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/ADT/SmallPtrSet.h"
27 #include "llvm/ADT/SmallVector.h"
28 #include "llvm/ADT/Statistic.h"
29 #include "llvm/Analysis/TargetTransformInfo.h"
30 #include "llvm/IR/Attributes.h"
31 #include "llvm/IR/Constants.h"
32 #include "llvm/IR/DataLayout.h"
33 #include "llvm/IR/Instructions.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/IR/Module.h"
36 #include "llvm/Pass.h"
37 #include "llvm/Support/CFG.h"
38 #include "llvm/Transforms/Utils/Local.h"
41 STATISTIC(NumSimpl, "Number of blocks simplified");
44 struct CFGSimplifyPass : public FunctionPass {
45 static char ID; // Pass identification, replacement for typeid
46 CFGSimplifyPass() : FunctionPass(ID) {
47 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
50 virtual bool runOnFunction(Function &F);
54 char CFGSimplifyPass::ID = 0;
55 INITIALIZE_PASS(CFGSimplifyPass, "simplifycfg",
56 "Simplify the CFG", false, false)
58 // Public interface to the CFGSimplification pass
59 FunctionPass *llvm::createCFGSimplificationPass() {
60 return new CFGSimplifyPass();
63 /// changeToUnreachable - Insert an unreachable instruction before the specified
64 /// instruction, making it and the rest of the code in the block dead.
65 static void changeToUnreachable(Instruction *I, bool UseLLVMTrap) {
66 BasicBlock *BB = I->getParent();
67 // Loop over all of the successors, removing BB's entry from any PHI
69 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
70 (*SI)->removePredecessor(BB);
72 // Insert a call to llvm.trap right before this. This turns the undefined
73 // behavior into a hard fail instead of falling through into random code.
76 Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
77 CallInst *CallTrap = CallInst::Create(TrapFn, "", I);
78 CallTrap->setDebugLoc(I->getDebugLoc());
80 new UnreachableInst(I->getContext(), I);
82 // All instructions after this are dead.
83 BasicBlock::iterator BBI = I, BBE = BB->end();
85 if (!BBI->use_empty())
86 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
87 BB->getInstList().erase(BBI++);
91 /// changeToCall - Convert the specified invoke into a normal call.
92 static void changeToCall(InvokeInst *II) {
93 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
94 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args, "", II);
95 NewCall->takeName(II);
96 NewCall->setCallingConv(II->getCallingConv());
97 NewCall->setAttributes(II->getAttributes());
98 NewCall->setDebugLoc(II->getDebugLoc());
99 II->replaceAllUsesWith(NewCall);
101 // Follow the call by a branch to the normal destination.
102 BranchInst::Create(II->getNormalDest(), II);
104 // Update PHI nodes in the unwind destination
105 II->getUnwindDest()->removePredecessor(II->getParent());
106 II->eraseFromParent();
109 static bool markAliveBlocks(BasicBlock *BB,
110 SmallPtrSet<BasicBlock*, 128> &Reachable) {
112 SmallVector<BasicBlock*, 128> Worklist;
113 Worklist.push_back(BB);
114 Reachable.insert(BB);
115 bool Changed = false;
117 BB = Worklist.pop_back_val();
119 // Do a quick scan of the basic block, turning any obviously unreachable
120 // instructions into LLVM unreachable insts. The instruction combining pass
121 // canonicalizes unreachable insts into stores to null or undef.
122 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
123 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
124 if (CI->doesNotReturn()) {
125 // If we found a call to a no-return function, insert an unreachable
126 // instruction after it. Make sure there isn't *already* one there
129 if (!isa<UnreachableInst>(BBI)) {
130 // Don't insert a call to llvm.trap right before the unreachable.
131 changeToUnreachable(BBI, false);
138 // Store to undef and store to null are undefined and used to signal that
139 // they should be changed to unreachable by passes that can't modify the
141 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
142 // Don't touch volatile stores.
143 if (SI->isVolatile()) continue;
145 Value *Ptr = SI->getOperand(1);
147 if (isa<UndefValue>(Ptr) ||
148 (isa<ConstantPointerNull>(Ptr) &&
149 SI->getPointerAddressSpace() == 0)) {
150 changeToUnreachable(SI, true);
157 // Turn invokes that call 'nounwind' functions into ordinary calls.
158 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
159 Value *Callee = II->getCalledValue();
160 if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
161 changeToUnreachable(II, true);
163 } else if (II->doesNotThrow()) {
164 if (II->use_empty() && II->onlyReadsMemory()) {
165 // jump to the normal destination branch.
166 BranchInst::Create(II->getNormalDest(), II);
167 II->getUnwindDest()->removePredecessor(II->getParent());
168 II->eraseFromParent();
175 Changed |= ConstantFoldTerminator(BB, true);
176 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
177 if (Reachable.insert(*SI))
178 Worklist.push_back(*SI);
179 } while (!Worklist.empty());
183 /// removeUnreachableBlocksFromFn - Remove blocks that are not reachable, even
184 /// if they are in a dead cycle. Return true if a change was made, false
186 static bool removeUnreachableBlocksFromFn(Function &F) {
187 SmallPtrSet<BasicBlock*, 128> Reachable;
188 bool Changed = markAliveBlocks(F.begin(), Reachable);
190 // If there are unreachable blocks in the CFG...
191 if (Reachable.size() == F.size())
194 assert(Reachable.size() < F.size());
195 NumSimpl += F.size()-Reachable.size();
197 // Loop over all of the basic blocks that are not reachable, dropping all of
198 // their internal references...
199 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
200 if (Reachable.count(BB))
203 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
204 if (Reachable.count(*SI))
205 (*SI)->removePredecessor(BB);
206 BB->dropAllReferences();
209 for (Function::iterator I = ++F.begin(); I != F.end();)
210 if (!Reachable.count(I))
211 I = F.getBasicBlockList().erase(I);
218 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
219 /// node) return blocks, merge them together to promote recursive block merging.
220 static bool mergeEmptyReturnBlocks(Function &F) {
221 bool Changed = false;
223 BasicBlock *RetBlock = 0;
225 // Scan all the blocks in the function, looking for empty return blocks.
226 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
227 BasicBlock &BB = *BBI++;
229 // Only look at return blocks.
230 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
231 if (Ret == 0) continue;
233 // Only look at the block if it is empty or the only other thing in it is a
234 // single PHI node that is the operand to the return.
235 if (Ret != &BB.front()) {
236 // Check for something else in the block.
237 BasicBlock::iterator I = Ret;
239 // Skip over debug info.
240 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
242 if (!isa<DbgInfoIntrinsic>(I) &&
243 (!isa<PHINode>(I) || I != BB.begin() ||
244 Ret->getNumOperands() == 0 ||
245 Ret->getOperand(0) != I))
249 // If this is the first returning block, remember it and keep going.
255 // Otherwise, we found a duplicate return block. Merge the two.
258 // Case when there is no input to the return or when the returned values
259 // agree is trivial. Note that they can't agree if there are phis in the
261 if (Ret->getNumOperands() == 0 ||
262 Ret->getOperand(0) ==
263 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
264 BB.replaceAllUsesWith(RetBlock);
265 BB.eraseFromParent();
269 // If the canonical return block has no PHI node, create one now.
270 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
271 if (RetBlockPHI == 0) {
272 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
273 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
274 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
275 std::distance(PB, PE), "merge",
278 for (pred_iterator PI = PB; PI != PE; ++PI)
279 RetBlockPHI->addIncoming(InVal, *PI);
280 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
283 // Turn BB into a block that just unconditionally branches to the return
284 // block. This handles the case when the two return blocks have a common
285 // predecessor but that return different things.
286 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
287 BB.getTerminator()->eraseFromParent();
288 BranchInst::Create(RetBlock, &BB);
294 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
295 /// iterating until no more changes are made.
296 static bool iterativelySimplifyCFG(Function &F, const DataLayout *TD,
297 const TargetTransformInfo *TTI) {
298 bool Changed = false;
299 bool LocalChange = true;
300 while (LocalChange) {
303 // Loop over all of the basic blocks and remove them if they are unneeded...
305 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
306 if (SimplifyCFG(BBIt++, TD, TTI)) {
311 Changed |= LocalChange;
316 // It is possible that we may require multiple passes over the code to fully
319 bool CFGSimplifyPass::runOnFunction(Function &F) {
320 const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
321 const TargetTransformInfo *TTI =
322 getAnalysisIfAvailable<TargetTransformInfo>();
323 bool EverChanged = removeUnreachableBlocksFromFn(F);
324 EverChanged |= mergeEmptyReturnBlocks(F);
325 EverChanged |= iterativelySimplifyCFG(F, TD, TTI);
327 // If neither pass changed anything, we're done.
328 if (!EverChanged) return false;
330 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
331 // removeUnreachableBlocksFromFn is needed to nuke them, which means we should
332 // iterate between the two optimizations. We structure the code like this to
333 // avoid reruning iterativelySimplifyCFG if the second pass of
334 // removeUnreachableBlocksFromFn doesn't do anything.
335 if (!removeUnreachableBlocksFromFn(F))
339 EverChanged = iterativelySimplifyCFG(F, TD, TTI);
340 EverChanged |= removeUnreachableBlocksFromFn(F);
341 } while (EverChanged);