1 //===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements dead code elimination and basic block merging, along
11 // with a collection of other peephole control flow optimizations. For example:
13 // * Removes basic blocks with no predecessors.
14 // * Merges a basic block into its predecessor if there is only one and the
15 // predecessor only has one successor.
16 // * Eliminates PHI nodes for basic blocks with a single predecessor.
17 // * Eliminates a basic block that only contains an unconditional branch.
18 // * Changes invoke instructions to nounwind functions to be calls.
19 // * Change things like "if (x) if (y)" into "if (x&y)".
22 //===----------------------------------------------------------------------===//
24 #include "llvm/Transforms/Scalar.h"
25 #include "llvm/ADT/SmallPtrSet.h"
26 #include "llvm/ADT/SmallVector.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/Analysis/AssumptionTracker.h"
29 #include "llvm/Analysis/TargetTransformInfo.h"
30 #include "llvm/IR/Attributes.h"
31 #include "llvm/IR/CFG.h"
32 #include "llvm/IR/Constants.h"
33 #include "llvm/IR/DataLayout.h"
34 #include "llvm/IR/Instructions.h"
35 #include "llvm/IR/IntrinsicInst.h"
36 #include "llvm/IR/Module.h"
37 #include "llvm/Pass.h"
38 #include "llvm/Transforms/Utils/Local.h"
41 #define DEBUG_TYPE "simplifycfg"
43 STATISTIC(NumSimpl, "Number of blocks simplified");
46 struct CFGSimplifyPass : public FunctionPass {
47 static char ID; // Pass identification, replacement for typeid
48 CFGSimplifyPass() : FunctionPass(ID) {
49 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
51 bool runOnFunction(Function &F) override;
53 void getAnalysisUsage(AnalysisUsage &AU) const override {
54 AU.addRequired<AssumptionTracker>();
55 AU.addRequired<TargetTransformInfo>();
60 char CFGSimplifyPass::ID = 0;
61 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
63 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
64 INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
65 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
68 // Public interface to the CFGSimplification pass
69 FunctionPass *llvm::createCFGSimplificationPass() {
70 return new CFGSimplifyPass();
73 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
74 /// node) return blocks, merge them together to promote recursive block merging.
75 static bool mergeEmptyReturnBlocks(Function &F) {
78 BasicBlock *RetBlock = nullptr;
80 // Scan all the blocks in the function, looking for empty return blocks.
81 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
82 BasicBlock &BB = *BBI++;
84 // Only look at return blocks.
85 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
88 // Only look at the block if it is empty or the only other thing in it is a
89 // single PHI node that is the operand to the return.
90 if (Ret != &BB.front()) {
91 // Check for something else in the block.
92 BasicBlock::iterator I = Ret;
94 // Skip over debug info.
95 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
97 if (!isa<DbgInfoIntrinsic>(I) &&
98 (!isa<PHINode>(I) || I != BB.begin() ||
99 Ret->getNumOperands() == 0 ||
100 Ret->getOperand(0) != I))
104 // If this is the first returning block, remember it and keep going.
110 // Otherwise, we found a duplicate return block. Merge the two.
113 // Case when there is no input to the return or when the returned values
114 // agree is trivial. Note that they can't agree if there are phis in the
116 if (Ret->getNumOperands() == 0 ||
117 Ret->getOperand(0) ==
118 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
119 BB.replaceAllUsesWith(RetBlock);
120 BB.eraseFromParent();
124 // If the canonical return block has no PHI node, create one now.
125 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
127 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
128 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
129 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
130 std::distance(PB, PE), "merge",
133 for (pred_iterator PI = PB; PI != PE; ++PI)
134 RetBlockPHI->addIncoming(InVal, *PI);
135 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
138 // Turn BB into a block that just unconditionally branches to the return
139 // block. This handles the case when the two return blocks have a common
140 // predecessor but that return different things.
141 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
142 BB.getTerminator()->eraseFromParent();
143 BranchInst::Create(RetBlock, &BB);
149 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
150 /// iterating until no more changes are made.
151 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
152 const DataLayout *DL,
153 AssumptionTracker *AT) {
154 bool Changed = false;
155 bool LocalChange = true;
156 while (LocalChange) {
159 // Loop over all of the basic blocks and remove them if they are unneeded...
161 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
162 if (SimplifyCFG(BBIt++, TTI, DL, AT)) {
167 Changed |= LocalChange;
172 // It is possible that we may require multiple passes over the code to fully
175 bool CFGSimplifyPass::runOnFunction(Function &F) {
176 if (skipOptnoneFunction(F))
179 AssumptionTracker *AT = &getAnalysis<AssumptionTracker>();
180 const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
181 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
182 const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
183 bool EverChanged = removeUnreachableBlocks(F);
184 EverChanged |= mergeEmptyReturnBlocks(F);
185 EverChanged |= iterativelySimplifyCFG(F, TTI, DL, AT);
187 // If neither pass changed anything, we're done.
188 if (!EverChanged) return false;
190 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
191 // removeUnreachableBlocks is needed to nuke them, which means we should
192 // iterate between the two optimizations. We structure the code like this to
193 // avoid reruning iterativelySimplifyCFG if the second pass of
194 // removeUnreachableBlocks doesn't do anything.
195 if (!removeUnreachableBlocks(F))
199 EverChanged = iterativelySimplifyCFG(F, TTI, DL, AT);
200 EverChanged |= removeUnreachableBlocks(F);
201 } while (EverChanged);