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/Support/CommandLine.h"
39 #include "llvm/Transforms/Utils/Local.h"
42 #define DEBUG_TYPE "simplifycfg"
44 static cl::opt<unsigned>
45 UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
46 cl::desc("Control the number of bonus instructions (default = 1)"));
48 STATISTIC(NumSimpl, "Number of blocks simplified");
51 struct CFGSimplifyPass : public FunctionPass {
52 static char ID; // Pass identification, replacement for typeid
53 unsigned BonusInstThreshold;
54 CFGSimplifyPass(int T = -1) : FunctionPass(ID) {
55 BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
56 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
58 bool runOnFunction(Function &F) override;
60 void getAnalysisUsage(AnalysisUsage &AU) const override {
61 AU.addRequired<AssumptionTracker>();
62 AU.addRequired<TargetTransformInfo>();
67 char CFGSimplifyPass::ID = 0;
68 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
70 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
71 INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
72 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
75 // Public interface to the CFGSimplification pass
76 FunctionPass *llvm::createCFGSimplificationPass(int Threshold) {
77 return new CFGSimplifyPass(Threshold);
80 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
81 /// node) return blocks, merge them together to promote recursive block merging.
82 static bool mergeEmptyReturnBlocks(Function &F) {
85 BasicBlock *RetBlock = nullptr;
87 // Scan all the blocks in the function, looking for empty return blocks.
88 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
89 BasicBlock &BB = *BBI++;
91 // Only look at return blocks.
92 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
95 // Only look at the block if it is empty or the only other thing in it is a
96 // single PHI node that is the operand to the return.
97 if (Ret != &BB.front()) {
98 // Check for something else in the block.
99 BasicBlock::iterator I = Ret;
101 // Skip over debug info.
102 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
104 if (!isa<DbgInfoIntrinsic>(I) &&
105 (!isa<PHINode>(I) || I != BB.begin() ||
106 Ret->getNumOperands() == 0 ||
107 Ret->getOperand(0) != I))
111 // If this is the first returning block, remember it and keep going.
117 // Otherwise, we found a duplicate return block. Merge the two.
120 // Case when there is no input to the return or when the returned values
121 // agree is trivial. Note that they can't agree if there are phis in the
123 if (Ret->getNumOperands() == 0 ||
124 Ret->getOperand(0) ==
125 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
126 BB.replaceAllUsesWith(RetBlock);
127 BB.eraseFromParent();
131 // If the canonical return block has no PHI node, create one now.
132 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
134 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
135 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
136 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
137 std::distance(PB, PE), "merge",
140 for (pred_iterator PI = PB; PI != PE; ++PI)
141 RetBlockPHI->addIncoming(InVal, *PI);
142 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
145 // Turn BB into a block that just unconditionally branches to the return
146 // block. This handles the case when the two return blocks have a common
147 // predecessor but that return different things.
148 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
149 BB.getTerminator()->eraseFromParent();
150 BranchInst::Create(RetBlock, &BB);
156 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
157 /// iterating until no more changes are made.
158 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
159 const DataLayout *DL,
160 AssumptionTracker *AT,
161 unsigned BonusInstThreshold) {
162 bool Changed = false;
163 bool LocalChange = true;
164 while (LocalChange) {
167 // Loop over all of the basic blocks and remove them if they are unneeded...
169 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
170 if (SimplifyCFG(BBIt++, TTI, BonusInstThreshold, DL, AT)) {
175 Changed |= LocalChange;
180 // It is possible that we may require multiple passes over the code to fully
183 bool CFGSimplifyPass::runOnFunction(Function &F) {
184 if (skipOptnoneFunction(F))
187 AssumptionTracker *AT = &getAnalysis<AssumptionTracker>();
188 const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
189 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
190 const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
191 bool EverChanged = removeUnreachableBlocks(F);
192 EverChanged |= mergeEmptyReturnBlocks(F);
193 EverChanged |= iterativelySimplifyCFG(F, TTI, DL, AT, BonusInstThreshold);
195 // If neither pass changed anything, we're done.
196 if (!EverChanged) return false;
198 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
199 // removeUnreachableBlocks is needed to nuke them, which means we should
200 // iterate between the two optimizations. We structure the code like this to
201 // avoid reruning iterativelySimplifyCFG if the second pass of
202 // removeUnreachableBlocks doesn't do anything.
203 if (!removeUnreachableBlocks(F))
207 EverChanged = iterativelySimplifyCFG(F, TTI, DL, AT, BonusInstThreshold);
208 EverChanged |= removeUnreachableBlocks(F);
209 } while (EverChanged);