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//===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file implements dead code elimination and basic block merging, along
// with a collection of other peephole control flow optimizations. For example:
// * Removes basic blocks with no predecessors.
// * Merges a basic block into its predecessor if there is only one and the
// predecessor only has one successor.
// * Eliminates PHI nodes for basic blocks with a single predecessor.
// * Eliminates a basic block that only contains an unconditional branch.
// * Changes invoke instructions to nounwind functions to be calls.
// * Change things like "if (x) if (y)" into "if (x&y)".
// * etc..
#include "llvm/Transforms/Scalar/SimplifyCFG.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.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/Support/CommandLine.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Scalar.h"
using namespace llvm;
#define DEBUG_TYPE "simplifycfg"
static cl::opt<unsigned>
UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
cl::desc("Control the number of bonus instructions (default = 1)"));
STATISTIC(NumSimpl, "Number of blocks simplified");
/// If we have more than one empty (other than phi node) return blocks,
/// merge them together to promote recursive block merging.
static bool mergeEmptyReturnBlocks(Function &F) {
bool Changed = false;
BasicBlock *RetBlock = nullptr;
// Scan all the blocks in the function, looking for empty return blocks.
for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
BasicBlock &BB = *BBI++;
// Only look at return blocks.
ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
if (!Ret) continue;
// Only look at the block if it is empty or the only other thing in it is a
// single PHI node that is the operand to the return.
if (Ret != &BB.front()) {
// Check for something else in the block.
BasicBlock::iterator I = Ret;
// Skip over debug info.
while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
if (!isa<DbgInfoIntrinsic>(I) &&
(!isa<PHINode>(I) || I != BB.begin() ||
Ret->getNumOperands() == 0 ||
Ret->getOperand(0) != I))
// If this is the first returning block, remember it and keep going.
if (!RetBlock) {
RetBlock = &BB;
// Otherwise, we found a duplicate return block. Merge the two.
Changed = true;
// Case when there is no input to the return or when the returned values
// agree is trivial. Note that they can't agree if there are phis in the
// blocks.
if (Ret->getNumOperands() == 0 ||
Ret->getOperand(0) ==
cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
// If the canonical return block has no PHI node, create one now.
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",
for (pred_iterator PI = PB; PI != PE; ++PI)
RetBlockPHI->addIncoming(InVal, *PI);
RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
// Turn BB into a block that just unconditionally branches to the return
// block. This handles the case when the two return blocks have a common
// predecessor but that return different things.
RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
BranchInst::Create(RetBlock, &BB);
return Changed;
/// Call SimplifyCFG on all the blocks in the function,
/// iterating until no more changes are made.
static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
AssumptionCache *AC,
unsigned BonusInstThreshold) {
bool Changed = false;
bool LocalChange = true;
while (LocalChange) {
LocalChange = false;
// Loop over all of the basic blocks and remove them if they are unneeded.
for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
if (SimplifyCFG(BBIt++, TTI, BonusInstThreshold, AC)) {
LocalChange = true;
Changed |= LocalChange;
return Changed;
static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
AssumptionCache *AC, int BonusInstThreshold) {
bool EverChanged = removeUnreachableBlocks(F);
EverChanged |= mergeEmptyReturnBlocks(F);
EverChanged |= iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
// If neither pass changed anything, we're done.
if (!EverChanged) return false;
// iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
// removeUnreachableBlocks is needed to nuke them, which means we should
// iterate between the two optimizations. We structure the code like this to
// avoid rerunning iterativelySimplifyCFG if the second pass of
// removeUnreachableBlocks doesn't do anything.
if (!removeUnreachableBlocks(F))
return true;
do {
EverChanged = iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
EverChanged |= removeUnreachableBlocks(F);
} while (EverChanged);
return true;
: BonusInstThreshold(UserBonusInstThreshold) {}
SimplifyCFGPass::SimplifyCFGPass(int BonusInstThreshold)
: BonusInstThreshold(BonusInstThreshold) {}
PreservedAnalyses SimplifyCFGPass::run(Function &F,
AnalysisManager<Function> *AM) {
auto &TTI = AM->getResult<TargetIRAnalysis>(F);
auto &AC = AM->getResult<AssumptionAnalysis>(F);
if (!simplifyFunctionCFG(F, TTI, &AC, BonusInstThreshold))
return PreservedAnalyses::none();
return PreservedAnalyses::all();
namespace {
struct CFGSimplifyPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
unsigned BonusInstThreshold;
std::function<bool(const Function &)> PredicateFtor;
CFGSimplifyPass(int T = -1,
std::function<bool(const Function &)> Ftor = nullptr)
: FunctionPass(ID), PredicateFtor(Ftor) {
BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
bool runOnFunction(Function &F) override {
if (PredicateFtor && !PredicateFtor(F))
return false;
if (skipOptnoneFunction(F))
return false;
AssumptionCache *AC =
const TargetTransformInfo &TTI =
return simplifyFunctionCFG(F, TTI, AC, BonusInstThreshold);
void getAnalysisUsage(AnalysisUsage &AU) const override {
char CFGSimplifyPass::ID = 0;
INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
// Public interface to the CFGSimplification pass
FunctionPass *
llvm::createCFGSimplificationPass(int Threshold,
std::function<bool(const Function &)> Ftor) {
return new CFGSimplifyPass(Threshold, Ftor);