an approximation with the assumption that the
123  // footprint increases with the tile size linearly in that dimension (i.e.,
124  // assumes one-to-one access function).
125  std::optional<int64_t> fp = getMemoryFootprintBytes(band[0], 0);
126  if (!fp) {
127  // Fill with default tile sizes if footprint is unknown.
128  std::fill(tileSizes->begin(), tileSizes->end(),
129  LoopTiling::kDefaultTileSize);
130  if (avoidMaxMinBounds)
131  adjustToDivisorsOfTripCounts(band, tileSizes);
132  LLVM_DEBUG(
133  rootForOp.emitWarning("memory footprint unknown: using default tile "
134  "sizes adjusted to trip count divisors"));
135  return;
136  }
137 
138  // Check how many times larger the cache size is when compared to footprint.
139  uint64_t cacheSizeBytes = cacheSizeInKiB * 1024;
140  uint64_t excessFactor = llvm::divideCeil(*fp, cacheSizeBytes);
141  if (excessFactor <= 1) {
142  // No need of any tiling - set tile size to 1.
143  std::fill(tileSizes->begin(), tileSizes->end(), 1);
144  return;
145  }
146 
147  // Divide all loops equally in an attempt to reduce footprint.
148  // TODO: this is approximate. Ideally, obtain reuse factor /
149  // profitability along each dimension and weight tile sizes based on that as
150  // one possible approach. Or compute a polynomial in tile sizes and solve for
151  // it.
152 
153  // For an n-d tileable band, compute the n^th root of the excess.
154  unsigned tSize =
155  static_cast<unsigned>(floorl(std::pow(excessFactor, 1.0 / band.size())));
156  // We'll keep a running product to determine the last tile size better.
157  unsigned cumulProductOfTileSizes = 1;
158  for (unsigned i = 0, e = band.size(); i < e; i++) {
159  if (i < e - 1)
160  (*tileSizes)[i] = tSize;
161  else
162  // Set last tile size to cover the balance.
163  (*tileSizes)[i] = std::max(
164  1U, static_cast<unsigned>(excessFactor / cumulProductOfTileSizes));
165  cumulProductOfTileSizes *= (*tileSizes)[i];
166  }
167  if (avoidMaxMinBounds)
168  adjustToDivisorsOfTripCounts(band, tileSizes);
169 }
170 
171 void LoopTiling::runOnOperation() {
172  // Bands of loops to tile.
173  std::vector<SmallVector<AffineForOp, 6>> bands;
174  getTileableBands(getOperation(), &bands);
175 
176  // Tile each band.
177  for (auto &band : bands) {
178  // Set up tile sizes; fill missing tile sizes at the end with default tile
179  // size or tileSize if one was provided.
180  SmallVector<unsigned, 6> tileSizes;
181  getTileSizes(band, &tileSizes);
182  if (llvm::DebugFlag) {
183  auto diag = band[0].emitRemark("using tile sizes [");
184  for (unsigned tSize : tileSizes)
185  diag << tSize << ' ';
186  diag << "]\n";
187  }
188  SmallVector<AffineForOp, 6> tiledNest;
189  if (failed(tilePerfectlyNested(band, tileSizes, &tiledNest))) {
190  // An empty band always succeeds.
191  assert(!band.empty() && "guaranteed to succeed on empty bands");
192  LLVM_DEBUG(band.front()->emitRemark("loop tiling failed!\n"));
193  continue;
194  }
195 
196  // Separate full and partial tiles.
197  if (separate) {
198  auto intraTileLoops =
199  MutableArrayRef<AffineForOp>(tiledNest).drop_front(band.size());
200  if (failed(separateFullTiles(intraTileLoops))) {
201  assert(!intraTileLoops.empty() &&
202  "guaranteed to succeed on empty bands");
203  LLVM_DEBUG(intraTileLoops.front()->emitRemark(
204  "separation post tiling failed!\n"));
205  }
206  }
207  }
208 }
209 
210 constexpr unsigned LoopTiling::kDefaultTileSize;
static void adjustToDivisorsOfTripCounts(ArrayRef< AffineForOp > band, SmallVectorImpl< unsigned > *tileSizes)
Reduces each tile size to the largest divisor of the corresponding trip count (if the trip count is k...
Definition: LoopTiling.cpp:74
static std::string diag(const llvm::Value &value)
static Value max(ImplicitLocOpBuilder &builder, Value value, Value bound)
Include the generated interface declarations.
LogicalResult separateFullTiles(MutableArrayRef< AffineForOp > nest, SmallVectorImpl< AffineForOp > *fullTileNest=nullptr)
Separates full tiles from partial tiles for a perfect nest nest by generating a conditional guard tha...
Definition: LoopUtils.cpp:2796
void getTileableBands(func::FuncOp f, std::vector< SmallVector< AffineForOp, 6 >> *bands)
Identify valid and profitable bands of loops to tile.
Definition: LoopUtils.cpp:962
std::optional< uint64_t > getConstantTripCount(AffineForOp forOp)
Returns the trip count of the loop if it's a constant, std::nullopt otherwise.
std::optional< int64_t > getMemoryFootprintBytes(AffineForOp forOp, int memorySpace=-1)
Gets the memory footprint of all data touched in the specified memory space in bytes; if the memory s...
Definition: Utils.cpp:1351
std::unique_ptr< OperationPass< func::FuncOp > > createLoopTilingPass(uint64_t cacheSizeBytes)
Creates a pass to perform tiling on loop nests.
Definition: LoopTiling.cpp:65
bool failed(LogicalResult result)
Utility function that returns true if the provided LogicalResult corresponds to a failure value.
Definition: LogicalResult.h:72
LogicalResult tilePerfectlyNested(MutableArrayRef< AffineForOp > input, ArrayRef< unsigned > tileSizes, SmallVectorImpl< AffineForOp > *tiledNest=nullptr)
Tiles the specified band of perfectly nested loops creating tile-space loops and intra-tile loops.
Definition: LoopUtils.cpp:859