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/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 CFGSimplifyPass : public FunctionPass {
43 static char ID; // Pass identification, replacement for typeid
44 CFGSimplifyPass() : FunctionPass(&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, LLVMContext &Context) {
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->getContext(), 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->setAttributes(II->getAttributes());
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,
99 LLVMContext &Context) {
101 SmallVector<BasicBlock*, 128> Worklist;
102 Worklist.push_back(BB);
103 bool Changed = false;
104 while (!Worklist.empty()) {
105 BB = Worklist.back();
108 if (!Reachable.insert(BB))
111 // Do a quick scan of the basic block, turning any obviously unreachable
112 // instructions into LLVM unreachable insts. The instruction combining pass
113 // canonicalizes unreachable insts into stores to null or undef.
114 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
115 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
116 if (CI->doesNotReturn()) {
117 // If we found a call to a no-return function, insert an unreachable
118 // instruction after it. Make sure there isn't *already* one there
121 if (!isa<UnreachableInst>(BBI)) {
122 ChangeToUnreachable(BBI, Context);
129 // Store to undef and store to null are undefined and used to signal that
130 // they should be changed to unreachable by passes that can't modify the
132 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
133 Value *Ptr = SI->getOperand(1);
135 if (isa<UndefValue>(Ptr) ||
136 (isa<ConstantPointerNull>(Ptr) &&
137 SI->getPointerAddressSpace() == 0)) {
138 ChangeToUnreachable(SI, Context);
145 // Turn invokes that call 'nounwind' functions into ordinary calls.
146 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
147 if (II->doesNotThrow()) {
152 Changed |= ConstantFoldTerminator(BB);
153 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
154 Worklist.push_back(*SI);
159 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
160 /// if they are in a dead cycle. Return true if a change was made, false
162 static bool RemoveUnreachableBlocksFromFn(Function &F) {
163 SmallPtrSet<BasicBlock*, 128> Reachable;
164 bool Changed = MarkAliveBlocks(F.begin(), Reachable, F.getContext());
166 // If there are unreachable blocks in the CFG...
167 if (Reachable.size() == F.size())
170 assert(Reachable.size() < F.size());
171 NumSimpl += F.size()-Reachable.size();
173 // Loop over all of the basic blocks that are not reachable, dropping all of
174 // their internal references...
175 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
176 if (Reachable.count(BB))
179 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
180 if (Reachable.count(*SI))
181 (*SI)->removePredecessor(BB);
182 BB->dropAllReferences();
185 for (Function::iterator I = ++F.begin(); I != F.end();)
186 if (!Reachable.count(I))
187 I = F.getBasicBlockList().erase(I);
194 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
195 /// iterating until no more changes are made.
196 static bool IterativeSimplifyCFG(Function &F) {
197 bool Changed = false;
198 bool LocalChange = true;
199 while (LocalChange) {
202 // Loop over all of the basic blocks (except the first one) and remove them
203 // if they are unneeded...
205 for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
206 if (SimplifyCFG(BBIt++)) {
211 Changed |= LocalChange;
216 // It is possible that we may require multiple passes over the code to fully
219 bool CFGSimplifyPass::runOnFunction(Function &F) {
220 bool EverChanged = RemoveUnreachableBlocksFromFn(F);
221 EverChanged |= IterativeSimplifyCFG(F);
223 // If neither pass changed anything, we're done.
224 if (!EverChanged) return false;
226 // IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
227 // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
228 // iterate between the two optimizations. We structure the code like this to
229 // avoid reruning IterativeSimplifyCFG if the second pass of
230 // RemoveUnreachableBlocksFromFn doesn't do anything.
231 if (!RemoveUnreachableBlocksFromFn(F))
235 EverChanged = IterativeSimplifyCFG(F);
236 EverChanged |= RemoveUnreachableBlocksFromFn(F);
237 } while (EverChanged);