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/SimplifyCFG.h"
25 #include "llvm/ADT/SmallPtrSet.h"
26 #include "llvm/ADT/SmallVector.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/Analysis/AssumptionCache.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/Support/CommandLine.h"
39 #include "llvm/Transforms/Utils/Local.h"
40 #include "llvm/Transforms/Scalar.h"
43 #define DEBUG_TYPE "simplifycfg"
45 static cl::opt<unsigned>
46 UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
47 cl::desc("Control the number of bonus instructions (default = 1)"));
49 STATISTIC(NumSimpl, "Number of blocks simplified");
51 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
52 /// node) return blocks, merge them together to promote recursive block merging.
53 static bool mergeEmptyReturnBlocks(Function &F) {
56 BasicBlock *RetBlock = nullptr;
58 // Scan all the blocks in the function, looking for empty return blocks.
59 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
60 BasicBlock &BB = *BBI++;
62 // Only look at return blocks.
63 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
66 // Only look at the block if it is empty or the only other thing in it is a
67 // single PHI node that is the operand to the return.
68 if (Ret != &BB.front()) {
69 // Check for something else in the block.
70 BasicBlock::iterator I = Ret;
72 // Skip over debug info.
73 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
75 if (!isa<DbgInfoIntrinsic>(I) &&
76 (!isa<PHINode>(I) || I != BB.begin() ||
77 Ret->getNumOperands() == 0 ||
78 Ret->getOperand(0) != I))
82 // If this is the first returning block, remember it and keep going.
88 // Otherwise, we found a duplicate return block. Merge the two.
91 // Case when there is no input to the return or when the returned values
92 // agree is trivial. Note that they can't agree if there are phis in the
94 if (Ret->getNumOperands() == 0 ||
96 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
97 BB.replaceAllUsesWith(RetBlock);
102 // If the canonical return block has no PHI node, create one now.
103 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
105 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
106 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
107 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
108 std::distance(PB, PE), "merge",
111 for (pred_iterator PI = PB; PI != PE; ++PI)
112 RetBlockPHI->addIncoming(InVal, *PI);
113 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
116 // Turn BB into a block that just unconditionally branches to the return
117 // block. This handles the case when the two return blocks have a common
118 // predecessor but that return different things.
119 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
120 BB.getTerminator()->eraseFromParent();
121 BranchInst::Create(RetBlock, &BB);
127 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
128 /// iterating until no more changes are made.
129 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
130 const DataLayout *DL, AssumptionCache *AC,
131 unsigned BonusInstThreshold) {
132 bool Changed = false;
133 bool LocalChange = true;
134 while (LocalChange) {
137 // Loop over all of the basic blocks and remove them if they are unneeded...
139 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
140 if (SimplifyCFG(BBIt++, TTI, BonusInstThreshold, DL, AC)) {
145 Changed |= LocalChange;
150 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
151 const DataLayout *DL, AssumptionCache *AC,
152 int BonusInstThreshold) {
153 bool EverChanged = removeUnreachableBlocks(F);
154 EverChanged |= mergeEmptyReturnBlocks(F);
155 EverChanged |= iterativelySimplifyCFG(F, TTI, DL, AC, BonusInstThreshold);
157 // If neither pass changed anything, we're done.
158 if (!EverChanged) return false;
160 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
161 // removeUnreachableBlocks is needed to nuke them, which means we should
162 // iterate between the two optimizations. We structure the code like this to
163 // avoid reruning iterativelySimplifyCFG if the second pass of
164 // removeUnreachableBlocks doesn't do anything.
165 if (!removeUnreachableBlocks(F))
169 EverChanged = iterativelySimplifyCFG(F, TTI, DL, AC, BonusInstThreshold);
170 EverChanged |= removeUnreachableBlocks(F);
171 } while (EverChanged);
176 SimplifyCFGPass::SimplifyCFGPass()
177 : BonusInstThreshold(UserBonusInstThreshold) {}
179 SimplifyCFGPass::SimplifyCFGPass(int BonusInstThreshold)
180 : BonusInstThreshold(BonusInstThreshold) {}
182 PreservedAnalyses SimplifyCFGPass::run(Function &F,
183 AnalysisManager<Function> *AM) {
184 auto &DL = F.getParent()->getDataLayout();
185 auto &TTI = AM->getResult<TargetIRAnalysis>(F);
186 auto &AC = AM->getResult<AssumptionAnalysis>(F);
188 if (!simplifyFunctionCFG(F, TTI, &DL, &AC, BonusInstThreshold))
189 return PreservedAnalyses::none();
191 return PreservedAnalyses::all();
195 struct CFGSimplifyPass : public FunctionPass {
196 static char ID; // Pass identification, replacement for typeid
197 unsigned BonusInstThreshold;
198 CFGSimplifyPass(int T = -1) : FunctionPass(ID) {
199 BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
200 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
202 bool runOnFunction(Function &F) override {
203 if (skipOptnoneFunction(F))
206 AssumptionCache *AC =
207 &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
208 const TargetTransformInfo &TTI =
209 getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
210 const DataLayout &DL = F.getParent()->getDataLayout();
211 return simplifyFunctionCFG(F, TTI, &DL, AC, BonusInstThreshold);
214 void getAnalysisUsage(AnalysisUsage &AU) const override {
215 AU.addRequired<AssumptionCacheTracker>();
216 AU.addRequired<TargetTransformInfoWrapperPass>();
221 char CFGSimplifyPass::ID = 0;
222 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
224 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
225 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
226 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
229 // Public interface to the CFGSimplification pass
230 FunctionPass *llvm::createCFGSimplificationPass(int Threshold) {
231 return new CFGSimplifyPass(Threshold);