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/Module.h"
30 #include "llvm/ParameterAttributes.h"
31 #include "llvm/Support/CFG.h"
32 #include "llvm/Support/Compiler.h"
33 #include "llvm/Pass.h"
34 #include "llvm/ADT/SmallVector.h"
35 #include "llvm/ADT/SmallPtrSet.h"
36 #include "llvm/ADT/Statistic.h"
39 STATISTIC(NumSimpl, "Number of blocks simplified");
42 struct VISIBILITY_HIDDEN CFGSimplifyPass : public FunctionPass {
43 static char ID; // Pass identification, replacement for typeid
44 CFGSimplifyPass() : FunctionPass((intptr_t)&ID) {}
46 virtual bool runOnFunction(Function &F);
50 char CFGSimplifyPass::ID = 0;
51 static RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
53 // Public interface to the CFGSimplification pass
54 FunctionPass *llvm::createCFGSimplificationPass() {
55 return new CFGSimplifyPass();
58 /// ChangeToUnreachable - Insert an unreachable instruction before the specified
59 /// instruction, making it and the rest of the code in the block dead.
60 static void ChangeToUnreachable(Instruction *I) {
61 BasicBlock *BB = I->getParent();
62 // Loop over all of the successors, removing BB's entry from any PHI
64 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
65 (*SI)->removePredecessor(BB);
67 new UnreachableInst(I);
69 // All instructions after this are dead.
70 BasicBlock::iterator BBI = I, BBE = BB->end();
72 if (!BBI->use_empty())
73 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
74 BB->getInstList().erase(BBI++);
78 /// ChangeToCall - Convert the specified invoke into a normal call.
79 static void ChangeToCall(InvokeInst *II) {
80 BasicBlock *BB = II->getParent();
81 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
82 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
84 NewCall->takeName(II);
85 NewCall->setCallingConv(II->getCallingConv());
86 NewCall->setParamAttrs(II->getParamAttrs());
87 II->replaceAllUsesWith(NewCall);
89 // Follow the call by a branch to the normal destination.
90 BranchInst::Create(II->getNormalDest(), II);
92 // Update PHI nodes in the unwind destination
93 II->getUnwindDest()->removePredecessor(BB);
94 BB->getInstList().erase(II);
97 static bool MarkAliveBlocks(BasicBlock *BB,
98 SmallPtrSet<BasicBlock*, 128> &Reachable) {
100 SmallVector<BasicBlock*, 128> Worklist;
101 Worklist.push_back(BB);
102 bool Changed = false;
103 while (!Worklist.empty()) {
104 BB = Worklist.back();
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 if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
129 if (isa<ConstantPointerNull>(SI->getOperand(1)) ||
130 isa<UndefValue>(SI->getOperand(1))) {
131 ChangeToUnreachable(SI);
137 // Turn invokes that call 'nounwind' functions into ordinary calls.
138 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
139 if (II->doesNotThrow()) {
144 Changed |= ConstantFoldTerminator(BB);
145 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
146 Worklist.push_back(*SI);
151 /// RemoveUnreachableBlocks - Remove blocks that are not reachable, even if they
152 /// are in a dead cycle. Return true if a change was made, false otherwise.
153 static bool RemoveUnreachableBlocks(Function &F) {
154 SmallPtrSet<BasicBlock*, 128> Reachable;
155 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
157 // If there are unreachable blocks in the CFG...
158 if (Reachable.size() == F.size())
161 assert(Reachable.size() < F.size());
162 NumSimpl += F.size()-Reachable.size();
164 // Loop over all of the basic blocks that are not reachable, dropping all of
165 // their internal references...
166 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
167 if (Reachable.count(BB))
170 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
171 if (Reachable.count(*SI))
172 (*SI)->removePredecessor(BB);
173 BB->dropAllReferences();
176 for (Function::iterator I = ++F.begin(); I != F.end();)
177 if (!Reachable.count(I))
178 I = F.getBasicBlockList().erase(I);
185 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
186 /// iterating until no more changes are made.
187 static bool IterativeSimplifyCFG(Function &F) {
188 bool Changed = false;
189 bool LocalChange = true;
190 while (LocalChange) {
193 // Loop over all of the basic blocks (except the first one) and remove them
194 // if they are unneeded...
196 for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
197 if (SimplifyCFG(BBIt++)) {
202 Changed |= LocalChange;
207 // It is possible that we may require multiple passes over the code to fully
210 bool CFGSimplifyPass::runOnFunction(Function &F) {
211 bool EverChanged = RemoveUnreachableBlocks(F);
212 EverChanged |= IterativeSimplifyCFG(F);
214 // If neither pass changed anything, we're done.
215 if (!EverChanged) return false;
217 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
218 // RemoveUnreachableBlocks is needed to nuke them, which means we should
219 // iterate between the two optimizations. We structure the code like this to
220 // avoid reruning IterativeSimplifyCFG if the second pass of
221 // RemoveUnreachableBlocks doesn't do anything.
222 if (!RemoveUnreachableBlocks(F))
226 EverChanged = IterativeSimplifyCFG(F);
227 EverChanged |= RemoveUnreachableBlocks(F);
228 } while (EverChanged);