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 Value *Ptr = SI->getOperand(1);
142 if (isa<UndefValue>(Ptr) ||
143 (isa<ConstantPointerNull>(Ptr) &&
144 SI->getPointerAddressSpace() == 0)) {
145 ChangeToUnreachable(SI, true);
152 // Turn invokes that call 'nounwind' functions into ordinary calls.
153 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
154 if (II->doesNotThrow()) {
159 Changed |= ConstantFoldTerminator(BB);
160 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
161 Worklist.push_back(*SI);
162 } while (!Worklist.empty());
166 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
167 /// if they are in a dead cycle. Return true if a change was made, false
169 static bool RemoveUnreachableBlocksFromFn(Function &F) {
170 SmallPtrSet<BasicBlock*, 128> Reachable;
171 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
173 // If there are unreachable blocks in the CFG...
174 if (Reachable.size() == F.size())
177 assert(Reachable.size() < F.size());
178 NumSimpl += F.size()-Reachable.size();
180 // Loop over all of the basic blocks that are not reachable, dropping all of
181 // their internal references...
182 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
183 if (Reachable.count(BB))
186 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
187 if (Reachable.count(*SI))
188 (*SI)->removePredecessor(BB);
189 BB->dropAllReferences();
192 for (Function::iterator I = ++F.begin(); I != F.end();)
193 if (!Reachable.count(I))
194 I = F.getBasicBlockList().erase(I);
201 /// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
202 /// node) return blocks, merge them together to promote recursive block merging.
203 static bool MergeEmptyReturnBlocks(Function &F) {
204 bool Changed = false;
206 BasicBlock *RetBlock = 0;
208 // Scan all the blocks in the function, looking for empty return blocks.
209 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
210 BasicBlock &BB = *BBI++;
212 // Only look at return blocks.
213 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
214 if (Ret == 0) continue;
216 // Only look at the block if it is empty or the only other thing in it is a
217 // single PHI node that is the operand to the return.
218 if (Ret != &BB.front()) {
219 // Check for something else in the block.
220 BasicBlock::iterator I = Ret;
222 // Skip over debug info.
223 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
225 if (!isa<DbgInfoIntrinsic>(I) &&
226 (!isa<PHINode>(I) || I != BB.begin() ||
227 Ret->getNumOperands() == 0 ||
228 Ret->getOperand(0) != I))
232 // If this is the first returning block, remember it and keep going.
238 // Otherwise, we found a duplicate return block. Merge the two.
241 // Case when there is no input to the return or when the returned values
242 // agree is trivial. Note that they can't agree if there are phis in the
244 if (Ret->getNumOperands() == 0 ||
245 Ret->getOperand(0) ==
246 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
247 BB.replaceAllUsesWith(RetBlock);
248 BB.eraseFromParent();
252 // If the canonical return block has no PHI node, create one now.
253 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
254 if (RetBlockPHI == 0) {
255 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
256 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), "merge",
259 for (pred_iterator PI = pred_begin(RetBlock), E = pred_end(RetBlock);
261 RetBlockPHI->addIncoming(InVal, *PI);
262 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
265 // Turn BB into a block that just unconditionally branches to the return
266 // block. This handles the case when the two return blocks have a common
267 // predecessor but that return different things.
268 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
269 BB.getTerminator()->eraseFromParent();
270 BranchInst::Create(RetBlock, &BB);
276 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
277 /// iterating until no more changes are made.
278 static bool IterativeSimplifyCFG(Function &F, const TargetData *TD) {
279 bool Changed = false;
280 bool LocalChange = true;
281 while (LocalChange) {
284 // Loop over all of the basic blocks (except the first one) and remove them
285 // if they are unneeded...
287 for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
288 if (SimplifyCFG(BBIt++, TD)) {
293 Changed |= LocalChange;
298 // It is possible that we may require multiple passes over the code to fully
301 bool CFGSimplifyPass::runOnFunction(Function &F) {
302 const TargetData *TD = getAnalysisIfAvailable<TargetData>();
303 bool EverChanged = RemoveUnreachableBlocksFromFn(F);
304 EverChanged |= MergeEmptyReturnBlocks(F);
305 EverChanged |= IterativeSimplifyCFG(F, TD);
307 // If neither pass changed anything, we're done.
308 if (!EverChanged) return false;
310 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
311 // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
312 // iterate between the two optimizations. We structure the code like this to
313 // avoid reruning IterativeSimplifyCFG if the second pass of
314 // RemoveUnreachableBlocksFromFn doesn't do anything.
315 if (!RemoveUnreachableBlocksFromFn(F))
319 EverChanged = IterativeSimplifyCFG(F, TD);
320 EverChanged |= RemoveUnreachableBlocksFromFn(F);
321 } while (EverChanged);