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) {
46 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
49 virtual bool runOnFunction(Function &F);
53 char CFGSimplifyPass::ID = 0;
54 INITIALIZE_PASS(CFGSimplifyPass, "simplifycfg",
55 "Simplify the CFG", false, false)
57 // Public interface to the CFGSimplification pass
58 FunctionPass *llvm::createCFGSimplificationPass() {
59 return new CFGSimplifyPass();
62 /// ChangeToUnreachable - Insert an unreachable instruction before the specified
63 /// instruction, making it and the rest of the code in the block dead.
64 static void ChangeToUnreachable(Instruction *I, bool UseLLVMTrap) {
65 BasicBlock *BB = I->getParent();
66 // Loop over all of the successors, removing BB's entry from any PHI
68 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
69 (*SI)->removePredecessor(BB);
71 // Insert a call to llvm.trap right before this. This turns the undefined
72 // behavior into a hard fail instead of falling through into random code.
75 Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
76 CallInst *CallTrap = CallInst::Create(TrapFn, "", I);
77 CallTrap->setDebugLoc(I->getDebugLoc());
79 new UnreachableInst(I->getContext(), I);
81 // All instructions after this are dead.
82 BasicBlock::iterator BBI = I, BBE = BB->end();
84 if (!BBI->use_empty())
85 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
86 BB->getInstList().erase(BBI++);
90 /// ChangeToCall - Convert the specified invoke into a normal call.
91 static void ChangeToCall(InvokeInst *II) {
92 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
93 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args, "", II);
94 NewCall->takeName(II);
95 NewCall->setCallingConv(II->getCallingConv());
96 NewCall->setAttributes(II->getAttributes());
97 NewCall->setDebugLoc(II->getDebugLoc());
98 II->replaceAllUsesWith(NewCall);
100 // Follow the call by a branch to the normal destination.
101 BranchInst::Create(II->getNormalDest(), II);
102 II->eraseFromParent();
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 Value *Callee = II->getCalledValue();
158 if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
159 ChangeToUnreachable(II, true);
161 } else if (II->doesNotThrow()) {
162 if (II->use_empty() && II->onlyReadsMemory()) {
163 // jump to the normal destination branch.
164 BranchInst::Create(II->getNormalDest(), II);
165 II->eraseFromParent();
172 Changed |= ConstantFoldTerminator(BB, true);
173 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
174 Worklist.push_back(*SI);
175 } while (!Worklist.empty());
179 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
180 /// if they are in a dead cycle. Return true if a change was made, false
182 static bool RemoveUnreachableBlocksFromFn(Function &F) {
183 SmallPtrSet<BasicBlock*, 128> Reachable;
184 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
186 // If there are unreachable blocks in the CFG...
187 if (Reachable.size() == F.size())
190 assert(Reachable.size() < F.size());
191 NumSimpl += F.size()-Reachable.size();
193 // Loop over all of the basic blocks that are not reachable, dropping all of
194 // their internal references...
195 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
196 if (Reachable.count(BB))
199 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
200 if (Reachable.count(*SI))
201 (*SI)->removePredecessor(BB);
202 BB->dropAllReferences();
205 for (Function::iterator I = ++F.begin(); I != F.end();)
206 if (!Reachable.count(I))
207 I = F.getBasicBlockList().erase(I);
214 /// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
215 /// node) return blocks, merge them together to promote recursive block merging.
216 static bool MergeEmptyReturnBlocks(Function &F) {
217 bool Changed = false;
219 BasicBlock *RetBlock = 0;
221 // Scan all the blocks in the function, looking for empty return blocks.
222 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
223 BasicBlock &BB = *BBI++;
225 // Only look at return blocks.
226 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
227 if (Ret == 0) continue;
229 // Only look at the block if it is empty or the only other thing in it is a
230 // single PHI node that is the operand to the return.
231 if (Ret != &BB.front()) {
232 // Check for something else in the block.
233 BasicBlock::iterator I = Ret;
235 // Skip over debug info.
236 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
238 if (!isa<DbgInfoIntrinsic>(I) &&
239 (!isa<PHINode>(I) || I != BB.begin() ||
240 Ret->getNumOperands() == 0 ||
241 Ret->getOperand(0) != I))
245 // If this is the first returning block, remember it and keep going.
251 // Otherwise, we found a duplicate return block. Merge the two.
254 // Case when there is no input to the return or when the returned values
255 // agree is trivial. Note that they can't agree if there are phis in the
257 if (Ret->getNumOperands() == 0 ||
258 Ret->getOperand(0) ==
259 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
260 BB.replaceAllUsesWith(RetBlock);
261 BB.eraseFromParent();
265 // If the canonical return block has no PHI node, create one now.
266 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
267 if (RetBlockPHI == 0) {
268 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
269 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
270 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
271 std::distance(PB, PE), "merge",
274 for (pred_iterator PI = PB; PI != PE; ++PI)
275 RetBlockPHI->addIncoming(InVal, *PI);
276 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
279 // Turn BB into a block that just unconditionally branches to the return
280 // block. This handles the case when the two return blocks have a common
281 // predecessor but that return different things.
282 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
283 BB.getTerminator()->eraseFromParent();
284 BranchInst::Create(RetBlock, &BB);
290 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
291 /// iterating until no more changes are made.
292 static bool IterativeSimplifyCFG(Function &F, const TargetData *TD) {
293 bool Changed = false;
294 bool LocalChange = true;
295 while (LocalChange) {
298 // Loop over all of the basic blocks and remove them if they are unneeded...
300 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
301 if (SimplifyCFG(BBIt++, TD)) {
306 Changed |= LocalChange;
311 // It is possible that we may require multiple passes over the code to fully
314 bool CFGSimplifyPass::runOnFunction(Function &F) {
315 const TargetData *TD = getAnalysisIfAvailable<TargetData>();
316 bool EverChanged = RemoveUnreachableBlocksFromFn(F);
317 EverChanged |= MergeEmptyReturnBlocks(F);
318 EverChanged |= IterativeSimplifyCFG(F, TD);
320 // If neither pass changed anything, we're done.
321 if (!EverChanged) return false;
323 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
324 // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
325 // iterate between the two optimizations. We structure the code like this to
326 // avoid reruning IterativeSimplifyCFG if the second pass of
327 // RemoveUnreachableBlocksFromFn doesn't do anything.
328 if (!RemoveUnreachableBlocksFromFn(F))
332 EverChanged = IterativeSimplifyCFG(F, TD);
333 EverChanged |= RemoveUnreachableBlocksFromFn(F);
334 } while (EverChanged);