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/Attributes.h"
30 #include "llvm/Constants.h"
31 #include "llvm/DataLayout.h"
32 #include "llvm/Instructions.h"
33 #include "llvm/IntrinsicInst.h"
34 #include "llvm/Module.h"
35 #include "llvm/Pass.h"
36 #include "llvm/Support/CFG.h"
37 #include "llvm/TargetTransformInfo.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);
54 char CFGSimplifyPass::ID = 0;
55 INITIALIZE_PASS(CFGSimplifyPass, "simplifycfg",
56 "Simplify the CFG", false, false)
58 // Public interface to the CFGSimplification pass
59 FunctionPass *llvm::createCFGSimplificationPass() {
60 return new CFGSimplifyPass();
63 /// changeToUnreachable - Insert an unreachable instruction before the specified
64 /// instruction, making it and the rest of the code in the block dead.
65 static void changeToUnreachable(Instruction *I, bool UseLLVMTrap) {
66 BasicBlock *BB = I->getParent();
67 // Loop over all of the successors, removing BB's entry from any PHI
69 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
70 (*SI)->removePredecessor(BB);
72 // Insert a call to llvm.trap right before this. This turns the undefined
73 // behavior into a hard fail instead of falling through into random code.
76 Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
77 CallInst *CallTrap = CallInst::Create(TrapFn, "", I);
78 CallTrap->setDebugLoc(I->getDebugLoc());
80 new UnreachableInst(I->getContext(), I);
82 // All instructions after this are dead.
83 BasicBlock::iterator BBI = I, BBE = BB->end();
85 if (!BBI->use_empty())
86 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
87 BB->getInstList().erase(BBI++);
91 /// changeToCall - Convert the specified invoke into a normal call.
92 static void changeToCall(InvokeInst *II) {
93 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
94 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args, "", II);
95 NewCall->takeName(II);
96 NewCall->setCallingConv(II->getCallingConv());
97 NewCall->setAttributes(II->getAttributes());
98 NewCall->setDebugLoc(II->getDebugLoc());
99 II->replaceAllUsesWith(NewCall);
101 // Follow the call by a branch to the normal destination.
102 BranchInst::Create(II->getNormalDest(), II);
104 // Update PHI nodes in the unwind destination
105 II->getUnwindDest()->removePredecessor(II->getParent());
106 II->eraseFromParent();
109 static bool markAliveBlocks(BasicBlock *BB,
110 SmallPtrSet<BasicBlock*, 128> &Reachable) {
112 SmallVector<BasicBlock*, 128> Worklist;
113 Worklist.push_back(BB);
114 bool Changed = false;
116 BB = Worklist.pop_back_val();
118 if (!Reachable.insert(BB))
121 // Do a quick scan of the basic block, turning any obviously unreachable
122 // instructions into LLVM unreachable insts. The instruction combining pass
123 // canonicalizes unreachable insts into stores to null or undef.
124 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
125 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
126 if (CI->doesNotReturn()) {
127 // If we found a call to a no-return function, insert an unreachable
128 // instruction after it. Make sure there isn't *already* one there
131 if (!isa<UnreachableInst>(BBI)) {
132 // Don't insert a call to llvm.trap right before the unreachable.
133 changeToUnreachable(BBI, false);
140 // Store to undef and store to null are undefined and used to signal that
141 // they should be changed to unreachable by passes that can't modify the
143 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
144 // Don't touch volatile stores.
145 if (SI->isVolatile()) continue;
147 Value *Ptr = SI->getOperand(1);
149 if (isa<UndefValue>(Ptr) ||
150 (isa<ConstantPointerNull>(Ptr) &&
151 SI->getPointerAddressSpace() == 0)) {
152 changeToUnreachable(SI, true);
159 // Turn invokes that call 'nounwind' functions into ordinary calls.
160 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
161 Value *Callee = II->getCalledValue();
162 if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
163 changeToUnreachable(II, true);
165 } else if (II->doesNotThrow()) {
166 if (II->use_empty() && II->onlyReadsMemory()) {
167 // jump to the normal destination branch.
168 BranchInst::Create(II->getNormalDest(), II);
169 II->getUnwindDest()->removePredecessor(II->getParent());
170 II->eraseFromParent();
177 Changed |= ConstantFoldTerminator(BB, true);
178 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
179 Worklist.push_back(*SI);
180 } while (!Worklist.empty());
184 /// removeUnreachableBlocksFromFn - Remove blocks that are not reachable, even
185 /// if they are in a dead cycle. Return true if a change was made, false
187 static bool removeUnreachableBlocksFromFn(Function &F) {
188 SmallPtrSet<BasicBlock*, 128> Reachable;
189 bool Changed = markAliveBlocks(F.begin(), Reachable);
191 // If there are unreachable blocks in the CFG...
192 if (Reachable.size() == F.size())
195 assert(Reachable.size() < F.size());
196 NumSimpl += F.size()-Reachable.size();
198 // Loop over all of the basic blocks that are not reachable, dropping all of
199 // their internal references...
200 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
201 if (Reachable.count(BB))
204 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
205 if (Reachable.count(*SI))
206 (*SI)->removePredecessor(BB);
207 BB->dropAllReferences();
210 for (Function::iterator I = ++F.begin(); I != F.end();)
211 if (!Reachable.count(I))
212 I = F.getBasicBlockList().erase(I);
219 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
220 /// node) return blocks, merge them together to promote recursive block merging.
221 static bool mergeEmptyReturnBlocks(Function &F) {
222 bool Changed = false;
224 BasicBlock *RetBlock = 0;
226 // Scan all the blocks in the function, looking for empty return blocks.
227 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
228 BasicBlock &BB = *BBI++;
230 // Only look at return blocks.
231 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
232 if (Ret == 0) continue;
234 // Only look at the block if it is empty or the only other thing in it is a
235 // single PHI node that is the operand to the return.
236 if (Ret != &BB.front()) {
237 // Check for something else in the block.
238 BasicBlock::iterator I = Ret;
240 // Skip over debug info.
241 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
243 if (!isa<DbgInfoIntrinsic>(I) &&
244 (!isa<PHINode>(I) || I != BB.begin() ||
245 Ret->getNumOperands() == 0 ||
246 Ret->getOperand(0) != I))
250 // If this is the first returning block, remember it and keep going.
256 // Otherwise, we found a duplicate return block. Merge the two.
259 // Case when there is no input to the return or when the returned values
260 // agree is trivial. Note that they can't agree if there are phis in the
262 if (Ret->getNumOperands() == 0 ||
263 Ret->getOperand(0) ==
264 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
265 BB.replaceAllUsesWith(RetBlock);
266 BB.eraseFromParent();
270 // If the canonical return block has no PHI node, create one now.
271 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
272 if (RetBlockPHI == 0) {
273 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
274 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
275 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
276 std::distance(PB, PE), "merge",
279 for (pred_iterator PI = PB; PI != PE; ++PI)
280 RetBlockPHI->addIncoming(InVal, *PI);
281 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
284 // Turn BB into a block that just unconditionally branches to the return
285 // block. This handles the case when the two return blocks have a common
286 // predecessor but that return different things.
287 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
288 BB.getTerminator()->eraseFromParent();
289 BranchInst::Create(RetBlock, &BB);
295 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
296 /// iterating until no more changes are made.
297 static bool iterativelySimplifyCFG(Function &F, const DataLayout *TD,
298 const TargetTransformInfo *TTI) {
299 bool Changed = false;
300 bool LocalChange = true;
301 while (LocalChange) {
304 // Loop over all of the basic blocks and remove them if they are unneeded...
306 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
307 if (SimplifyCFG(BBIt++, TD, TTI)) {
312 Changed |= LocalChange;
317 // It is possible that we may require multiple passes over the code to fully
320 bool CFGSimplifyPass::runOnFunction(Function &F) {
321 const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
322 const TargetTransformInfo *TTI =
323 getAnalysisIfAvailable<TargetTransformInfo>();
324 bool EverChanged = removeUnreachableBlocksFromFn(F);
325 EverChanged |= mergeEmptyReturnBlocks(F);
326 EverChanged |= iterativelySimplifyCFG(F, TD, TTI);
328 // If neither pass changed anything, we're done.
329 if (!EverChanged) return false;
331 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
332 // removeUnreachableBlocksFromFn is needed to nuke them, which means we should
333 // iterate between the two optimizations. We structure the code like this to
334 // avoid reruning iterativelySimplifyCFG if the second pass of
335 // removeUnreachableBlocksFromFn doesn't do anything.
336 if (!removeUnreachableBlocksFromFn(F))
340 EverChanged = iterativelySimplifyCFG(F, TD, TTI);
341 EverChanged |= removeUnreachableBlocksFromFn(F);
342 } while (EverChanged);