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/Transforms/Utils/Local.h"
27 #include "llvm/Constants.h"
28 #include "llvm/Instructions.h"
29 #include "llvm/IntrinsicInst.h"
30 #include "llvm/Module.h"
31 #include "llvm/Attributes.h"
32 #include "llvm/Support/CFG.h"
33 #include "llvm/Pass.h"
34 #include "llvm/Target/TargetData.h"
35 #include "llvm/ADT/SmallVector.h"
36 #include "llvm/ADT/SmallPtrSet.h"
37 #include "llvm/ADT/Statistic.h"
40 STATISTIC(NumSimpl, "Number of blocks simplified");
43 struct CFGSimplifyPass : public FunctionPass {
44 static char ID; // Pass identification, replacement for typeid
45 CFGSimplifyPass() : FunctionPass(&ID) {}
47 virtual bool runOnFunction(Function &F);
51 char CFGSimplifyPass::ID = 0;
52 static RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
54 // Public interface to the CFGSimplification pass
55 FunctionPass *llvm::createCFGSimplificationPass() {
56 return new CFGSimplifyPass();
59 /// ChangeToUnreachable - Insert an unreachable instruction before the specified
60 /// instruction, making it and the rest of the code in the block dead.
61 static void ChangeToUnreachable(Instruction *I, bool UseLLVMTrap) {
62 BasicBlock *BB = I->getParent();
63 // Loop over all of the successors, removing BB's entry from any PHI
65 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
66 (*SI)->removePredecessor(BB);
68 // Insert a call to llvm.trap right before this. This turns the undefined
69 // behavior into a hard fail instead of falling through into random code.
72 Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
73 CallInst::Create(TrapFn, "", I);
75 new UnreachableInst(I->getContext(), I);
77 // All instructions after this are dead.
78 BasicBlock::iterator BBI = I, BBE = BB->end();
80 if (!BBI->use_empty())
81 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
82 BB->getInstList().erase(BBI++);
86 /// ChangeToCall - Convert the specified invoke into a normal call.
87 static void ChangeToCall(InvokeInst *II) {
88 BasicBlock *BB = II->getParent();
89 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
90 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
92 NewCall->takeName(II);
93 NewCall->setCallingConv(II->getCallingConv());
94 NewCall->setAttributes(II->getAttributes());
95 II->replaceAllUsesWith(NewCall);
97 // Follow the call by a branch to the normal destination.
98 BranchInst::Create(II->getNormalDest(), II);
100 // Update PHI nodes in the unwind destination
101 II->getUnwindDest()->removePredecessor(BB);
102 BB->getInstList().erase(II);
105 static bool MarkAliveBlocks(BasicBlock *BB,
106 SmallPtrSet<BasicBlock*, 128> &Reachable) {
108 SmallVector<BasicBlock*, 128> Worklist;
109 Worklist.push_back(BB);
110 bool Changed = false;
112 BB = Worklist.pop_back_val();
114 if (!Reachable.insert(BB))
117 // Do a quick scan of the basic block, turning any obviously unreachable
118 // instructions into LLVM unreachable insts. The instruction combining pass
119 // canonicalizes unreachable insts into stores to null or undef.
120 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
121 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
122 if (CI->doesNotReturn()) {
123 // If we found a call to a no-return function, insert an unreachable
124 // instruction after it. Make sure there isn't *already* one there
127 if (!isa<UnreachableInst>(BBI)) {
128 // Don't insert a call to llvm.trap right before the unreachable.
129 ChangeToUnreachable(BBI, false);
136 // Store to undef and store to null are undefined and used to signal that
137 // they should be changed to unreachable by passes that can't modify the
139 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
140 // Don't touch volatile stores.
141 if (SI->isVolatile()) continue;
143 Value *Ptr = SI->getOperand(1);
145 if (isa<UndefValue>(Ptr) ||
146 (isa<ConstantPointerNull>(Ptr) &&
147 SI->getPointerAddressSpace() == 0)) {
148 ChangeToUnreachable(SI, true);
155 // Turn invokes that call 'nounwind' functions into ordinary calls.
156 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
157 if (II->doesNotThrow()) {
162 Changed |= ConstantFoldTerminator(BB);
163 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
164 Worklist.push_back(*SI);
165 } while (!Worklist.empty());
169 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
170 /// if they are in a dead cycle. Return true if a change was made, false
172 static bool RemoveUnreachableBlocksFromFn(Function &F) {
173 SmallPtrSet<BasicBlock*, 128> Reachable;
174 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
176 // If there are unreachable blocks in the CFG...
177 if (Reachable.size() == F.size())
180 assert(Reachable.size() < F.size());
181 NumSimpl += F.size()-Reachable.size();
183 // Loop over all of the basic blocks that are not reachable, dropping all of
184 // their internal references...
185 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
186 if (Reachable.count(BB))
189 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
190 if (Reachable.count(*SI))
191 (*SI)->removePredecessor(BB);
192 BB->dropAllReferences();
195 for (Function::iterator I = ++F.begin(); I != F.end();)
196 if (!Reachable.count(I))
197 I = F.getBasicBlockList().erase(I);
204 /// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
205 /// node) return blocks, merge them together to promote recursive block merging.
206 static bool MergeEmptyReturnBlocks(Function &F) {
207 bool Changed = false;
209 BasicBlock *RetBlock = 0;
211 // Scan all the blocks in the function, looking for empty return blocks.
212 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
213 BasicBlock &BB = *BBI++;
215 // Only look at return blocks.
216 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
217 if (Ret == 0) continue;
219 // Only look at the block if it is empty or the only other thing in it is a
220 // single PHI node that is the operand to the return.
221 if (Ret != &BB.front()) {
222 // Check for something else in the block.
223 BasicBlock::iterator I = Ret;
225 // Skip over debug info.
226 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
228 if (!isa<DbgInfoIntrinsic>(I) &&
229 (!isa<PHINode>(I) || I != BB.begin() ||
230 Ret->getNumOperands() == 0 ||
231 Ret->getOperand(0) != I))
235 // If this is the first returning block, remember it and keep going.
241 // Otherwise, we found a duplicate return block. Merge the two.
244 // Case when there is no input to the return or when the returned values
245 // agree is trivial. Note that they can't agree if there are phis in the
247 if (Ret->getNumOperands() == 0 ||
248 Ret->getOperand(0) ==
249 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
250 BB.replaceAllUsesWith(RetBlock);
251 BB.eraseFromParent();
255 // If the canonical return block has no PHI node, create one now.
256 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
257 if (RetBlockPHI == 0) {
258 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
259 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), "merge",
262 for (pred_iterator PI = pred_begin(RetBlock), E = pred_end(RetBlock);
264 RetBlockPHI->addIncoming(InVal, *PI);
265 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
268 // Turn BB into a block that just unconditionally branches to the return
269 // block. This handles the case when the two return blocks have a common
270 // predecessor but that return different things.
271 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
272 BB.getTerminator()->eraseFromParent();
273 BranchInst::Create(RetBlock, &BB);
279 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
280 /// iterating until no more changes are made.
281 static bool IterativeSimplifyCFG(Function &F, const TargetData *TD) {
282 bool Changed = false;
283 bool LocalChange = true;
284 while (LocalChange) {
287 // Loop over all of the basic blocks (except the first one) and remove them
288 // if they are unneeded...
290 for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
291 if (SimplifyCFG(BBIt++, TD)) {
296 Changed |= LocalChange;
301 // It is possible that we may require multiple passes over the code to fully
304 bool CFGSimplifyPass::runOnFunction(Function &F) {
305 const TargetData *TD = getAnalysisIfAvailable<TargetData>();
306 bool EverChanged = RemoveUnreachableBlocksFromFn(F);
307 EverChanged |= MergeEmptyReturnBlocks(F);
308 EverChanged |= IterativeSimplifyCFG(F, TD);
310 // If neither pass changed anything, we're done.
311 if (!EverChanged) return false;
313 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
314 // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
315 // iterate between the two optimizations. We structure the code like this to
316 // avoid reruning IterativeSimplifyCFG if the second pass of
317 // RemoveUnreachableBlocksFromFn doesn't do anything.
318 if (!RemoveUnreachableBlocksFromFn(F))
322 EverChanged = IterativeSimplifyCFG(F, TD);
323 EverChanged |= RemoveUnreachableBlocksFromFn(F);
324 } while (EverChanged);