1 //===- UnifyLoopExits.cpp - Redirect exiting edges to one block -*- C++ -*-===//
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
7 //===----------------------------------------------------------------------===//
9 // For each natural loop with multiple exit blocks, this pass creates a new
10 // block N such that all exiting blocks now branch to N, and then control flow
11 // is redistributed to all the original exit blocks.
13 // Limitation: This assumes that all terminators in the CFG are direct branches
14 // (the "br" instruction). The presence of any other control flow
15 // such as indirectbr, switch or callbr will cause an assert.
17 //===----------------------------------------------------------------------===//
19 #include "llvm/Analysis/LoopInfo.h"
20 #include "llvm/IR/Dominators.h"
21 #include "llvm/InitializePasses.h"
22 #include "llvm/Transforms/Utils.h"
23 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
25 #define DEBUG_TYPE "unify-loop-exits"
30 struct UnifyLoopExits : public FunctionPass {
32 UnifyLoopExits() : FunctionPass(ID) {
33 initializeUnifyLoopExitsPass(*PassRegistry::getPassRegistry());
36 void getAnalysisUsage(AnalysisUsage &AU) const override {
37 AU.addRequiredID(LowerSwitchID);
38 AU.addRequired<LoopInfoWrapperPass>();
39 AU.addRequired<DominatorTreeWrapperPass>();
40 AU.addPreservedID(LowerSwitchID);
41 AU.addPreserved<LoopInfoWrapperPass>();
42 AU.addPreserved<DominatorTreeWrapperPass>();
45 bool runOnFunction(Function &F) override;
49 char UnifyLoopExits::ID = 0;
51 FunctionPass *llvm::createUnifyLoopExitsPass() { return new UnifyLoopExits(); }
53 INITIALIZE_PASS_BEGIN(UnifyLoopExits, "unify-loop-exits",
54 "Fixup each natural loop to have a single exit block",
55 false /* Only looks at CFG */, false /* Analysis Pass */)
56 INITIALIZE_PASS_DEPENDENCY(LowerSwitch)
57 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
58 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
59 INITIALIZE_PASS_END(UnifyLoopExits, "unify-loop-exits",
60 "Fixup each natural loop to have a single exit block",
61 false /* Only looks at CFG */, false /* Analysis Pass */)
63 // The current transform introduces new control flow paths which may break the
64 // SSA requirement that every def must dominate all its uses. For example,
65 // consider a value D defined inside the loop that is used by some instruction
66 // U outside the loop. It follows that D dominates U, since the original
67 // program has valid SSA form. After merging the exits, all paths from D to U
68 // now flow through the unified exit block. In addition, there may be other
69 // paths that do not pass through D, but now reach the unified exit
70 // block. Thus, D no longer dominates U.
72 // Restore the dominance by creating a phi for each such D at the new unified
73 // loop exit. But when doing this, ignore any uses U that are in the new unified
74 // loop exit, since those were introduced specially when the block was created.
76 // The use of SSAUpdater seems like overkill for this operation. The location
77 // for creating the new PHI is well-known, and also the set of incoming blocks
79 static void restoreSSA(const DominatorTree &DT, const Loop *L,
80 const SetVector<BasicBlock *> &Incoming,
81 BasicBlock *LoopExitBlock) {
82 using InstVector = SmallVector<Instruction *, 8>;
83 using IIMap = DenseMap<Instruction *, InstVector>;
85 for (auto BB : L->blocks()) {
87 for (auto &U : I.uses()) {
88 auto UserInst = cast<Instruction>(U.getUser());
89 auto UserBlock = UserInst->getParent();
90 if (UserBlock == LoopExitBlock)
92 if (L->contains(UserBlock))
94 LLVM_DEBUG(dbgs() << "added ext use for " << I.getName() << "("
95 << BB->getName() << ")"
96 << ": " << UserInst->getName() << "("
97 << UserBlock->getName() << ")"
99 ExternalUsers[&I].push_back(UserInst);
104 for (auto II : ExternalUsers) {
105 // For each Def used outside the loop, create NewPhi in
106 // LoopExitBlock. NewPhi receives Def only along exiting blocks that
107 // dominate it, while the remaining values are undefined since those paths
108 // didn't exist in the original CFG.
110 LLVM_DEBUG(dbgs() << "externally used: " << Def->getName() << "\n");
111 auto NewPhi = PHINode::Create(Def->getType(), Incoming.size(),
112 Def->getName() + ".moved",
113 LoopExitBlock->getTerminator());
114 for (auto In : Incoming) {
115 LLVM_DEBUG(dbgs() << "predecessor " << In->getName() << ": ");
116 if (Def->getParent() == In || DT.dominates(Def, In)) {
117 LLVM_DEBUG(dbgs() << "dominated\n");
118 NewPhi->addIncoming(Def, In);
120 LLVM_DEBUG(dbgs() << "not dominated\n");
121 NewPhi->addIncoming(UndefValue::get(Def->getType()), In);
125 LLVM_DEBUG(dbgs() << "external users:");
126 for (auto U : II.second) {
127 LLVM_DEBUG(dbgs() << " " << U->getName());
128 U->replaceUsesOfWith(Def, NewPhi);
130 LLVM_DEBUG(dbgs() << "\n");
134 static bool unifyLoopExits(DominatorTree &DT, LoopInfo &LI, Loop *L) {
135 // To unify the loop exits, we need a list of the exiting blocks as
136 // well as exit blocks. The functions for locating these lists both
137 // traverse the entire loop body. It is more efficient to first
138 // locate the exiting blocks and then examine their successors to
139 // locate the exit blocks.
140 SetVector<BasicBlock *> ExitingBlocks;
141 SetVector<BasicBlock *> Exits;
143 // We need SetVectors, but the Loop API takes a vector, so we use a temporary.
144 SmallVector<BasicBlock *, 8> Temp;
145 L->getExitingBlocks(Temp);
146 for (auto BB : Temp) {
147 ExitingBlocks.insert(BB);
148 for (auto S : successors(BB)) {
149 auto SL = LI.getLoopFor(S);
150 // A successor is not an exit if it is directly or indirectly in the
152 if (SL == L || L->contains(SL))
159 dbgs() << "Found exit blocks:";
160 for (auto Exit : Exits) {
161 dbgs() << " " << Exit->getName();
165 dbgs() << "Found exiting blocks:";
166 for (auto EB : ExitingBlocks) {
167 dbgs() << " " << EB->getName();
171 if (Exits.size() <= 1) {
172 LLVM_DEBUG(dbgs() << "loop does not have multiple exits; nothing to do\n");
176 SmallVector<BasicBlock *, 8> GuardBlocks;
177 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
178 auto LoopExitBlock = CreateControlFlowHub(&DTU, GuardBlocks, ExitingBlocks,
181 restoreSSA(DT, L, ExitingBlocks, LoopExitBlock);
183 #if defined(EXPENSIVE_CHECKS)
184 assert(DT.verify(DominatorTree::VerificationLevel::Full));
186 assert(DT.verify(DominatorTree::VerificationLevel::Fast));
187 #endif // EXPENSIVE_CHECKS
190 // The guard blocks were created outside the loop, so they need to become
191 // members of the parent loop.
192 if (auto ParentLoop = L->getParentLoop()) {
193 for (auto G : GuardBlocks) {
194 ParentLoop->addBasicBlockToLoop(G, LI);
196 ParentLoop->verifyLoop();
199 #if defined(EXPENSIVE_CHECKS)
201 #endif // EXPENSIVE_CHECKS
206 bool UnifyLoopExits::runOnFunction(Function &F) {
207 LLVM_DEBUG(dbgs() << "===== Unifying loop exits in function " << F.getName()
209 auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
210 auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
212 bool Changed = false;
213 auto Loops = LI.getLoopsInPreorder();
214 for (auto L : Loops) {
215 LLVM_DEBUG(dbgs() << "Loop: " << L->getHeader()->getName() << " (depth: "
216 << LI.getLoopDepth(L->getHeader()) << ")\n");
217 Changed |= unifyLoopExits(DT, LI, L);