xref: /llvm-project/mlir/lib/Dialect/Affine/Transforms/AffineLoopInvariantCodeMotion.cpp (revision 5d001f58f241a0e15751feea8f087ede0ca6371f) 
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/IR/AffineExpr.h"
22 #include "mlir/IR/AffineMap.h"
23 #include "mlir/IR/Builders.h"
24 #include "mlir/Transforms/LoopUtils.h"
25 #include "mlir/Transforms/Utils.h"
26 #include "llvm/ADT/DenseMap.h"
27 #include "llvm/ADT/DenseSet.h"
28 #include "llvm/ADT/SmallPtrSet.h"
29 #include "llvm/Support/CommandLine.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/raw_ostream.h"
32 
33 #define DEBUG_TYPE "licm"
34 
35 using namespace mlir;
36 
37 namespace {
38 
39 /// Loop invariant code motion (LICM) pass.
40 /// TODO: The pass is missing zero-trip tests.
41 /// TODO: Check for the presence of side effects before hoisting.
42 /// TODO: This code should be removed once the new LICM pass can handle its
43 ///       uses.
44 struct LoopInvariantCodeMotion
45     : public AffineLoopInvariantCodeMotionBase<LoopInvariantCodeMotion> {
46   void runOnFunction() override;
47   void runOnAffineForOp(AffineForOp forOp);
48 };
49 } // end anonymous namespace
50 
51 static bool
52 checkInvarianceOfNestedIfOps(Operation *op, Value indVar, ValueRange iterArgs,
53                              SmallPtrSetImpl<Operation *> &opsWithUsers,
54                              SmallPtrSetImpl<Operation *> &opsToHoist);
55 static bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
56                               SmallPtrSetImpl<Operation *> &opsWithUsers,
57                               SmallPtrSetImpl<Operation *> &opsToHoist);
58 
59 static bool
60 areAllOpsInTheBlockListInvariant(Region &blockList, Value indVar,
61                                  ValueRange iterArgs,
62                                  SmallPtrSetImpl<Operation *> &opsWithUsers,
63                                  SmallPtrSetImpl<Operation *> &opsToHoist);
64 
65 // Returns true if the individual op is loop invariant.
66 bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
67                        SmallPtrSetImpl<Operation *> &opsWithUsers,
68                        SmallPtrSetImpl<Operation *> &opsToHoist) {
69   LLVM_DEBUG(llvm::dbgs() << "iterating on op: " << op;);
70 
71   if (isa<AffineIfOp>(op)) {
72     if (!checkInvarianceOfNestedIfOps(&op, indVar, iterArgs, opsWithUsers,
73                                       opsToHoist)) {
74       return false;
75     }
76   } else if (auto forOp = dyn_cast<AffineForOp>(op)) {
77     if (!areAllOpsInTheBlockListInvariant(forOp.getLoopBody(), indVar, iterArgs,
78                                           opsWithUsers, opsToHoist)) {
79       return false;
80     }
81   } else if (isa<AffineDmaStartOp, AffineDmaWaitOp>(op)) {
82     // TODO: Support DMA ops.
83     return false;
84   } else if (!isa<ConstantOp>(op)) {
85     // Register op in the set of ops that have users.
86     opsWithUsers.insert(&op);
87     if (isa<AffineMapAccessInterface>(op)) {
88       Value memref = isa<AffineReadOpInterface>(op)
89                          ? cast<AffineReadOpInterface>(op).getMemRef()
90                          : cast<AffineWriteOpInterface>(op).getMemRef();
91       for (auto *user : memref.getUsers()) {
92         // If this memref has a user that is a DMA, give up because these
93         // operations write to this memref.
94         if (isa<AffineDmaStartOp, AffineDmaWaitOp>(op)) {
95           return false;
96         }
97         // If the memref used by the load/store is used in a store elsewhere in
98         // the loop nest, we do not hoist. Similarly, if the memref used in a
99         // load is also being stored too, we do not hoist the load.
100         if (isa<AffineWriteOpInterface>(user) ||
101             (isa<AffineReadOpInterface>(user) &&
102              isa<AffineWriteOpInterface>(op))) {
103           if (&op != user) {
104             SmallVector<AffineForOp, 8> userIVs;
105             getLoopIVs(*user, &userIVs);
106             // Check that userIVs don't contain the for loop around the op.
107             if (llvm::is_contained(userIVs, getForInductionVarOwner(indVar))) {
108               return false;
109             }
110           }
111         }
112       }
113     }
114 
115     if (op.getNumOperands() == 0 && !isa<AffineYieldOp>(op)) {
116       LLVM_DEBUG(llvm::dbgs() << "\nNon-constant op with 0 operands\n");
117       return false;
118     }
119   }
120 
121   // Check operands.
122   for (unsigned int i = 0; i < op.getNumOperands(); ++i) {
123     auto *operandSrc = op.getOperand(i).getDefiningOp();
124 
125     LLVM_DEBUG(
126         op.getOperand(i).print(llvm::dbgs() << "\nIterating on operand\n"));
127 
128     // If the loop IV is the operand, this op isn't loop invariant.
129     if (indVar == op.getOperand(i)) {
130       LLVM_DEBUG(llvm::dbgs() << "\nLoop IV is the operand\n");
131       return false;
132     }
133 
134     // If the one of the iter_args is the operand, this op isn't loop invariant.
135     if (llvm::is_contained(iterArgs, op.getOperand(i))) {
136       LLVM_DEBUG(llvm::dbgs() << "\nOne of the iter_args is the operand\n");
137       return false;
138     }
139 
140     if (operandSrc != nullptr) {
141       LLVM_DEBUG(llvm::dbgs() << *operandSrc << "\nIterating on operand src\n");
142 
143       // If the value was defined in the loop (outside of the
144       // if/else region), and that operation itself wasn't meant to
145       // be hoisted, then mark this operation loop dependent.
146       if (opsWithUsers.count(operandSrc) && opsToHoist.count(operandSrc) == 0) {
147         return false;
148       }
149     }
150   }
151 
152   // If no operand was loop variant, mark this op for motion.
153   opsToHoist.insert(&op);
154   return true;
155 }
156 
157 // Checks if all ops in a region (i.e. list of blocks) are loop invariant.
158 bool areAllOpsInTheBlockListInvariant(
159     Region &blockList, Value indVar, ValueRange iterArgs,
160     SmallPtrSetImpl<Operation *> &opsWithUsers,
161     SmallPtrSetImpl<Operation *> &opsToHoist) {
162 
163   for (auto &b : blockList) {
164     for (auto &op : b) {
165       if (!isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist)) {
166         return false;
167       }
168     }
169   }
170 
171   return true;
172 }
173 
174 // Returns true if the affine.if op can be hoisted.
175 bool checkInvarianceOfNestedIfOps(Operation *op, Value indVar,
176                                   ValueRange iterArgs,
177                                   SmallPtrSetImpl<Operation *> &opsWithUsers,
178                                   SmallPtrSetImpl<Operation *> &opsToHoist) {
179   assert(isa<AffineIfOp>(op));
180   auto ifOp = cast<AffineIfOp>(op);
181 
182   if (!areAllOpsInTheBlockListInvariant(ifOp.thenRegion(), indVar, iterArgs,
183                                         opsWithUsers, opsToHoist)) {
184     return false;
185   }
186 
187   if (!areAllOpsInTheBlockListInvariant(ifOp.elseRegion(), indVar, iterArgs,
188                                         opsWithUsers, opsToHoist)) {
189     return false;
190   }
191 
192   return true;
193 }
194 
195 void LoopInvariantCodeMotion::runOnAffineForOp(AffineForOp forOp) {
196   auto *loopBody = forOp.getBody();
197   auto indVar = forOp.getInductionVar();
198   ValueRange iterArgs = forOp.getRegionIterArgs();
199 
200   // This is the place where hoisted instructions would reside.
201   OpBuilder b(forOp.getOperation());
202 
203   SmallPtrSet<Operation *, 8> opsToHoist;
204   SmallVector<Operation *, 8> opsToMove;
205   SmallPtrSet<Operation *, 8> opsWithUsers;
206 
207   for (auto &op : *loopBody) {
208     // Register op in the set of ops that have users. This set is used
209     // to prevent hoisting ops that depend on these ops that are
210     // not being hoisted.
211     if (!op.use_empty())
212       opsWithUsers.insert(&op);
213     if (!isa<AffineYieldOp>(op)) {
214       if (isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist)) {
215         opsToMove.push_back(&op);
216       }
217     }
218   }
219 
220   // For all instructions that we found to be invariant, place sequentially
221   // right before the for loop.
222   for (auto *op : opsToMove) {
223     op->moveBefore(forOp);
224   }
225 
226   LLVM_DEBUG(forOp->print(llvm::dbgs() << "Modified loop\n"));
227 }
228 
229 void LoopInvariantCodeMotion::runOnFunction() {
230   // Walk through all loops in a function in innermost-loop-first order.  This
231   // way, we first LICM from the inner loop, and place the ops in
232   // the outer loop, which in turn can be further LICM'ed.
233   getFunction().walk([&](AffineForOp op) {
234     LLVM_DEBUG(op->print(llvm::dbgs() << "\nOriginal loop\n"));
235     runOnAffineForOp(op);
236   });
237 }
238 
239 std::unique_ptr<OperationPass<FuncOp>>
240 mlir::createAffineLoopInvariantCodeMotionPass() {
241   return std::make_unique<LoopInvariantCodeMotion>();
242 }
243