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/TargetTransformInfo.h"
29 #include "llvm/IR/Attributes.h"
30 #include "llvm/IR/CFG.h"
31 #include "llvm/IR/Constants.h"
32 #include "llvm/IR/DataLayout.h"
33 #include "llvm/IR/Instructions.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/IR/Module.h"
36 #include "llvm/Pass.h"
37 #include "llvm/Transforms/Utils/Local.h"
40 #define DEBUG_TYPE "simplifycfg"
42 STATISTIC(NumSimpl, "Number of blocks simplified");
45 struct CFGSimplifyPass : public FunctionPass {
46 static char ID; // Pass identification, replacement for typeid
47 CFGSimplifyPass() : FunctionPass(ID) {
48 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
50 bool runOnFunction(Function &F) override;
52 void getAnalysisUsage(AnalysisUsage &AU) const override {
53 AU.addRequired<TargetTransformInfo>();
58 char CFGSimplifyPass::ID = 0;
59 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
61 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
62 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
65 // Public interface to the CFGSimplification pass
66 FunctionPass *llvm::createCFGSimplificationPass() {
67 return new CFGSimplifyPass();
70 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
71 /// node) return blocks, merge them together to promote recursive block merging.
72 static bool mergeEmptyReturnBlocks(Function &F) {
75 BasicBlock *RetBlock = nullptr;
77 // Scan all the blocks in the function, looking for empty return blocks.
78 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
79 BasicBlock &BB = *BBI++;
81 // Only look at return blocks.
82 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
85 // Only look at the block if it is empty or the only other thing in it is a
86 // single PHI node that is the operand to the return.
87 if (Ret != &BB.front()) {
88 // Check for something else in the block.
89 BasicBlock::iterator I = Ret;
91 // Skip over debug info.
92 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
94 if (!isa<DbgInfoIntrinsic>(I) &&
95 (!isa<PHINode>(I) || I != BB.begin() ||
96 Ret->getNumOperands() == 0 ||
97 Ret->getOperand(0) != I))
101 // If this is the first returning block, remember it and keep going.
107 // Otherwise, we found a duplicate return block. Merge the two.
110 // Case when there is no input to the return or when the returned values
111 // agree is trivial. Note that they can't agree if there are phis in the
113 if (Ret->getNumOperands() == 0 ||
114 Ret->getOperand(0) ==
115 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
116 BB.replaceAllUsesWith(RetBlock);
117 BB.eraseFromParent();
121 // If the canonical return block has no PHI node, create one now.
122 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
124 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
125 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
126 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
127 std::distance(PB, PE), "merge",
130 for (pred_iterator PI = PB; PI != PE; ++PI)
131 RetBlockPHI->addIncoming(InVal, *PI);
132 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
135 // Turn BB into a block that just unconditionally branches to the return
136 // block. This handles the case when the two return blocks have a common
137 // predecessor but that return different things.
138 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
139 BB.getTerminator()->eraseFromParent();
140 BranchInst::Create(RetBlock, &BB);
146 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
147 /// iterating until no more changes are made.
148 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
149 const DataLayout *DL) {
150 bool Changed = false;
151 bool LocalChange = true;
152 while (LocalChange) {
155 // Loop over all of the basic blocks and remove them if they are unneeded...
157 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
158 if (SimplifyCFG(BBIt++, TTI, DL)) {
163 Changed |= LocalChange;
168 // It is possible that we may require multiple passes over the code to fully
171 bool CFGSimplifyPass::runOnFunction(Function &F) {
172 if (skipOptnoneFunction(F))
175 const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
176 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
177 const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
178 bool EverChanged = removeUnreachableBlocks(F);
179 EverChanged |= mergeEmptyReturnBlocks(F);
180 EverChanged |= iterativelySimplifyCFG(F, TTI, DL);
182 // If neither pass changed anything, we're done.
183 if (!EverChanged) return false;
185 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
186 // removeUnreachableBlocks is needed to nuke them, which means we should
187 // iterate between the two optimizations. We structure the code like this to
188 // avoid reruning iterativelySimplifyCFG if the second pass of
189 // removeUnreachableBlocks doesn't do anything.
190 if (!removeUnreachableBlocks(F))
194 EverChanged = iterativelySimplifyCFG(F, TTI, DL);
195 EverChanged |= removeUnreachableBlocks(F);
196 } while (EverChanged);