xref: /llvm-project/mlir/lib/Dialect/Affine/Transforms/AffineLoopInvariantCodeMotion.cpp (revision be0a7e9f27083ada6072fcc0711ffa5630daa5ec) 
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 
13 #include "PassDetail.h"
14 #include "mlir/Analysis/AffineAnalysis.h"
15 #include "mlir/Analysis/AffineStructures.h"
16 #include "mlir/Analysis/LoopAnalysis.h"
17 #include "mlir/Analysis/SliceAnalysis.h"
18 #include "mlir/Analysis/Utils.h"
19 #include "mlir/Dialect/Affine/IR/AffineOps.h"
20 #include "mlir/Dialect/Affine/Passes.h"
21 #include "mlir/Dialect/Arithmetic/IR/Arithmetic.h"
22 #include "mlir/IR/AffineExpr.h"
23 #include "mlir/IR/AffineMap.h"
24 #include "mlir/IR/Builders.h"
25 #include "mlir/Transforms/LoopUtils.h"
26 #include "mlir/Transforms/Utils.h"
27 #include "llvm/ADT/DenseMap.h"
28 #include "llvm/ADT/DenseSet.h"
29 #include "llvm/ADT/SmallPtrSet.h"
30 #include "llvm/Support/CommandLine.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/raw_ostream.h"
33 
34 #define DEBUG_TYPE "licm"
35 
36 using namespace mlir;
37 
38 namespace {
39 
40 /// Loop invariant code motion (LICM) pass.
41 /// TODO: The pass is missing zero-trip tests.
42 /// TODO: Check for the presence of side effects before hoisting.
43 /// TODO: This code should be removed once the new LICM pass can handle its
44 ///       uses.
45 struct LoopInvariantCodeMotion
46     : public AffineLoopInvariantCodeMotionBase<LoopInvariantCodeMotion> {
47   void runOnFunction() override;
48   void runOnAffineForOp(AffineForOp forOp);
49 };
50 } // namespace
51 
52 static bool
53 checkInvarianceOfNestedIfOps(Operation *op, Value indVar, ValueRange iterArgs,
54                              SmallPtrSetImpl<Operation *> &opsWithUsers,
55                              SmallPtrSetImpl<Operation *> &opsToHoist);
56 static bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
57                               SmallPtrSetImpl<Operation *> &opsWithUsers,
58                               SmallPtrSetImpl<Operation *> &opsToHoist);
59 
60 static bool
61 areAllOpsInTheBlockListInvariant(Region &blockList, Value indVar,
62                                  ValueRange iterArgs,
63                                  SmallPtrSetImpl<Operation *> &opsWithUsers,
64                                  SmallPtrSetImpl<Operation *> &opsToHoist);
65 
66 // Returns true if the individual op is loop invariant.
67 bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
68                        SmallPtrSetImpl<Operation *> &opsWithUsers,
69                        SmallPtrSetImpl<Operation *> &opsToHoist) {
70   LLVM_DEBUG(llvm::dbgs() << "iterating on op: " << op;);
71 
72   if (isa<AffineIfOp>(op)) {
73     if (!checkInvarianceOfNestedIfOps(&op, indVar, iterArgs, opsWithUsers,
74                                       opsToHoist)) {
75       return false;
76     }
77   } else if (auto forOp = dyn_cast<AffineForOp>(op)) {
78     if (!areAllOpsInTheBlockListInvariant(forOp.getLoopBody(), indVar, iterArgs,
79                                           opsWithUsers, opsToHoist)) {
80       return false;
81     }
82   } else if (isa<AffineDmaStartOp, AffineDmaWaitOp>(op)) {
83     // TODO: Support DMA ops.
84     return false;
85   } else if (!isa<arith::ConstantOp, ConstantOp>(op)) {
86     // Register op in the set of ops that have users.
87     opsWithUsers.insert(&op);
88     if (isa<AffineMapAccessInterface>(op)) {
89       Value memref = isa<AffineReadOpInterface>(op)
90                          ? cast<AffineReadOpInterface>(op).getMemRef()
91                          : cast<AffineWriteOpInterface>(op).getMemRef();
92       for (auto *user : memref.getUsers()) {
93         // If this memref has a user that is a DMA, give up because these
94         // operations write to this memref.
95         if (isa<AffineDmaStartOp, AffineDmaWaitOp>(op)) {
96           return false;
97         }
98         // If the memref used by the load/store is used in a store elsewhere in
99         // the loop nest, we do not hoist. Similarly, if the memref used in a
100         // load is also being stored too, we do not hoist the load.
101         if (isa<AffineWriteOpInterface>(user) ||
102             (isa<AffineReadOpInterface>(user) &&
103              isa<AffineWriteOpInterface>(op))) {
104           if (&op != user) {
105             SmallVector<AffineForOp, 8> userIVs;
106             getLoopIVs(*user, &userIVs);
107             // Check that userIVs don't contain the for loop around the op.
108             if (llvm::is_contained(userIVs, getForInductionVarOwner(indVar))) {
109               return false;
110             }
111           }
112         }
113       }
114     }
115 
116     if (op.getNumOperands() == 0 && !isa<AffineYieldOp>(op)) {
117       LLVM_DEBUG(llvm::dbgs() << "\nNon-constant op with 0 operands\n");
118       return false;
119     }
120   }
121 
122   // Check operands.
123   for (unsigned int i = 0; i < op.getNumOperands(); ++i) {
124     auto *operandSrc = op.getOperand(i).getDefiningOp();
125 
126     LLVM_DEBUG(
127         op.getOperand(i).print(llvm::dbgs() << "\nIterating on operand\n"));
128 
129     // If the loop IV is the operand, this op isn't loop invariant.
130     if (indVar == op.getOperand(i)) {
131       LLVM_DEBUG(llvm::dbgs() << "\nLoop IV is the operand\n");
132       return false;
133     }
134 
135     // If the one of the iter_args is the operand, this op isn't loop invariant.
136     if (llvm::is_contained(iterArgs, op.getOperand(i))) {
137       LLVM_DEBUG(llvm::dbgs() << "\nOne of the iter_args is the operand\n");
138       return false;
139     }
140 
141     if (operandSrc != nullptr) {
142       LLVM_DEBUG(llvm::dbgs() << *operandSrc << "\nIterating on operand src\n");
143 
144       // If the value was defined in the loop (outside of the
145       // if/else region), and that operation itself wasn't meant to
146       // be hoisted, then mark this operation loop dependent.
147       if (opsWithUsers.count(operandSrc) && opsToHoist.count(operandSrc) == 0) {
148         return false;
149       }
150     }
151   }
152 
153   // If no operand was loop variant, mark this op for motion.
154   opsToHoist.insert(&op);
155   return true;
156 }
157 
158 // Checks if all ops in a region (i.e. list of blocks) are loop invariant.
159 bool areAllOpsInTheBlockListInvariant(
160     Region &blockList, Value indVar, ValueRange iterArgs,
161     SmallPtrSetImpl<Operation *> &opsWithUsers,
162     SmallPtrSetImpl<Operation *> &opsToHoist) {
163 
164   for (auto &b : blockList) {
165     for (auto &op : b) {
166       if (!isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist)) {
167         return false;
168       }
169     }
170   }
171 
172   return true;
173 }
174 
175 // Returns true if the affine.if op can be hoisted.
176 bool checkInvarianceOfNestedIfOps(Operation *op, Value indVar,
177                                   ValueRange iterArgs,
178                                   SmallPtrSetImpl<Operation *> &opsWithUsers,
179                                   SmallPtrSetImpl<Operation *> &opsToHoist) {
180   assert(isa<AffineIfOp>(op));
181   auto ifOp = cast<AffineIfOp>(op);
182 
183   if (!areAllOpsInTheBlockListInvariant(ifOp.thenRegion(), indVar, iterArgs,
184                                         opsWithUsers, opsToHoist)) {
185     return false;
186   }
187 
188   if (!areAllOpsInTheBlockListInvariant(ifOp.elseRegion(), indVar, iterArgs,
189                                         opsWithUsers, opsToHoist)) {
190     return false;
191   }
192 
193   return true;
194 }
195 
196 void LoopInvariantCodeMotion::runOnAffineForOp(AffineForOp forOp) {
197   auto *loopBody = forOp.getBody();
198   auto indVar = forOp.getInductionVar();
199   ValueRange iterArgs = forOp.getRegionIterArgs();
200 
201   // This is the place where hoisted instructions would reside.
202   OpBuilder b(forOp.getOperation());
203 
204   SmallPtrSet<Operation *, 8> opsToHoist;
205   SmallVector<Operation *, 8> opsToMove;
206   SmallPtrSet<Operation *, 8> opsWithUsers;
207 
208   for (auto &op : *loopBody) {
209     // Register op in the set of ops that have users. This set is used
210     // to prevent hoisting ops that depend on these ops that are
211     // not being hoisted.
212     if (!op.use_empty())
213       opsWithUsers.insert(&op);
214     if (!isa<AffineYieldOp>(op)) {
215       if (isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist)) {
216         opsToMove.push_back(&op);
217       }
218     }
219   }
220 
221   // For all instructions that we found to be invariant, place sequentially
222   // right before the for loop.
223   for (auto *op : opsToMove) {
224     op->moveBefore(forOp);
225   }
226 
227   LLVM_DEBUG(forOp->print(llvm::dbgs() << "Modified loop\n"));
228 }
229 
230 void LoopInvariantCodeMotion::runOnFunction() {
231   // Walk through all loops in a function in innermost-loop-first order.  This
232   // way, we first LICM from the inner loop, and place the ops in
233   // the outer loop, which in turn can be further LICM'ed.
234   getFunction().walk([&](AffineForOp op) {
235     LLVM_DEBUG(op->print(llvm::dbgs() << "\nOriginal loop\n"));
236     runOnAffineForOp(op);
237   });
238 }
239 
240 std::unique_ptr<OperationPass<FuncOp>>
241 mlir::createAffineLoopInvariantCodeMotionPass() {
242   return std::make_unique<LoopInvariantCodeMotion>();
243 }
244