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 #define DEBUG_TYPE "simplifycfg"
25 #include "llvm/Transforms/Scalar.h"
26 #include "llvm/ADT/SmallPtrSet.h"
27 #include "llvm/ADT/SmallVector.h"
28 #include "llvm/ADT/Statistic.h"
29 #include "llvm/Analysis/TargetTransformInfo.h"
30 #include "llvm/IR/Attributes.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/Support/CFG.h"
38 #include "llvm/Transforms/Utils/Local.h"
41 STATISTIC(NumSimpl, "Number of blocks simplified");
44 struct CFGSimplifyPass : public FunctionPass {
45 static char ID; // Pass identification, replacement for typeid
46 CFGSimplifyPass() : FunctionPass(ID) {
47 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
50 virtual bool runOnFunction(Function &F);
52 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
53 AU.addRequired<TargetTransformInfo>();
58 char CFGSimplifyPass::ID = 0;
59 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG",
61 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
62 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG",
65 // Public interface to the CFGSimplification pass
66 FunctionPass *llvm::createCFGSimplificationPass() {
67 return new CFGSimplifyPass();
70 /// changeToUnreachable - Insert an unreachable instruction before the specified
71 /// instruction, making it and the rest of the code in the block dead.
72 static void changeToUnreachable(Instruction *I, bool UseLLVMTrap) {
73 BasicBlock *BB = I->getParent();
74 // Loop over all of the successors, removing BB's entry from any PHI
76 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
77 (*SI)->removePredecessor(BB);
79 // Insert a call to llvm.trap right before this. This turns the undefined
80 // behavior into a hard fail instead of falling through into random code.
83 Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
84 CallInst *CallTrap = CallInst::Create(TrapFn, "", I);
85 CallTrap->setDebugLoc(I->getDebugLoc());
87 new UnreachableInst(I->getContext(), I);
89 // All instructions after this are dead.
90 BasicBlock::iterator BBI = I, BBE = BB->end();
92 if (!BBI->use_empty())
93 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
94 BB->getInstList().erase(BBI++);
98 /// changeToCall - Convert the specified invoke into a normal call.
99 static void changeToCall(InvokeInst *II) {
100 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
101 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args, "", II);
102 NewCall->takeName(II);
103 NewCall->setCallingConv(II->getCallingConv());
104 NewCall->setAttributes(II->getAttributes());
105 NewCall->setDebugLoc(II->getDebugLoc());
106 II->replaceAllUsesWith(NewCall);
108 // Follow the call by a branch to the normal destination.
109 BranchInst::Create(II->getNormalDest(), II);
111 // Update PHI nodes in the unwind destination
112 II->getUnwindDest()->removePredecessor(II->getParent());
113 II->eraseFromParent();
116 static bool markAliveBlocks(BasicBlock *BB,
117 SmallPtrSet<BasicBlock*, 128> &Reachable) {
119 SmallVector<BasicBlock*, 128> Worklist;
120 Worklist.push_back(BB);
121 Reachable.insert(BB);
122 bool Changed = false;
124 BB = Worklist.pop_back_val();
126 // Do a quick scan of the basic block, turning any obviously unreachable
127 // instructions into LLVM unreachable insts. The instruction combining pass
128 // canonicalizes unreachable insts into stores to null or undef.
129 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
130 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
131 if (CI->doesNotReturn()) {
132 // If we found a call to a no-return function, insert an unreachable
133 // instruction after it. Make sure there isn't *already* one there
136 if (!isa<UnreachableInst>(BBI)) {
137 // Don't insert a call to llvm.trap right before the unreachable.
138 changeToUnreachable(BBI, false);
145 // Store to undef and store to null are undefined and used to signal that
146 // they should be changed to unreachable by passes that can't modify the
148 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
149 // Don't touch volatile stores.
150 if (SI->isVolatile()) continue;
152 Value *Ptr = SI->getOperand(1);
154 if (isa<UndefValue>(Ptr) ||
155 (isa<ConstantPointerNull>(Ptr) &&
156 SI->getPointerAddressSpace() == 0)) {
157 changeToUnreachable(SI, true);
164 // Turn invokes that call 'nounwind' functions into ordinary calls.
165 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
166 Value *Callee = II->getCalledValue();
167 if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
168 changeToUnreachable(II, true);
170 } else if (II->doesNotThrow()) {
171 if (II->use_empty() && II->onlyReadsMemory()) {
172 // jump to the normal destination branch.
173 BranchInst::Create(II->getNormalDest(), II);
174 II->getUnwindDest()->removePredecessor(II->getParent());
175 II->eraseFromParent();
182 Changed |= ConstantFoldTerminator(BB, true);
183 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
184 if (Reachable.insert(*SI))
185 Worklist.push_back(*SI);
186 } while (!Worklist.empty());
190 /// removeUnreachableBlocksFromFn - Remove blocks that are not reachable, even
191 /// if they are in a dead cycle. Return true if a change was made, false
193 static bool removeUnreachableBlocksFromFn(Function &F) {
194 SmallPtrSet<BasicBlock*, 128> Reachable;
195 bool Changed = markAliveBlocks(F.begin(), Reachable);
197 // If there are unreachable blocks in the CFG...
198 if (Reachable.size() == F.size())
201 assert(Reachable.size() < F.size());
202 NumSimpl += F.size()-Reachable.size();
204 // Loop over all of the basic blocks that are not reachable, dropping all of
205 // their internal references...
206 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
207 if (Reachable.count(BB))
210 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
211 if (Reachable.count(*SI))
212 (*SI)->removePredecessor(BB);
213 BB->dropAllReferences();
216 for (Function::iterator I = ++F.begin(); I != F.end();)
217 if (!Reachable.count(I))
218 I = F.getBasicBlockList().erase(I);
225 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
226 /// node) return blocks, merge them together to promote recursive block merging.
227 static bool mergeEmptyReturnBlocks(Function &F) {
228 bool Changed = false;
230 BasicBlock *RetBlock = 0;
232 // Scan all the blocks in the function, looking for empty return blocks.
233 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
234 BasicBlock &BB = *BBI++;
236 // Only look at return blocks.
237 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
238 if (Ret == 0) continue;
240 // Only look at the block if it is empty or the only other thing in it is a
241 // single PHI node that is the operand to the return.
242 if (Ret != &BB.front()) {
243 // Check for something else in the block.
244 BasicBlock::iterator I = Ret;
246 // Skip over debug info.
247 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
249 if (!isa<DbgInfoIntrinsic>(I) &&
250 (!isa<PHINode>(I) || I != BB.begin() ||
251 Ret->getNumOperands() == 0 ||
252 Ret->getOperand(0) != I))
256 // If this is the first returning block, remember it and keep going.
262 // Otherwise, we found a duplicate return block. Merge the two.
265 // Case when there is no input to the return or when the returned values
266 // agree is trivial. Note that they can't agree if there are phis in the
268 if (Ret->getNumOperands() == 0 ||
269 Ret->getOperand(0) ==
270 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
271 BB.replaceAllUsesWith(RetBlock);
272 BB.eraseFromParent();
276 // If the canonical return block has no PHI node, create one now.
277 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
278 if (RetBlockPHI == 0) {
279 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
280 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
281 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
282 std::distance(PB, PE), "merge",
285 for (pred_iterator PI = PB; PI != PE; ++PI)
286 RetBlockPHI->addIncoming(InVal, *PI);
287 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
290 // Turn BB into a block that just unconditionally branches to the return
291 // block. This handles the case when the two return blocks have a common
292 // predecessor but that return different things.
293 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
294 BB.getTerminator()->eraseFromParent();
295 BranchInst::Create(RetBlock, &BB);
301 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
302 /// iterating until no more changes are made.
303 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
304 const DataLayout *TD) {
305 bool Changed = false;
306 bool LocalChange = true;
307 while (LocalChange) {
310 // Loop over all of the basic blocks and remove them if they are unneeded...
312 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
313 if (SimplifyCFG(BBIt++, TTI, TD)) {
318 Changed |= LocalChange;
323 // It is possible that we may require multiple passes over the code to fully
326 bool CFGSimplifyPass::runOnFunction(Function &F) {
327 const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
328 const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
329 bool EverChanged = removeUnreachableBlocksFromFn(F);
330 EverChanged |= mergeEmptyReturnBlocks(F);
331 EverChanged |= iterativelySimplifyCFG(F, TTI, TD);
333 // If neither pass changed anything, we're done.
334 if (!EverChanged) return false;
336 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
337 // removeUnreachableBlocksFromFn is needed to nuke them, which means we should
338 // iterate between the two optimizations. We structure the code like this to
339 // avoid reruning iterativelySimplifyCFG if the second pass of
340 // removeUnreachableBlocksFromFn doesn't do anything.
341 if (!removeUnreachableBlocksFromFn(F))
345 EverChanged = iterativelySimplifyCFG(F, TTI, TD);
346 EverChanged |= removeUnreachableBlocksFromFn(F);
347 } while (EverChanged);