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 INITIALIZE_PASS(CFGSimplifyPass, "simplifycfg",
53 "Simplify the CFG", false, false);
55 // Public interface to the CFGSimplification pass
56 FunctionPass *llvm::createCFGSimplificationPass() {
57 return new CFGSimplifyPass();
60 /// ChangeToUnreachable - Insert an unreachable instruction before the specified
61 /// instruction, making it and the rest of the code in the block dead.
62 static void ChangeToUnreachable(Instruction *I, bool UseLLVMTrap) {
63 BasicBlock *BB = I->getParent();
64 // Loop over all of the successors, removing BB's entry from any PHI
66 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
67 (*SI)->removePredecessor(BB);
69 // Insert a call to llvm.trap right before this. This turns the undefined
70 // behavior into a hard fail instead of falling through into random code.
73 Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
74 CallInst::Create(TrapFn, "", I);
76 new UnreachableInst(I->getContext(), I);
78 // All instructions after this are dead.
79 BasicBlock::iterator BBI = I, BBE = BB->end();
81 if (!BBI->use_empty())
82 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
83 BB->getInstList().erase(BBI++);
87 /// ChangeToCall - Convert the specified invoke into a normal call.
88 static void ChangeToCall(InvokeInst *II) {
89 BasicBlock *BB = II->getParent();
90 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
91 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
93 NewCall->takeName(II);
94 NewCall->setCallingConv(II->getCallingConv());
95 NewCall->setAttributes(II->getAttributes());
96 II->replaceAllUsesWith(NewCall);
98 // Follow the call by a branch to the normal destination.
99 BranchInst::Create(II->getNormalDest(), II);
101 // Update PHI nodes in the unwind destination
102 II->getUnwindDest()->removePredecessor(BB);
103 BB->getInstList().erase(II);
106 static bool MarkAliveBlocks(BasicBlock *BB,
107 SmallPtrSet<BasicBlock*, 128> &Reachable) {
109 SmallVector<BasicBlock*, 128> Worklist;
110 Worklist.push_back(BB);
111 bool Changed = false;
113 BB = Worklist.pop_back_val();
115 if (!Reachable.insert(BB))
118 // Do a quick scan of the basic block, turning any obviously unreachable
119 // instructions into LLVM unreachable insts. The instruction combining pass
120 // canonicalizes unreachable insts into stores to null or undef.
121 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
122 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
123 if (CI->doesNotReturn()) {
124 // If we found a call to a no-return function, insert an unreachable
125 // instruction after it. Make sure there isn't *already* one there
128 if (!isa<UnreachableInst>(BBI)) {
129 // Don't insert a call to llvm.trap right before the unreachable.
130 ChangeToUnreachable(BBI, false);
137 // Store to undef and store to null are undefined and used to signal that
138 // they should be changed to unreachable by passes that can't modify the
140 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
141 // Don't touch volatile stores.
142 if (SI->isVolatile()) continue;
144 Value *Ptr = SI->getOperand(1);
146 if (isa<UndefValue>(Ptr) ||
147 (isa<ConstantPointerNull>(Ptr) &&
148 SI->getPointerAddressSpace() == 0)) {
149 ChangeToUnreachable(SI, true);
156 // Turn invokes that call 'nounwind' functions into ordinary calls.
157 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
158 if (II->doesNotThrow()) {
163 Changed |= ConstantFoldTerminator(BB);
164 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
165 Worklist.push_back(*SI);
166 } while (!Worklist.empty());
170 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
171 /// if they are in a dead cycle. Return true if a change was made, false
173 static bool RemoveUnreachableBlocksFromFn(Function &F) {
174 SmallPtrSet<BasicBlock*, 128> Reachable;
175 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
177 // If there are unreachable blocks in the CFG...
178 if (Reachable.size() == F.size())
181 assert(Reachable.size() < F.size());
182 NumSimpl += F.size()-Reachable.size();
184 // Loop over all of the basic blocks that are not reachable, dropping all of
185 // their internal references...
186 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
187 if (Reachable.count(BB))
190 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
191 if (Reachable.count(*SI))
192 (*SI)->removePredecessor(BB);
193 BB->dropAllReferences();
196 for (Function::iterator I = ++F.begin(); I != F.end();)
197 if (!Reachable.count(I))
198 I = F.getBasicBlockList().erase(I);
205 /// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
206 /// node) return blocks, merge them together to promote recursive block merging.
207 static bool MergeEmptyReturnBlocks(Function &F) {
208 bool Changed = false;
210 BasicBlock *RetBlock = 0;
212 // Scan all the blocks in the function, looking for empty return blocks.
213 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
214 BasicBlock &BB = *BBI++;
216 // Only look at return blocks.
217 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
218 if (Ret == 0) continue;
220 // Only look at the block if it is empty or the only other thing in it is a
221 // single PHI node that is the operand to the return.
222 if (Ret != &BB.front()) {
223 // Check for something else in the block.
224 BasicBlock::iterator I = Ret;
226 // Skip over debug info.
227 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
229 if (!isa<DbgInfoIntrinsic>(I) &&
230 (!isa<PHINode>(I) || I != BB.begin() ||
231 Ret->getNumOperands() == 0 ||
232 Ret->getOperand(0) != I))
236 // If this is the first returning block, remember it and keep going.
242 // Otherwise, we found a duplicate return block. Merge the two.
245 // Case when there is no input to the return or when the returned values
246 // agree is trivial. Note that they can't agree if there are phis in the
248 if (Ret->getNumOperands() == 0 ||
249 Ret->getOperand(0) ==
250 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
251 BB.replaceAllUsesWith(RetBlock);
252 BB.eraseFromParent();
256 // If the canonical return block has no PHI node, create one now.
257 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
258 if (RetBlockPHI == 0) {
259 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
260 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), "merge",
263 for (pred_iterator PI = pred_begin(RetBlock), E = pred_end(RetBlock);
265 RetBlockPHI->addIncoming(InVal, *PI);
266 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
269 // Turn BB into a block that just unconditionally branches to the return
270 // block. This handles the case when the two return blocks have a common
271 // predecessor but that return different things.
272 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
273 BB.getTerminator()->eraseFromParent();
274 BranchInst::Create(RetBlock, &BB);
280 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
281 /// iterating until no more changes are made.
282 static bool IterativeSimplifyCFG(Function &F, const TargetData *TD) {
283 bool Changed = false;
284 bool LocalChange = true;
285 while (LocalChange) {
288 // Loop over all of the basic blocks and remove them if they are unneeded...
290 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
291 if (SimplifyCFG(BBIt++, TD)) {
296 Changed |= LocalChange;
301 // It is possible that we may require multiple passes over the code to fully
304 bool CFGSimplifyPass::runOnFunction(Function &F) {
305 const TargetData *TD = getAnalysisIfAvailable<TargetData>();
306 bool EverChanged = RemoveUnreachableBlocksFromFn(F);
307 EverChanged |= MergeEmptyReturnBlocks(F);
308 EverChanged |= IterativeSimplifyCFG(F, TD);
310 // If neither pass changed anything, we're done.
311 if (!EverChanged) return false;
313 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
314 // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
315 // iterate between the two optimizations. We structure the code like this to
316 // avoid reruning IterativeSimplifyCFG if the second pass of
317 // RemoveUnreachableBlocksFromFn doesn't do anything.
318 if (!RemoveUnreachableBlocksFromFn(F))
322 EverChanged = IterativeSimplifyCFG(F, TD);
323 EverChanged |= RemoveUnreachableBlocksFromFn(F);
324 } while (EverChanged);