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, 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);
48 char CFGSimplifyPass::ID = 0;
49 RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
52 // Public interface to the CFGSimplification pass
53 FunctionPass *llvm::createCFGSimplificationPass() {
54 return new CFGSimplifyPass();
57 /// ChangeToUnreachable - Insert an unreachable instruction before the specified
58 /// instruction, making it and the rest of the code in the block dead.
59 static void ChangeToUnreachable(Instruction *I) {
60 BasicBlock *BB = I->getParent();
61 // Loop over all of the successors, removing BB's entry from any PHI
63 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
64 (*SI)->removePredecessor(BB);
66 new UnreachableInst(I);
68 // All instructions after this are dead.
69 BasicBlock::iterator BBI = I, BBE = BB->end();
71 if (!BBI->use_empty())
72 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
73 BB->getInstList().erase(BBI++);
77 /// ChangeToCall - Convert the specified invoke into a normal call.
78 static void ChangeToCall(InvokeInst *II) {
79 BasicBlock *BB = II->getParent();
80 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
81 CallInst *NewCall = new CallInst(II->getCalledValue(), Args.begin(),
83 NewCall->takeName(II);
84 NewCall->setCallingConv(II->getCallingConv());
85 NewCall->setParamAttrs(II->getParamAttrs());
86 II->replaceAllUsesWith(NewCall);
88 // Follow the call by a branch to the normal destination.
89 new BranchInst(II->getNormalDest(), II);
91 // Update PHI nodes in the unwind destination
92 II->getUnwindDest()->removePredecessor(BB);
93 BB->getInstList().erase(II);
96 static bool MarkAliveBlocks(BasicBlock *BB,
97 SmallPtrSet<BasicBlock*, 128> &Reachable) {
99 SmallVector<BasicBlock*, 128> Worklist;
100 Worklist.push_back(BB);
101 bool Changed = false;
102 while (!Worklist.empty()) {
103 BB = Worklist.back();
106 if (!Reachable.insert(BB))
109 // Do a quick scan of the basic block, turning any obviously unreachable
110 // instructions into LLVM unreachable insts. The instruction combining pass
111 // canonicalizes unreachable insts into stores to null or undef.
112 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
113 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
114 if (CI->doesNotReturn()) {
115 // If we found a call to a no-return function, insert an unreachable
116 // instruction after it. Make sure there isn't *already* one there
119 if (!isa<UnreachableInst>(BBI)) {
120 ChangeToUnreachable(BBI);
127 if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
128 if (isa<ConstantPointerNull>(SI->getOperand(1)) ||
129 isa<UndefValue>(SI->getOperand(1))) {
130 ChangeToUnreachable(SI);
136 // Turn invokes that call 'nounwind' functions into ordinary calls.
137 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
138 if (II->doesNotThrow()) {
143 Changed |= ConstantFoldTerminator(BB);
144 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
145 Worklist.push_back(*SI);
150 /// RemoveUnreachableBlocks - Remove blocks that are not reachable, even if they
151 /// are in a dead cycle. Return true if a change was made, false otherwise.
152 static bool RemoveUnreachableBlocks(Function &F) {
153 SmallPtrSet<BasicBlock*, 128> Reachable;
154 bool Changed = MarkAliveBlocks(F.begin(), Reachable);
156 // If there are unreachable blocks in the CFG...
157 if (Reachable.size() == F.size())
160 assert(Reachable.size() < F.size());
161 NumSimpl += F.size()-Reachable.size();
163 // Loop over all of the basic blocks that are not reachable, dropping all of
164 // their internal references...
165 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB)
166 if (!Reachable.count(BB)) {
167 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI!=SE; ++SI)
168 if (Reachable.count(*SI))
169 (*SI)->removePredecessor(BB);
170 BB->dropAllReferences();
173 for (Function::iterator I = ++F.begin(); I != F.end();)
174 if (!Reachable.count(I))
175 I = F.getBasicBlockList().erase(I);
182 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
183 /// iterating until no more changes are made.
184 static bool IterativeSimplifyCFG(Function &F) {
185 bool Changed = false;
186 bool LocalChange = true;
187 while (LocalChange) {
190 // Loop over all of the basic blocks (except the first one) and remove them
191 // if they are unneeded...
193 for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
194 if (SimplifyCFG(BBIt++)) {
199 Changed |= LocalChange;
204 // It is possible that we may require multiple passes over the code to fully
207 bool CFGSimplifyPass::runOnFunction(Function &F) {
208 bool EverChanged = RemoveUnreachableBlocks(F);
209 EverChanged |= IterativeSimplifyCFG(F);
211 // If neither pass changed anything, we're done.
212 if (!EverChanged) return false;
214 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
215 // RemoveUnreachableBlocks is needed to nuke them, which means we should
216 // iterate between the two optimizations. We structure the code like this to
217 // avoid reruning IterativeSimplifyCFG if the second pass of
218 // RemoveUnreachableBlocks doesn't do anything.
219 if (!RemoveUnreachableBlocks(F))
223 EverChanged = IterativeSimplifyCFG(F);
224 EverChanged |= RemoveUnreachableBlocks(F);
225 } while (EverChanged);