SimplifyCFGPass.cpp [plain text]
#define DEBUG_TYPE "simplifycfg"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Module.h"
#include "llvm/Attributes.h"
#include "llvm/Support/CFG.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumSimpl, "Number of blocks simplified");
namespace {
struct CFGSimplifyPass : public FunctionPass {
static char ID; CFGSimplifyPass() : FunctionPass(ID) {
initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnFunction(Function &F);
};
}
char CFGSimplifyPass::ID = 0;
INITIALIZE_PASS(CFGSimplifyPass, "simplifycfg",
"Simplify the CFG", false, false)
FunctionPass *llvm::createCFGSimplificationPass() {
return new CFGSimplifyPass();
}
static void ChangeToUnreachable(Instruction *I, bool UseLLVMTrap) {
BasicBlock *BB = I->getParent();
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
(*SI)->removePredecessor(BB);
if (UseLLVMTrap) {
Function *TrapFn =
Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
CallInst *CallTrap = CallInst::Create(TrapFn, "", I);
CallTrap->setDebugLoc(I->getDebugLoc());
}
new UnreachableInst(I->getContext(), I);
BasicBlock::iterator BBI = I, BBE = BB->end();
while (BBI != BBE) {
if (!BBI->use_empty())
BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
BB->getInstList().erase(BBI++);
}
}
static void ChangeToCall(InvokeInst *II) {
BasicBlock *BB = II->getParent();
SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args, "", II);
NewCall->takeName(II);
NewCall->setCallingConv(II->getCallingConv());
NewCall->setAttributes(II->getAttributes());
NewCall->setDebugLoc(II->getDebugLoc());
II->replaceAllUsesWith(NewCall);
BranchInst::Create(II->getNormalDest(), II);
II->getUnwindDest()->removePredecessor(BB);
BB->getInstList().erase(II);
}
static bool MarkAliveBlocks(BasicBlock *BB,
SmallPtrSet<BasicBlock*, 128> &Reachable) {
SmallVector<BasicBlock*, 128> Worklist;
Worklist.push_back(BB);
bool Changed = false;
do {
BB = Worklist.pop_back_val();
if (!Reachable.insert(BB))
continue;
for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
if (CI->doesNotReturn()) {
++BBI;
if (!isa<UnreachableInst>(BBI)) {
ChangeToUnreachable(BBI, false);
Changed = true;
}
break;
}
}
if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
if (SI->isVolatile()) continue;
Value *Ptr = SI->getOperand(1);
if (isa<UndefValue>(Ptr) ||
(isa<ConstantPointerNull>(Ptr) &&
SI->getPointerAddressSpace() == 0)) {
ChangeToUnreachable(SI, true);
Changed = true;
break;
}
}
}
if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
if (II->doesNotThrow()) {
ChangeToCall(II);
Changed = true;
}
Changed |= ConstantFoldTerminator(BB, true);
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
Worklist.push_back(*SI);
} while (!Worklist.empty());
return Changed;
}
static bool RemoveUnreachableBlocksFromFn(Function &F) {
SmallPtrSet<BasicBlock*, 128> Reachable;
bool Changed = MarkAliveBlocks(F.begin(), Reachable);
if (Reachable.size() == F.size())
return Changed;
assert(Reachable.size() < F.size());
NumSimpl += F.size()-Reachable.size();
for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
if (Reachable.count(BB))
continue;
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
if (Reachable.count(*SI))
(*SI)->removePredecessor(BB);
BB->dropAllReferences();
}
for (Function::iterator I = ++F.begin(); I != F.end();)
if (!Reachable.count(I))
I = F.getBasicBlockList().erase(I);
else
++I;
return true;
}
static bool MergeEmptyReturnBlocks(Function &F) {
bool Changed = false;
BasicBlock *RetBlock = 0;
for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
BasicBlock &BB = *BBI++;
ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
if (Ret == 0) continue;
if (Ret != &BB.front()) {
BasicBlock::iterator I = Ret;
--I;
while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
--I;
if (!isa<DbgInfoIntrinsic>(I) &&
(!isa<PHINode>(I) || I != BB.begin() ||
Ret->getNumOperands() == 0 ||
Ret->getOperand(0) != I))
continue;
}
if (RetBlock == 0) {
RetBlock = &BB;
continue;
}
Changed = true;
if (Ret->getNumOperands() == 0 ||
Ret->getOperand(0) ==
cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
BB.replaceAllUsesWith(RetBlock);
BB.eraseFromParent();
continue;
}
PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
if (RetBlockPHI == 0) {
Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
std::distance(PB, PE), "merge",
&RetBlock->front());
for (pred_iterator PI = PB; PI != PE; ++PI)
RetBlockPHI->addIncoming(InVal, *PI);
RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
}
RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
BB.getTerminator()->eraseFromParent();
BranchInst::Create(RetBlock, &BB);
}
return Changed;
}
static bool IterativeSimplifyCFG(Function &F, const TargetData *TD) {
bool Changed = false;
bool LocalChange = true;
while (LocalChange) {
LocalChange = false;
for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
if (SimplifyCFG(BBIt++, TD)) {
LocalChange = true;
++NumSimpl;
}
}
Changed |= LocalChange;
}
return Changed;
}
bool CFGSimplifyPass::runOnFunction(Function &F) {
const TargetData *TD = getAnalysisIfAvailable<TargetData>();
bool EverChanged = RemoveUnreachableBlocksFromFn(F);
EverChanged |= MergeEmptyReturnBlocks(F);
EverChanged |= IterativeSimplifyCFG(F, TD);
if (!EverChanged) return false;
if (!RemoveUnreachableBlocksFromFn(F))
return true;
do {
EverChanged = IterativeSimplifyCFG(F, TD);
EverChanged |= RemoveUnreachableBlocksFromFn(F);
} while (EverChanged);
return true;
}