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) {
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 new UnreachableInst(I->getContext(), I);
70 // All instructions after this are dead.
71 BasicBlock::iterator BBI = I, BBE = BB->end();
73 if (!BBI->use_empty())
74 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
75 BB->getInstList().erase(BBI++);
79 /// ChangeToCall - Convert the specified invoke into a normal call.
80 static void ChangeToCall(InvokeInst *II) {
81 BasicBlock *BB = II->getParent();
82 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
83 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
85 NewCall->takeName(II);
86 NewCall->setCallingConv(II->getCallingConv());
87 NewCall->setAttributes(II->getAttributes());
88 II->replaceAllUsesWith(NewCall);
90 // Follow the call by a branch to the normal destination.
91 BranchInst::Create(II->getNormalDest(), II);
93 // Update PHI nodes in the unwind destination
94 II->getUnwindDest()->removePredecessor(BB);
95 BB->getInstList().erase(II);
98 static bool MarkAliveBlocks(BasicBlock *BB,
99 SmallPtrSet<BasicBlock*, 128> &Reachable) {
101 SmallVector<BasicBlock*, 128> Worklist;
102 Worklist.push_back(BB);
103 bool Changed = false;
105 BB = Worklist.pop_back_val();
107 if (!Reachable.insert(BB))
110 // Do a quick scan of the basic block, turning any obviously unreachable
111 // instructions into LLVM unreachable insts. The instruction combining pass
112 // canonicalizes unreachable insts into stores to null or undef.
113 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
114 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
115 if (CI->doesNotReturn()) {
116 // If we found a call to a no-return function, insert an unreachable
117 // instruction after it. Make sure there isn't *already* one there
120 if (!isa<UnreachableInst>(BBI)) {
121 ChangeToUnreachable(BBI);
128 // Store to undef and store to null are undefined and used to signal that
129 // they should be changed to unreachable by passes that can't modify the
131 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
132 Value *Ptr = SI->getOperand(1);
134 if (isa<UndefValue>(Ptr) ||
135 (isa<ConstantPointerNull>(Ptr) &&
136 SI->getPointerAddressSpace() == 0)) {
137 ChangeToUnreachable(SI);
144 // Turn invokes that call 'nounwind' functions into ordinary calls.
145 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
146 if (II->doesNotThrow()) {
151 Changed |= ConstantFoldTerminator(BB);
152 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
153 Worklist.push_back(*SI);
154 } while (!Worklist.empty());
158 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
159 /// if they are in a dead cycle. Return true if a change was made, false
161 static bool RemoveUnreachableBlocksFromFn(Function &F) {
162 SmallPtrSet<BasicBlock*, 128> Reachable;
163 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
165 // If there are unreachable blocks in the CFG...
166 if (Reachable.size() == F.size())
169 assert(Reachable.size() < F.size());
170 NumSimpl += F.size()-Reachable.size();
172 // Loop over all of the basic blocks that are not reachable, dropping all of
173 // their internal references...
174 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
175 if (Reachable.count(BB))
178 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
179 if (Reachable.count(*SI))
180 (*SI)->removePredecessor(BB);
181 BB->dropAllReferences();
184 for (Function::iterator I = ++F.begin(); I != F.end();)
185 if (!Reachable.count(I))
186 I = F.getBasicBlockList().erase(I);
193 /// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
194 /// node) return blocks, merge them together to promote recursive block merging.
195 static bool MergeEmptyReturnBlocks(Function &F) {
196 bool Changed = false;
198 BasicBlock *RetBlock = 0;
200 // Scan all the blocks in the function, looking for empty return blocks.
201 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
202 BasicBlock &BB = *BBI++;
204 // Only look at return blocks.
205 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
206 if (Ret == 0) continue;
208 // Only look at the block if it is empty or the only other thing in it is a
209 // single PHI node that is the operand to the return.
210 if (Ret != &BB.front()) {
211 // Check for something else in the block.
212 BasicBlock::iterator I = Ret;
214 // Skip over debug info.
215 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
217 if (!isa<DbgInfoIntrinsic>(I) &&
218 (!isa<PHINode>(I) || I != BB.begin() ||
219 Ret->getNumOperands() == 0 ||
220 Ret->getOperand(0) != I))
224 // If this is the first returning block, remember it and keep going.
230 // Otherwise, we found a duplicate return block. Merge the two.
233 // Case when there is no input to the return or when the returned values
234 // agree is trivial. Note that they can't agree if there are phis in the
236 if (Ret->getNumOperands() == 0 ||
237 Ret->getOperand(0) ==
238 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
239 BB.replaceAllUsesWith(RetBlock);
240 BB.eraseFromParent();
244 // If the canonical return block has no PHI node, create one now.
245 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
246 if (RetBlockPHI == 0) {
247 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
248 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), "merge",
251 for (pred_iterator PI = pred_begin(RetBlock), E = pred_end(RetBlock);
253 RetBlockPHI->addIncoming(InVal, *PI);
254 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
257 // Turn BB into a block that just unconditionally branches to the return
258 // block. This handles the case when the two return blocks have a common
259 // predecessor but that return different things.
260 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
261 BB.getTerminator()->eraseFromParent();
262 BranchInst::Create(RetBlock, &BB);
268 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
269 /// iterating until no more changes are made.
270 static bool IterativeSimplifyCFG(Function &F, const TargetData *TD) {
271 bool Changed = false;
272 bool LocalChange = true;
273 while (LocalChange) {
276 // Loop over all of the basic blocks (except the first one) and remove them
277 // if they are unneeded...
279 for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
280 if (SimplifyCFG(BBIt++, TD)) {
285 Changed |= LocalChange;
290 // It is possible that we may require multiple passes over the code to fully
293 bool CFGSimplifyPass::runOnFunction(Function &F) {
294 const TargetData *TD = getAnalysisIfAvailable<TargetData>();
295 bool EverChanged = RemoveUnreachableBlocksFromFn(F);
296 EverChanged |= MergeEmptyReturnBlocks(F);
297 EverChanged |= IterativeSimplifyCFG(F, TD);
299 // If neither pass changed anything, we're done.
300 if (!EverChanged) return false;
302 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
303 // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
304 // iterate between the two optimizations. We structure the code like this to
305 // avoid reruning IterativeSimplifyCFG if the second pass of
306 // RemoveUnreachableBlocksFromFn doesn't do anything.
307 if (!RemoveUnreachableBlocksFromFn(F))
311 EverChanged = IterativeSimplifyCFG(F, TD);
312 EverChanged |= RemoveUnreachableBlocksFromFn(F);
313 } while (EverChanged);