MLIR  19.0.0git
AffineLoopInvariantCodeMotion.cpp
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1 //===- AffineLoopInvariantCodeMotion.cpp - Code to perform loop fusion-----===//
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 loop invariant code motion.
10 //
11 //===----------------------------------------------------------------------===//
12 
14 
25 #include "mlir/IR/AffineExpr.h"
26 #include "mlir/IR/AffineMap.h"
27 #include "mlir/IR/Builders.h"
28 #include "mlir/IR/Matchers.h"
30 #include "llvm/ADT/DenseMap.h"
31 #include "llvm/ADT/DenseSet.h"
32 #include "llvm/ADT/SmallPtrSet.h"
33 #include "llvm/Support/CommandLine.h"
34 #include "llvm/Support/Debug.h"
35 #include "llvm/Support/raw_ostream.h"
36 
37 namespace mlir {
38 namespace affine {
39 #define GEN_PASS_DEF_AFFINELOOPINVARIANTCODEMOTION
40 #include "mlir/Dialect/Affine/Passes.h.inc"
41 } // namespace affine
42 } // namespace mlir
43 
44 #define DEBUG_TYPE "licm"
45 
46 using namespace mlir;
47 using namespace mlir::affine;
48 
49 namespace {
50 
51 /// Affine loop invariant code motion (LICM) pass.
52 /// TODO: The pass is missing zero-trip tests.
53 /// TODO: This code should be removed once the new LICM pass can handle its
54 /// uses.
55 struct LoopInvariantCodeMotion
56  : public affine::impl::AffineLoopInvariantCodeMotionBase<
57  LoopInvariantCodeMotion> {
58  void runOnOperation() override;
59  void runOnAffineForOp(AffineForOp forOp);
60 };
61 } // namespace
62 
63 static bool
64 checkInvarianceOfNestedIfOps(AffineIfOp ifOp, Value indVar, ValueRange iterArgs,
65  SmallPtrSetImpl<Operation *> &opsWithUsers,
66  SmallPtrSetImpl<Operation *> &opsToHoist);
67 static bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
68  SmallPtrSetImpl<Operation *> &opsWithUsers,
69  SmallPtrSetImpl<Operation *> &opsToHoist);
70 
71 static bool
73  ValueRange iterArgs,
74  SmallPtrSetImpl<Operation *> &opsWithUsers,
75  SmallPtrSetImpl<Operation *> &opsToHoist);
76 
77 // Returns true if the individual op is loop invariant.
78 static bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
79  SmallPtrSetImpl<Operation *> &opsWithUsers,
80  SmallPtrSetImpl<Operation *> &opsToHoist) {
81  LLVM_DEBUG(llvm::dbgs() << "iterating on op: " << op;);
82 
83  if (auto ifOp = dyn_cast<AffineIfOp>(op)) {
84  if (!checkInvarianceOfNestedIfOps(ifOp, indVar, iterArgs, opsWithUsers,
85  opsToHoist))
86  return false;
87  } else if (auto forOp = dyn_cast<AffineForOp>(op)) {
88  if (!areAllOpsInTheBlockListInvariant(forOp.getRegion(), indVar, iterArgs,
89  opsWithUsers, opsToHoist))
90  return false;
91  } else if (auto parOp = dyn_cast<AffineParallelOp>(op)) {
92  if (!areAllOpsInTheBlockListInvariant(parOp.getRegion(), indVar, iterArgs,
93  opsWithUsers, opsToHoist))
94  return false;
95  } else if (!isMemoryEffectFree(&op) &&
96  !isa<AffineReadOpInterface, AffineWriteOpInterface,
97  AffinePrefetchOp>(&op)) {
98  // Check for side-effecting ops. Affine read/write ops are handled
99  // separately below.
100  return false;
101  } else if (!matchPattern(&op, m_Constant())) {
102  // Register op in the set of ops that have users.
103  opsWithUsers.insert(&op);
104  if (isa<AffineReadOpInterface, AffineWriteOpInterface>(op)) {
105  auto read = dyn_cast<AffineReadOpInterface>(op);
106  Value memref = read ? read.getMemRef()
107  : cast<AffineWriteOpInterface>(op).getMemRef();
108  for (auto *user : memref.getUsers()) {
109  // If this memref has a user that is a DMA, give up because these
110  // operations write to this memref.
111  if (isa<AffineDmaStartOp, AffineDmaWaitOp>(user))
112  return false;
113  // If the memref used by the load/store is used in a store elsewhere in
114  // the loop nest, we do not hoist. Similarly, if the memref used in a
115  // load is also being stored too, we do not hoist the load.
116  if (isa<AffineWriteOpInterface>(user) ||
117  (isa<AffineReadOpInterface>(user) &&
118  isa<AffineWriteOpInterface>(op))) {
119  if (&op != user) {
121  getAffineForIVs(*user, &userIVs);
122  // Check that userIVs don't contain the for loop around the op.
123  if (llvm::is_contained(userIVs, getForInductionVarOwner(indVar)))
124  return false;
125  }
126  }
127  }
128  }
129 
130  if (op.getNumOperands() == 0 && !isa<AffineYieldOp>(op)) {
131  LLVM_DEBUG(llvm::dbgs() << "Non-constant op with 0 operands\n");
132  return false;
133  }
134  }
135 
136  // Check operands.
137  for (unsigned int i = 0; i < op.getNumOperands(); ++i) {
138  auto *operandSrc = op.getOperand(i).getDefiningOp();
139 
140  LLVM_DEBUG(
141  op.getOperand(i).print(llvm::dbgs() << "Iterating on operand\n"));
142 
143  // If the loop IV is the operand, this op isn't loop invariant.
144  if (indVar == op.getOperand(i)) {
145  LLVM_DEBUG(llvm::dbgs() << "Loop IV is the operand\n");
146  return false;
147  }
148 
149  // If the one of the iter_args is the operand, this op isn't loop invariant.
150  if (llvm::is_contained(iterArgs, op.getOperand(i))) {
151  LLVM_DEBUG(llvm::dbgs() << "One of the iter_args is the operand\n");
152  return false;
153  }
154 
155  if (operandSrc) {
156  LLVM_DEBUG(llvm::dbgs() << *operandSrc << "Iterating on operand src\n");
157 
158  // If the value was defined in the loop (outside of the if/else region),
159  // and that operation itself wasn't meant to be hoisted, then mark this
160  // operation loop dependent.
161  if (opsWithUsers.count(operandSrc) && opsToHoist.count(operandSrc) == 0)
162  return false;
163  }
164  }
165 
166  // If no operand was loop variant, mark this op for motion.
167  opsToHoist.insert(&op);
168  return true;
169 }
170 
171 // Checks if all ops in a region (i.e. list of blocks) are loop invariant.
172 static bool
174  ValueRange iterArgs,
175  SmallPtrSetImpl<Operation *> &opsWithUsers,
176  SmallPtrSetImpl<Operation *> &opsToHoist) {
177 
178  for (auto &b : blockList) {
179  for (auto &op : b) {
180  if (!isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist))
181  return false;
182  }
183  }
184 
185  return true;
186 }
187 
188 // Returns true if the affine.if op can be hoisted.
189 static bool
190 checkInvarianceOfNestedIfOps(AffineIfOp ifOp, Value indVar, ValueRange iterArgs,
191  SmallPtrSetImpl<Operation *> &opsWithUsers,
192  SmallPtrSetImpl<Operation *> &opsToHoist) {
193  if (!areAllOpsInTheBlockListInvariant(ifOp.getThenRegion(), indVar, iterArgs,
194  opsWithUsers, opsToHoist))
195  return false;
196 
197  if (!areAllOpsInTheBlockListInvariant(ifOp.getElseRegion(), indVar, iterArgs,
198  opsWithUsers, opsToHoist))
199  return false;
200 
201  return true;
202 }
203 
204 void LoopInvariantCodeMotion::runOnAffineForOp(AffineForOp forOp) {
205  auto *loopBody = forOp.getBody();
206  auto indVar = forOp.getInductionVar();
207  ValueRange iterArgs = forOp.getRegionIterArgs();
208 
209  // This is the place where hoisted instructions would reside.
210  OpBuilder b(forOp.getOperation());
211 
212  SmallPtrSet<Operation *, 8> opsToHoist;
213  SmallVector<Operation *, 8> opsToMove;
214  SmallPtrSet<Operation *, 8> opsWithUsers;
215 
216  for (auto &op : *loopBody) {
217  // Register op in the set of ops that have users. This set is used
218  // to prevent hoisting ops that depend on these ops that are
219  // not being hoisted.
220  if (!op.use_empty())
221  opsWithUsers.insert(&op);
222  if (!isa<AffineYieldOp>(op)) {
223  if (isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist)) {
224  opsToMove.push_back(&op);
225  }
226  }
227  }
228 
229  // For all instructions that we found to be invariant, place sequentially
230  // right before the for loop.
231  for (auto *op : opsToMove) {
232  op->moveBefore(forOp);
233  }
234 
235  LLVM_DEBUG(forOp->print(llvm::dbgs() << "Modified loop\n"));
236 }
237 
238 void LoopInvariantCodeMotion::runOnOperation() {
239  // Walk through all loops in a function in innermost-loop-first order. This
240  // way, we first LICM from the inner loop, and place the ops in
241  // the outer loop, which in turn can be further LICM'ed.
242  getOperation().walk([&](AffineForOp op) {
243  LLVM_DEBUG(op->print(llvm::dbgs() << "\nOriginal loop\n"));
244  runOnAffineForOp(op);
245  });
246 }
247 
248 std::unique_ptr<OperationPass<func::FuncOp>>
250  return std::make_unique<LoopInvariantCodeMotion>();
251 }
static bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs, SmallPtrSetImpl< Operation * > &opsWithUsers, SmallPtrSetImpl< Operation * > &opsToHoist)
static bool areAllOpsInTheBlockListInvariant(Region &blockList, Value indVar, ValueRange iterArgs, SmallPtrSetImpl< Operation * > &opsWithUsers, SmallPtrSetImpl< Operation * > &opsToHoist)
static bool checkInvarianceOfNestedIfOps(AffineIfOp ifOp, Value indVar, ValueRange iterArgs, SmallPtrSetImpl< Operation * > &opsWithUsers, SmallPtrSetImpl< Operation * > &opsToHoist)
This class helps build Operations.
Definition: Builders.h:209
Operation is the basic unit of execution within MLIR.
Definition: Operation.h:88
bool use_empty()
Returns true if this operation has no uses.
Definition: Operation.h:848
Value getOperand(unsigned idx)
Definition: Operation.h:345
void print(raw_ostream &os, const OpPrintingFlags &flags=std::nullopt)
unsigned getNumOperands()
Definition: Operation.h:341
void moveBefore(Operation *existingOp)
Unlink this operation from its current block and insert it right before existingOp which may be in th...
Definition: Operation.cpp:555
This class contains a list of basic blocks and a link to the parent operation it is attached to.
Definition: Region.h:26
This class provides an abstraction over the different types of ranges over Values.
Definition: ValueRange.h:381
This class represents an instance of an SSA value in the MLIR system, representing a computable value...
Definition: Value.h:96
void print(raw_ostream &os) const
user_range getUsers() const
Definition: Value.h:228
Operation * getDefiningOp() const
If this value is the result of an operation, return the operation that defines it.
Definition: Value.cpp:20
std::unique_ptr< OperationPass< func::FuncOp > > createAffineLoopInvariantCodeMotionPass()
Creates a loop invariant code motion pass that hoists loop invariant operations out of affine loops.
AffineForOp getForInductionVarOwner(Value val)
Returns the loop parent of an induction variable.
Definition: AffineOps.cpp:2556
void getAffineForIVs(Operation &op, SmallVectorImpl< AffineForOp > *loops)
Populates 'loops' with IVs of the affine.for ops surrounding 'op' ordered from the outermost 'affine....
Definition: Utils.cpp:633
Include the generated interface declarations.
bool matchPattern(Value value, const Pattern &pattern)
Entry point for matching a pattern over a Value.
Definition: Matchers.h:401
bool isMemoryEffectFree(Operation *op)
Returns true if the given operation is free of memory effects.
detail::constant_op_matcher m_Constant()
Matches a constant foldable operation.
Definition: Matchers.h:310