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);
103 // Update PHI nodes in the unwind destination
104 II->getUnwindDest()->removePredecessor(II->getParent());
105 II->eraseFromParent();
108 static bool markAliveBlocks(BasicBlock *BB,
109 SmallPtrSet<BasicBlock*, 128> &Reachable) {
111 SmallVector<BasicBlock*, 128> Worklist;
112 Worklist.push_back(BB);
113 bool Changed = false;
115 BB = Worklist.pop_back_val();
117 if (!Reachable.insert(BB))
120 // Do a quick scan of the basic block, turning any obviously unreachable
121 // instructions into LLVM unreachable insts. The instruction combining pass
122 // canonicalizes unreachable insts into stores to null or undef.
123 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
124 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
125 if (CI->doesNotReturn()) {
126 // If we found a call to a no-return function, insert an unreachable
127 // instruction after it. Make sure there isn't *already* one there
130 if (!isa<UnreachableInst>(BBI)) {
131 // Don't insert a call to llvm.trap right before the unreachable.
132 changeToUnreachable(BBI, false);
139 // Store to undef and store to null are undefined and used to signal that
140 // they should be changed to unreachable by passes that can't modify the
142 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
143 // Don't touch volatile stores.
144 if (SI->isVolatile()) continue;
146 Value *Ptr = SI->getOperand(1);
148 if (isa<UndefValue>(Ptr) ||
149 (isa<ConstantPointerNull>(Ptr) &&
150 SI->getPointerAddressSpace() == 0)) {
151 changeToUnreachable(SI, true);
158 // Turn invokes that call 'nounwind' functions into ordinary calls.
159 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
160 Value *Callee = II->getCalledValue();
161 if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
162 changeToUnreachable(II, true);
164 } else if (II->doesNotThrow()) {
165 if (II->use_empty() && II->onlyReadsMemory()) {
166 // jump to the normal destination branch.
167 BranchInst::Create(II->getNormalDest(), II);
168 II->getUnwindDest()->removePredecessor(II->getParent());
169 II->eraseFromParent();
176 Changed |= ConstantFoldTerminator(BB, true);
177 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
178 Worklist.push_back(*SI);
179 } while (!Worklist.empty());
183 /// removeUnreachableBlocksFromFn - Remove blocks that are not reachable, even
184 /// if they are in a dead cycle. Return true if a change was made, false
186 static bool removeUnreachableBlocksFromFn(Function &F) {
187 SmallPtrSet<BasicBlock*, 128> Reachable;
188 bool Changed = markAliveBlocks(F.begin(), Reachable);
190 // If there are unreachable blocks in the CFG...
191 if (Reachable.size() == F.size())
194 assert(Reachable.size() < F.size());
195 NumSimpl += F.size()-Reachable.size();
197 // Loop over all of the basic blocks that are not reachable, dropping all of
198 // their internal references...
199 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
200 if (Reachable.count(BB))
203 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
204 if (Reachable.count(*SI))
205 (*SI)->removePredecessor(BB);
206 BB->dropAllReferences();
209 for (Function::iterator I = ++F.begin(); I != F.end();)
210 if (!Reachable.count(I))
211 I = F.getBasicBlockList().erase(I);
218 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
219 /// node) return blocks, merge them together to promote recursive block merging.
220 static bool mergeEmptyReturnBlocks(Function &F) {
221 bool Changed = false;
223 BasicBlock *RetBlock = 0;
225 // Scan all the blocks in the function, looking for empty return blocks.
226 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
227 BasicBlock &BB = *BBI++;
229 // Only look at return blocks.
230 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
231 if (Ret == 0) continue;
233 // Only look at the block if it is empty or the only other thing in it is a
234 // single PHI node that is the operand to the return.
235 if (Ret != &BB.front()) {
236 // Check for something else in the block.
237 BasicBlock::iterator I = Ret;
239 // Skip over debug info.
240 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
242 if (!isa<DbgInfoIntrinsic>(I) &&
243 (!isa<PHINode>(I) || I != BB.begin() ||
244 Ret->getNumOperands() == 0 ||
245 Ret->getOperand(0) != I))
249 // If this is the first returning block, remember it and keep going.
255 // Otherwise, we found a duplicate return block. Merge the two.
258 // Case when there is no input to the return or when the returned values
259 // agree is trivial. Note that they can't agree if there are phis in the
261 if (Ret->getNumOperands() == 0 ||
262 Ret->getOperand(0) ==
263 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
264 BB.replaceAllUsesWith(RetBlock);
265 BB.eraseFromParent();
269 // If the canonical return block has no PHI node, create one now.
270 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
271 if (RetBlockPHI == 0) {
272 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
273 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
274 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
275 std::distance(PB, PE), "merge",
278 for (pred_iterator PI = PB; PI != PE; ++PI)
279 RetBlockPHI->addIncoming(InVal, *PI);
280 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
283 // Turn BB into a block that just unconditionally branches to the return
284 // block. This handles the case when the two return blocks have a common
285 // predecessor but that return different things.
286 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
287 BB.getTerminator()->eraseFromParent();
288 BranchInst::Create(RetBlock, &BB);
294 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
295 /// iterating until no more changes are made.
296 static bool iterativelySimplifyCFG(Function &F, const TargetData *TD) {
297 bool Changed = false;
298 bool LocalChange = true;
299 while (LocalChange) {
302 // Loop over all of the basic blocks and remove them if they are unneeded...
304 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
305 if (SimplifyCFG(BBIt++, TD)) {
310 Changed |= LocalChange;
315 // It is possible that we may require multiple passes over the code to fully
318 bool CFGSimplifyPass::runOnFunction(Function &F) {
319 const TargetData *TD = getAnalysisIfAvailable<TargetData>();
320 bool EverChanged = removeUnreachableBlocksFromFn(F);
321 EverChanged |= mergeEmptyReturnBlocks(F);
322 EverChanged |= iterativelySimplifyCFG(F, TD);
324 // If neither pass changed anything, we're done.
325 if (!EverChanged) return false;
327 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
328 // removeUnreachableBlocksFromFn is needed to nuke them, which means we should
329 // iterate between the two optimizations. We structure the code like this to
330 // avoid reruning iterativelySimplifyCFG if the second pass of
331 // removeUnreachableBlocksFromFn doesn't do anything.
332 if (!removeUnreachableBlocksFromFn(F))
336 EverChanged = iterativelySimplifyCFG(F, TD);
337 EverChanged |= removeUnreachableBlocksFromFn(F);
338 } while (EverChanged);