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/SliceAnalysis.h"
15 #include "mlir/Dialect/Affine/Analysis/AffineAnalysis.h"
16 #include "mlir/Dialect/Affine/Analysis/AffineStructures.h"
17 #include "mlir/Dialect/Affine/Analysis/LoopAnalysis.h"
18 #include "mlir/Dialect/Affine/Analysis/Utils.h"
19 #include "mlir/Dialect/Affine/IR/AffineOps.h"
20 #include "mlir/Dialect/Affine/LoopUtils.h"
21 #include "mlir/Dialect/Affine/Passes.h"
22 #include "mlir/Dialect/Affine/Utils.h"
23 #include "mlir/Dialect/Arithmetic/IR/Arithmetic.h"
24 #include "mlir/IR/AffineExpr.h"
25 #include "mlir/IR/AffineMap.h"
26 #include "mlir/IR/Builders.h"
27 #include "mlir/IR/Matchers.h"
28 #include "llvm/ADT/DenseMap.h"
29 #include "llvm/ADT/DenseSet.h"
30 #include "llvm/ADT/SmallPtrSet.h"
31 #include "llvm/Support/CommandLine.h"
32 #include "llvm/Support/Debug.h"
33 #include "llvm/Support/raw_ostream.h"
34 
35 #define DEBUG_TYPE "licm"
36 
37 using namespace mlir;
38 
39 namespace {
40 
41 /// Loop invariant code motion (LICM) pass.
42 /// TODO: The pass is missing zero-trip tests.
43 /// TODO: Check for the presence of side effects before hoisting.
44 /// TODO: This code should be removed once the new LICM pass can handle its
45 ///       uses.
46 struct LoopInvariantCodeMotion
47     : public AffineLoopInvariantCodeMotionBase<LoopInvariantCodeMotion> {
48   void runOnOperation() override;
49   void runOnAffineForOp(AffineForOp forOp);
50 };
51 } // namespace
52 
53 static bool
54 checkInvarianceOfNestedIfOps(Operation *op, Value indVar, ValueRange iterArgs,
55                              SmallPtrSetImpl<Operation *> &opsWithUsers,
56                              SmallPtrSetImpl<Operation *> &opsToHoist);
57 static bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
58                               SmallPtrSetImpl<Operation *> &opsWithUsers,
59                               SmallPtrSetImpl<Operation *> &opsToHoist);
60 
61 static bool
62 areAllOpsInTheBlockListInvariant(Region &blockList, Value indVar,
63                                  ValueRange iterArgs,
64                                  SmallPtrSetImpl<Operation *> &opsWithUsers,
65                                  SmallPtrSetImpl<Operation *> &opsToHoist);
66 
67 // Returns true if the individual op is loop invariant.
isOpLoopInvariant(Operation & op,Value indVar,ValueRange iterArgs,SmallPtrSetImpl<Operation * > & opsWithUsers,SmallPtrSetImpl<Operation * > & opsToHoist)68 bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
69                        SmallPtrSetImpl<Operation *> &opsWithUsers,
70                        SmallPtrSetImpl<Operation *> &opsToHoist) {
71   LLVM_DEBUG(llvm::dbgs() << "iterating on op: " << op;);
72 
73   if (isa<AffineIfOp>(op)) {
74     if (!checkInvarianceOfNestedIfOps(&op, indVar, iterArgs, opsWithUsers,
75                                       opsToHoist)) {
76       return false;
77     }
78   } else if (auto forOp = dyn_cast<AffineForOp>(op)) {
79     if (!areAllOpsInTheBlockListInvariant(forOp.getLoopBody(), indVar, iterArgs,
80                                           opsWithUsers, opsToHoist)) {
81       return false;
82     }
83   } else if (isa<AffineDmaStartOp, AffineDmaWaitOp>(op)) {
84     // TODO: Support DMA ops.
85     return false;
86   } else if (!matchPattern(&op, m_Constant())) {
87     // Register op in the set of ops that have users.
88     opsWithUsers.insert(&op);
89     if (isa<AffineMapAccessInterface>(op)) {
90       Value memref = isa<AffineReadOpInterface>(op)
91                          ? cast<AffineReadOpInterface>(op).getMemRef()
92                          : cast<AffineWriteOpInterface>(op).getMemRef();
93       for (auto *user : memref.getUsers()) {
94         // If this memref has a user that is a DMA, give up because these
95         // operations write to this memref.
96         if (isa<AffineDmaStartOp, AffineDmaWaitOp>(op)) {
97           return false;
98         }
99         // If the memref used by the load/store is used in a store elsewhere in
100         // the loop nest, we do not hoist. Similarly, if the memref used in a
101         // load is also being stored too, we do not hoist the load.
102         if (isa<AffineWriteOpInterface>(user) ||
103             (isa<AffineReadOpInterface>(user) &&
104              isa<AffineWriteOpInterface>(op))) {
105           if (&op != user) {
106             SmallVector<AffineForOp, 8> userIVs;
107             getLoopIVs(*user, &userIVs);
108             // Check that userIVs don't contain the for loop around the op.
109             if (llvm::is_contained(userIVs, getForInductionVarOwner(indVar))) {
110               return false;
111             }
112           }
113         }
114       }
115     }
116 
117     if (op.getNumOperands() == 0 && !isa<AffineYieldOp>(op)) {
118       LLVM_DEBUG(llvm::dbgs() << "\nNon-constant op with 0 operands\n");
119       return false;
120     }
121   }
122 
123   // Check operands.
124   for (unsigned int i = 0; i < op.getNumOperands(); ++i) {
125     auto *operandSrc = op.getOperand(i).getDefiningOp();
126 
127     LLVM_DEBUG(
128         op.getOperand(i).print(llvm::dbgs() << "\nIterating on operand\n"));
129 
130     // If the loop IV is the operand, this op isn't loop invariant.
131     if (indVar == op.getOperand(i)) {
132       LLVM_DEBUG(llvm::dbgs() << "\nLoop IV is the operand\n");
133       return false;
134     }
135 
136     // If the one of the iter_args is the operand, this op isn't loop invariant.
137     if (llvm::is_contained(iterArgs, op.getOperand(i))) {
138       LLVM_DEBUG(llvm::dbgs() << "\nOne of the iter_args is the operand\n");
139       return false;
140     }
141 
142     if (operandSrc != nullptr) {
143       LLVM_DEBUG(llvm::dbgs() << *operandSrc << "\nIterating on operand src\n");
144 
145       // If the value was defined in the loop (outside of the
146       // if/else region), and that operation itself wasn't meant to
147       // be hoisted, then mark this operation loop dependent.
148       if (opsWithUsers.count(operandSrc) && opsToHoist.count(operandSrc) == 0) {
149         return false;
150       }
151     }
152   }
153 
154   // If no operand was loop variant, mark this op for motion.
155   opsToHoist.insert(&op);
156   return true;
157 }
158 
159 // Checks if all ops in a region (i.e. list of blocks) are loop invariant.
areAllOpsInTheBlockListInvariant(Region & blockList,Value indVar,ValueRange iterArgs,SmallPtrSetImpl<Operation * > & opsWithUsers,SmallPtrSetImpl<Operation * > & opsToHoist)160 bool areAllOpsInTheBlockListInvariant(
161     Region &blockList, Value indVar, ValueRange iterArgs,
162     SmallPtrSetImpl<Operation *> &opsWithUsers,
163     SmallPtrSetImpl<Operation *> &opsToHoist) {
164 
165   for (auto &b : blockList) {
166     for (auto &op : b) {
167       if (!isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist)) {
168         return false;
169       }
170     }
171   }
172 
173   return true;
174 }
175 
176 // Returns true if the affine.if op can be hoisted.
checkInvarianceOfNestedIfOps(Operation * op,Value indVar,ValueRange iterArgs,SmallPtrSetImpl<Operation * > & opsWithUsers,SmallPtrSetImpl<Operation * > & opsToHoist)177 bool checkInvarianceOfNestedIfOps(Operation *op, Value indVar,
178                                   ValueRange iterArgs,
179                                   SmallPtrSetImpl<Operation *> &opsWithUsers,
180                                   SmallPtrSetImpl<Operation *> &opsToHoist) {
181   assert(isa<AffineIfOp>(op));
182   auto ifOp = cast<AffineIfOp>(op);
183 
184   if (!areAllOpsInTheBlockListInvariant(ifOp.getThenRegion(), indVar, iterArgs,
185                                         opsWithUsers, opsToHoist)) {
186     return false;
187   }
188 
189   if (!areAllOpsInTheBlockListInvariant(ifOp.getElseRegion(), indVar, iterArgs,
190                                         opsWithUsers, opsToHoist)) {
191     return false;
192   }
193 
194   return true;
195 }
196 
runOnAffineForOp(AffineForOp forOp)197 void LoopInvariantCodeMotion::runOnAffineForOp(AffineForOp forOp) {
198   auto *loopBody = forOp.getBody();
199   auto indVar = forOp.getInductionVar();
200   ValueRange iterArgs = forOp.getRegionIterArgs();
201 
202   // This is the place where hoisted instructions would reside.
203   OpBuilder b(forOp.getOperation());
204 
205   SmallPtrSet<Operation *, 8> opsToHoist;
206   SmallVector<Operation *, 8> opsToMove;
207   SmallPtrSet<Operation *, 8> opsWithUsers;
208 
209   for (auto &op : *loopBody) {
210     // Register op in the set of ops that have users. This set is used
211     // to prevent hoisting ops that depend on these ops that are
212     // not being hoisted.
213     if (!op.use_empty())
214       opsWithUsers.insert(&op);
215     if (!isa<AffineYieldOp>(op)) {
216       if (isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist)) {
217         opsToMove.push_back(&op);
218       }
219     }
220   }
221 
222   // For all instructions that we found to be invariant, place sequentially
223   // right before the for loop.
224   for (auto *op : opsToMove) {
225     op->moveBefore(forOp);
226   }
227 
228   LLVM_DEBUG(forOp->print(llvm::dbgs() << "Modified loop\n"));
229 }
230 
runOnOperation()231 void LoopInvariantCodeMotion::runOnOperation() {
232   // Walk through all loops in a function in innermost-loop-first order.  This
233   // way, we first LICM from the inner loop, and place the ops in
234   // the outer loop, which in turn can be further LICM'ed.
235   getOperation().walk([&](AffineForOp op) {
236     LLVM_DEBUG(op->print(llvm::dbgs() << "\nOriginal loop\n"));
237     runOnAffineForOp(op);
238   });
239 }
240 
241 std::unique_ptr<OperationPass<func::FuncOp>>
createAffineLoopInvariantCodeMotionPass()242 mlir::createAffineLoopInvariantCodeMotionPass() {
243   return std::make_unique<LoopInvariantCodeMotion>();
244 }
245