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/LLVMContext.h"
30 #include "llvm/Module.h"
31 #include "llvm/Attributes.h"
32 #include "llvm/Support/CFG.h"
33 #include "llvm/Support/Compiler.h"
34 #include "llvm/Pass.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 VISIBILITY_HIDDEN 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, LLVMContext *Context) {
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);
70 // All instructions after this are dead.
71 BasicBlock::iterator BBI = I, BBE = BB->end();
73 if (!BBI->use_empty())
74 BBI->replaceAllUsesWith(Context->getUndef(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()+3, II->op_end());
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,
100 LLVMContext *Context) {
102 SmallVector<BasicBlock*, 128> Worklist;
103 Worklist.push_back(BB);
104 bool Changed = false;
105 while (!Worklist.empty()) {
106 BB = Worklist.back();
109 if (!Reachable.insert(BB))
112 bool AsmFound = false;
113 StoreInst *UndefStore = 0;
114 // Do a quick scan of the basic block, turning any obviously unreachable
115 // instructions into LLVM unreachable insts. The instruction combining pass
116 // canonicalizes unreachable insts into stores to null or undef.
117 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
118 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
119 if (CI->doesNotReturn()) {
120 // If we found a call to a no-return function, insert an unreachable
121 // instruction after it. Make sure there isn't *already* one there
124 if (!isa<UnreachableInst>(BBI)) {
125 ChangeToUnreachable(BBI, Context);
130 if (isa<InlineAsm>(CI->getOperand(0)))
134 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
135 Value *Ptr = SI->getOperand(1);
137 if ((isa<UndefValue>(Ptr) ||
138 (isa<ConstantPointerNull>(Ptr) &&
139 cast<PointerType>(Ptr->getType())->getAddressSpace() == 0)) &&
144 // We can't delete asm's just because their inputs are undefined;
145 // xor R, R is a common idiom for zeroing a register, for example.
146 // Assume user knows what he is doing.
147 if (UndefStore && !AsmFound) {
148 ChangeToUnreachable(UndefStore, Context);
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);
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, F.getContext());
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 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
202 /// iterating until no more changes are made.
203 static bool IterativeSimplifyCFG(Function &F) {
204 bool Changed = false;
205 bool LocalChange = true;
206 while (LocalChange) {
209 // Loop over all of the basic blocks (except the first one) and remove them
210 // if they are unneeded...
212 for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
213 if (SimplifyCFG(BBIt++)) {
218 Changed |= LocalChange;
223 // It is possible that we may require multiple passes over the code to fully
226 bool CFGSimplifyPass::runOnFunction(Function &F) {
227 bool EverChanged = RemoveUnreachableBlocksFromFn(F);
228 EverChanged |= IterativeSimplifyCFG(F);
230 // If neither pass changed anything, we're done.
231 if (!EverChanged) return false;
233 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
234 // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
235 // iterate between the two optimizations. We structure the code like this to
236 // avoid reruning IterativeSimplifyCFG if the second pass of
237 // RemoveUnreachableBlocksFromFn doesn't do anything.
238 if (!RemoveUnreachableBlocksFromFn(F))
242 EverChanged = IterativeSimplifyCFG(F);
243 EverChanged |= RemoveUnreachableBlocksFromFn(F);
244 } while (EverChanged);