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::Create(TrapFn, "", I);
78 new UnreachableInst(I->getContext(), I);
80 // All instructions after this are dead.
81 BasicBlock::iterator BBI = I, BBE = BB->end();
83 if (!BBI->use_empty())
84 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
85 BB->getInstList().erase(BBI++);
89 /// ChangeToCall - Convert the specified invoke into a normal call.
90 static void ChangeToCall(InvokeInst *II) {
91 BasicBlock *BB = II->getParent();
92 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
93 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
95 NewCall->takeName(II);
96 NewCall->setCallingConv(II->getCallingConv());
97 NewCall->setAttributes(II->getAttributes());
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(BB);
105 BB->getInstList().erase(II);
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 if (II->doesNotThrow()) {
165 Changed |= ConstantFoldTerminator(BB);
166 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
167 Worklist.push_back(*SI);
168 } while (!Worklist.empty());
172 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
173 /// if they are in a dead cycle. Return true if a change was made, false
175 static bool RemoveUnreachableBlocksFromFn(Function &F) {
176 SmallPtrSet<BasicBlock*, 128> Reachable;
177 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
179 // If there are unreachable blocks in the CFG...
180 if (Reachable.size() == F.size())
183 assert(Reachable.size() < F.size());
184 NumSimpl += F.size()-Reachable.size();
186 // Loop over all of the basic blocks that are not reachable, dropping all of
187 // their internal references...
188 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
189 if (Reachable.count(BB))
192 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
193 if (Reachable.count(*SI))
194 (*SI)->removePredecessor(BB);
195 BB->dropAllReferences();
198 for (Function::iterator I = ++F.begin(); I != F.end();)
199 if (!Reachable.count(I))
200 I = F.getBasicBlockList().erase(I);
207 /// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
208 /// node) return blocks, merge them together to promote recursive block merging.
209 static bool MergeEmptyReturnBlocks(Function &F) {
210 bool Changed = false;
212 BasicBlock *RetBlock = 0;
214 // Scan all the blocks in the function, looking for empty return blocks.
215 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
216 BasicBlock &BB = *BBI++;
218 // Only look at return blocks.
219 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
220 if (Ret == 0) continue;
222 // Only look at the block if it is empty or the only other thing in it is a
223 // single PHI node that is the operand to the return.
224 if (Ret != &BB.front()) {
225 // Check for something else in the block.
226 BasicBlock::iterator I = Ret;
228 // Skip over debug info.
229 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
231 if (!isa<DbgInfoIntrinsic>(I) &&
232 (!isa<PHINode>(I) || I != BB.begin() ||
233 Ret->getNumOperands() == 0 ||
234 Ret->getOperand(0) != I))
238 // If this is the first returning block, remember it and keep going.
244 // Otherwise, we found a duplicate return block. Merge the two.
247 // Case when there is no input to the return or when the returned values
248 // agree is trivial. Note that they can't agree if there are phis in the
250 if (Ret->getNumOperands() == 0 ||
251 Ret->getOperand(0) ==
252 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
253 BB.replaceAllUsesWith(RetBlock);
254 BB.eraseFromParent();
258 // If the canonical return block has no PHI node, create one now.
259 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
260 if (RetBlockPHI == 0) {
261 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
262 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), "merge",
265 for (pred_iterator PI = pred_begin(RetBlock), E = pred_end(RetBlock);
267 RetBlockPHI->addIncoming(InVal, *PI);
268 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
271 // Turn BB into a block that just unconditionally branches to the return
272 // block. This handles the case when the two return blocks have a common
273 // predecessor but that return different things.
274 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
275 BB.getTerminator()->eraseFromParent();
276 BranchInst::Create(RetBlock, &BB);
282 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
283 /// iterating until no more changes are made.
284 static bool IterativeSimplifyCFG(Function &F, const TargetData *TD) {
285 bool Changed = false;
286 bool LocalChange = true;
287 while (LocalChange) {
290 // Loop over all of the basic blocks and remove them if they are unneeded...
292 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
293 if (SimplifyCFG(BBIt++, TD)) {
298 Changed |= LocalChange;
303 // It is possible that we may require multiple passes over the code to fully
306 bool CFGSimplifyPass::runOnFunction(Function &F) {
307 const TargetData *TD = getAnalysisIfAvailable<TargetData>();
308 bool EverChanged = RemoveUnreachableBlocksFromFn(F);
309 EverChanged |= MergeEmptyReturnBlocks(F);
310 EverChanged |= IterativeSimplifyCFG(F, TD);
312 // If neither pass changed anything, we're done.
313 if (!EverChanged) return false;
315 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
316 // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
317 // iterate between the two optimizations. We structure the code like this to
318 // avoid reruning IterativeSimplifyCFG if the second pass of
319 // RemoveUnreachableBlocksFromFn doesn't do anything.
320 if (!RemoveUnreachableBlocksFromFn(F))
324 EverChanged = IterativeSimplifyCFG(F, TD);
325 EverChanged |= RemoveUnreachableBlocksFromFn(F);
326 } while (EverChanged);