SimplifyCFGPass.cpp [plain text]
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
#define DEBUG_TYPE "simplifycfg"
STATISTIC(NumSimpl, "Number of blocks simplified");
namespace {
struct CFGSimplifyPass : public FunctionPass {
static char ID; CFGSimplifyPass() : FunctionPass(ID) {
initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<TargetTransformInfo>();
}
};
}
char CFGSimplifyPass::ID = 0;
INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
false)
INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
false)
FunctionPass *llvm::createCFGSimplificationPass() {
return new CFGSimplifyPass();
}
static bool mergeEmptyReturnBlocks(Function &F) {
bool Changed = false;
BasicBlock *RetBlock = nullptr;
for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
BasicBlock &BB = *BBI++;
ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
if (!Ret) 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) {
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) {
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 iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
const DataLayout *DL) {
bool Changed = false;
bool LocalChange = true;
while (LocalChange) {
LocalChange = false;
for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
if (SimplifyCFG(BBIt++, TTI, DL)) {
LocalChange = true;
++NumSimpl;
}
}
Changed |= LocalChange;
}
return Changed;
}
bool CFGSimplifyPass::runOnFunction(Function &F) {
if (skipOptnoneFunction(F))
return false;
const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
bool EverChanged = removeUnreachableBlocks(F);
EverChanged |= mergeEmptyReturnBlocks(F);
EverChanged |= iterativelySimplifyCFG(F, TTI, DL);
if (!EverChanged) return false;
if (!removeUnreachableBlocks(F))
return true;
do {
EverChanged = iterativelySimplifyCFG(F, TTI, DL);
EverChanged |= removeUnreachableBlocks(F);
} while (EverChanged);
return true;
}