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/Module.h"
30 #include "llvm/Attributes.h"
31 #include "llvm/Support/CFG.h"
32 #include "llvm/Pass.h"
33 #include "llvm/ADT/SmallVector.h"
34 #include "llvm/ADT/SmallPtrSet.h"
35 #include "llvm/ADT/Statistic.h"
38 STATISTIC(NumSimpl, "Number of blocks simplified");
41 struct CFGSimplifyPass : public FunctionPass {
42 static char ID; // Pass identification, replacement for typeid
43 CFGSimplifyPass() : FunctionPass(&ID) {}
45 virtual bool runOnFunction(Function &F);
49 char CFGSimplifyPass::ID = 0;
50 static RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
52 // Public interface to the CFGSimplification pass
53 FunctionPass *llvm::createCFGSimplificationPass() {
54 return new CFGSimplifyPass();
57 /// ChangeToUnreachable - Insert an unreachable instruction before the specified
58 /// instruction, making it and the rest of the code in the block dead.
59 static void ChangeToUnreachable(Instruction *I) {
60 BasicBlock *BB = I->getParent();
61 // Loop over all of the successors, removing BB's entry from any PHI
63 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
64 (*SI)->removePredecessor(BB);
66 new UnreachableInst(I->getContext(), I);
68 // All instructions after this are dead.
69 BasicBlock::iterator BBI = I, BBE = BB->end();
71 if (!BBI->use_empty())
72 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
73 BB->getInstList().erase(BBI++);
77 /// ChangeToCall - Convert the specified invoke into a normal call.
78 static void ChangeToCall(InvokeInst *II) {
79 BasicBlock *BB = II->getParent();
80 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
81 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
83 NewCall->takeName(II);
84 NewCall->setCallingConv(II->getCallingConv());
85 NewCall->setAttributes(II->getAttributes());
86 II->replaceAllUsesWith(NewCall);
88 // Follow the call by a branch to the normal destination.
89 BranchInst::Create(II->getNormalDest(), II);
91 // Update PHI nodes in the unwind destination
92 II->getUnwindDest()->removePredecessor(BB);
93 BB->getInstList().erase(II);
96 static bool MarkAliveBlocks(BasicBlock *BB,
97 SmallPtrSet<BasicBlock*, 128> &Reachable) {
99 SmallVector<BasicBlock*, 128> Worklist;
100 Worklist.push_back(BB);
101 bool Changed = false;
103 BB = Worklist.pop_back_val();
105 if (!Reachable.insert(BB))
108 // Do a quick scan of the basic block, turning any obviously unreachable
109 // instructions into LLVM unreachable insts. The instruction combining pass
110 // canonicalizes unreachable insts into stores to null or undef.
111 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
112 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
113 if (CI->doesNotReturn()) {
114 // If we found a call to a no-return function, insert an unreachable
115 // instruction after it. Make sure there isn't *already* one there
118 if (!isa<UnreachableInst>(BBI)) {
119 ChangeToUnreachable(BBI);
126 // Store to undef and store to null are undefined and used to signal that
127 // they should be changed to unreachable by passes that can't modify the
129 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
130 Value *Ptr = SI->getOperand(1);
132 if (isa<UndefValue>(Ptr) ||
133 (isa<ConstantPointerNull>(Ptr) &&
134 SI->getPointerAddressSpace() == 0)) {
135 ChangeToUnreachable(SI);
142 // Turn invokes that call 'nounwind' functions into ordinary calls.
143 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
144 if (II->doesNotThrow()) {
149 Changed |= ConstantFoldTerminator(BB);
150 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
151 Worklist.push_back(*SI);
152 } while (!Worklist.empty());
156 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
157 /// if they are in a dead cycle. Return true if a change was made, false
159 static bool RemoveUnreachableBlocksFromFn(Function &F) {
160 SmallPtrSet<BasicBlock*, 128> Reachable;
161 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
163 // If there are unreachable blocks in the CFG...
164 if (Reachable.size() == F.size())
167 assert(Reachable.size() < F.size());
168 NumSimpl += F.size()-Reachable.size();
170 // Loop over all of the basic blocks that are not reachable, dropping all of
171 // their internal references...
172 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
173 if (Reachable.count(BB))
176 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
177 if (Reachable.count(*SI))
178 (*SI)->removePredecessor(BB);
179 BB->dropAllReferences();
182 for (Function::iterator I = ++F.begin(); I != F.end();)
183 if (!Reachable.count(I))
184 I = F.getBasicBlockList().erase(I);
191 /// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
192 /// node) return blocks, merge them together to promote recursive block merging.
193 static bool MergeEmptyReturnBlocks(Function &F) {
194 bool Changed = false;
196 BasicBlock *RetBlock = 0;
198 // Scan all the blocks in the function, looking for empty return blocks.
199 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
200 BasicBlock &BB = *BBI++;
202 // Only look at return blocks.
203 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
204 if (Ret == 0) continue;
206 // Only look at the block if it is empty or the only other thing in it is a
207 // single PHI node that is the operand to the return.
208 if (Ret != &BB.front()) {
209 // Check for something else in the block.
210 BasicBlock::iterator I = Ret;
212 if (!isa<PHINode>(I) || I != BB.begin() ||
213 Ret->getNumOperands() == 0 ||
214 Ret->getOperand(0) != I)
218 // If this is the first returning block, remember it and keep going.
224 // Otherwise, we found a duplicate return block. Merge the two.
227 // Case when there is no input to the return or when the returned values
228 // agree is trivial. Note that they can't agree if there are phis in the
230 if (Ret->getNumOperands() == 0 ||
231 Ret->getOperand(0) ==
232 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
233 BB.replaceAllUsesWith(RetBlock);
234 BB.eraseFromParent();
238 // If the canonical return block has no PHI node, create one now.
239 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
240 if (RetBlockPHI == 0) {
241 Value *InVal = cast<ReturnInst>(RetBlock->begin())->getOperand(0);
242 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), "merge",
245 for (pred_iterator PI = pred_begin(RetBlock), E = pred_end(RetBlock);
247 RetBlockPHI->addIncoming(InVal, *PI);
248 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
251 // Turn BB into a block that just unconditionally branches to the return
252 // block. This handles the case when the two return blocks have a common
253 // predecessor but that return different things.
254 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
255 BB.getTerminator()->eraseFromParent();
256 BranchInst::Create(RetBlock, &BB);
262 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
263 /// iterating until no more changes are made.
264 static bool IterativeSimplifyCFG(Function &F) {
265 bool Changed = false;
266 bool LocalChange = true;
267 while (LocalChange) {
270 // Loop over all of the basic blocks (except the first one) and remove them
271 // if they are unneeded...
273 for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
274 if (SimplifyCFG(BBIt++)) {
279 Changed |= LocalChange;
284 // It is possible that we may require multiple passes over the code to fully
287 bool CFGSimplifyPass::runOnFunction(Function &F) {
288 bool EverChanged = RemoveUnreachableBlocksFromFn(F);
289 EverChanged |= MergeEmptyReturnBlocks(F);
290 EverChanged |= IterativeSimplifyCFG(F);
292 // If neither pass changed anything, we're done.
293 if (!EverChanged) return false;
295 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
296 // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
297 // iterate between the two optimizations. We structure the code like this to
298 // avoid reruning IterativeSimplifyCFG if the second pass of
299 // RemoveUnreachableBlocksFromFn doesn't do anything.
300 if (!RemoveUnreachableBlocksFromFn(F))
304 EverChanged = IterativeSimplifyCFG(F);
305 EverChanged |= RemoveUnreachableBlocksFromFn(F);
306 } while (EverChanged);