1 //===- SimplifyCFG.cpp - CFG Simplification Pass --------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements dead code elimination and basic block merging.
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.
19 //===----------------------------------------------------------------------===//
21 #define DEBUG_TYPE "simplifycfg"
22 #include "llvm/Transforms/Scalar.h"
23 #include "llvm/Transforms/Utils/Local.h"
24 #include "llvm/Constants.h"
25 #include "llvm/Instructions.h"
26 #include "llvm/Module.h"
27 #include "llvm/ParameterAttributes.h"
28 #include "llvm/Support/CFG.h"
29 #include "llvm/Support/Compiler.h"
30 #include "llvm/Pass.h"
31 #include "llvm/ADT/SmallVector.h"
32 #include "llvm/ADT/SmallPtrSet.h"
33 #include "llvm/ADT/Statistic.h"
36 STATISTIC(NumSimpl, "Number of blocks simplified");
39 struct VISIBILITY_HIDDEN CFGSimplifyPass : public FunctionPass {
40 static char ID; // Pass identification, replacement for typeid
41 CFGSimplifyPass() : FunctionPass((intptr_t)&ID) {}
43 virtual bool runOnFunction(Function &F);
45 char CFGSimplifyPass::ID = 0;
46 RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
49 // Public interface to the CFGSimplification pass
50 FunctionPass *llvm::createCFGSimplificationPass() {
51 return new CFGSimplifyPass();
54 /// ChangeToUnreachable - Insert an unreachable instruction before the specified
55 /// instruction, making it and the rest of the code in the block dead.
56 static void ChangeToUnreachable(Instruction *I) {
57 BasicBlock *BB = I->getParent();
58 // Loop over all of the successors, removing BB's entry from any PHI
60 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
61 (*SI)->removePredecessor(BB);
63 new UnreachableInst(I);
65 // All instructions after this are dead.
66 BasicBlock::iterator BBI = I, BBE = BB->end();
68 if (!BBI->use_empty())
69 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
70 BB->getInstList().erase(BBI++);
74 /// ChangeToCall - Convert the specified invoke into a normal call.
75 static void ChangeToCall(InvokeInst *II) {
76 BasicBlock *BB = II->getParent();
77 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
78 CallInst *NewCall = new CallInst(II->getCalledValue(), Args.begin(),
80 NewCall->takeName(II);
81 NewCall->setCallingConv(II->getCallingConv());
82 NewCall->setParamAttrs(II->getParamAttrs());
83 II->replaceAllUsesWith(NewCall);
85 // Follow the call by a branch to the normal destination.
86 new BranchInst(II->getNormalDest(), II);
88 // Update PHI nodes in the unwind destination
89 II->getUnwindDest()->removePredecessor(BB);
90 BB->getInstList().erase(II);
93 static bool MarkAliveBlocks(BasicBlock *BB,
94 SmallPtrSet<BasicBlock*, 128> &Reachable) {
96 SmallVector<BasicBlock*, 128> Worklist;
97 Worklist.push_back(BB);
99 while (!Worklist.empty()) {
100 BB = Worklist.back();
103 if (!Reachable.insert(BB))
106 // Do a quick scan of the basic block, turning any obviously unreachable
107 // instructions into LLVM unreachable insts. The instruction combining pass
108 // canonicalizes unreachable insts into stores to null or undef.
109 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
110 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
111 if (CI->paramHasAttr(0, ParamAttr::NoReturn)) {
112 // If we found a call to a no-return function, insert an unreachable
113 // instruction after it. Make sure there isn't *already* one there
116 if (!isa<UnreachableInst>(BBI)) {
117 ChangeToUnreachable(BBI);
124 if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
125 if (isa<ConstantPointerNull>(SI->getOperand(1)) ||
126 isa<UndefValue>(SI->getOperand(1))) {
127 ChangeToUnreachable(SI);
133 // Turn invokes that call 'nounwind' functions into ordinary calls.
134 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
135 if (II->paramHasAttr(0, ParamAttr::NoUnwind)) {
140 Changed |= ConstantFoldTerminator(BB);
141 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
142 Worklist.push_back(*SI);
147 /// RemoveUnreachableBlocks - Remove blocks that are not reachable, even if they
148 /// are in a dead cycle. Return true if a change was made, false otherwise.
149 static bool RemoveUnreachableBlocks(Function &F) {
150 SmallPtrSet<BasicBlock*, 128> Reachable;
151 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
153 // If there are unreachable blocks in the CFG...
154 if (Reachable.size() == F.size())
157 assert(Reachable.size() < F.size());
158 NumSimpl += F.size()-Reachable.size();
160 // Loop over all of the basic blocks that are not reachable, dropping all of
161 // their internal references...
162 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB)
163 if (!Reachable.count(BB)) {
164 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI!=SE; ++SI)
165 if (Reachable.count(*SI))
166 (*SI)->removePredecessor(BB);
167 BB->dropAllReferences();
170 for (Function::iterator I = ++F.begin(); I != F.end();)
171 if (!Reachable.count(I))
172 I = F.getBasicBlockList().erase(I);
179 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
180 /// iterating until no more changes are made.
181 static bool IterativeSimplifyCFG(Function &F) {
182 bool Changed = false;
183 bool LocalChange = true;
184 while (LocalChange) {
187 // Loop over all of the basic blocks (except the first one) and remove them
188 // if they are unneeded...
190 for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
191 if (SimplifyCFG(BBIt++)) {
196 Changed |= LocalChange;
201 // It is possible that we may require multiple passes over the code to fully
204 bool CFGSimplifyPass::runOnFunction(Function &F) {
205 bool EverChanged = RemoveUnreachableBlocks(F);
206 EverChanged |= IterativeSimplifyCFG(F);
208 // If neither pass changed anything, we're done.
209 if (!EverChanged) return false;
211 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
212 // RemoveUnreachableBlocks is needed to nuke them, which means we should
213 // iterate between the two optimizations. We structure the code like this to
214 // avoid reruning IterativeSimplifyCFG if the second pass of
215 // RemoveUnreachableBlocks doesn't do anything.
216 if (!RemoveUnreachableBlocks(F))
220 EverChanged = IterativeSimplifyCFG(F);
221 EverChanged |= RemoveUnreachableBlocks(F);
222 } while (EverChanged);