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//===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// 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/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.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/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/SimplifyCFG.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SimplifyCFGOptions.h"
#include <utility>
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)"));
static cl::opt<bool> UserKeepLoops(
"keep-loops", cl::Hidden, cl::init(true),
cl::desc("Preserve canonical loop structure (default = true)"));
static cl::opt<bool> UserSwitchToLookup(
"switch-to-lookup", cl::Hidden, cl::init(false),
cl::desc("Convert switches to lookup tables (default = false)"));
static cl::opt<bool> UserForwardSwitchCond(
"forward-switch-cond", cl::Hidden, cl::init(false),
cl::desc("Forward switch condition to phi ops (default = false)"));
static cl::opt<bool> UserHoistCommonInsts(
"hoist-common-insts", cl::Hidden, cl::init(false),
cl::desc("hoist common instructions (default = false)"));
static cl::opt<bool> UserSinkCommonInsts(
"sink-common-insts", cl::Hidden, cl::init(false),
cl::desc("Sink common instructions (default = false)"));
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, DomTreeUpdater *DTU) {
bool Changed = false;
std::vector<DominatorTree::UpdateType> Updates;
SmallVector<BasicBlock *, 8> DeadBlocks;
BasicBlock *RetBlock = nullptr;
// Scan all the blocks in the function, looking for empty return blocks.
for (BasicBlock &BB : make_early_inc_range(F)) {
if (DTU && DTU->isBBPendingDeletion(&BB))
// 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;
// Skip merging if this would result in a CallBr instruction with a
// duplicate destination. FIXME: See note in CodeGenPrepare.cpp.
bool SkipCallBr = false;
for (pred_iterator PI = pred_begin(&BB), E = pred_end(&BB);
PI != E && !SkipCallBr; ++PI) {
if (auto *CBI = dyn_cast<CallBrInst>((*PI)->getTerminator()))
for (unsigned i = 0, e = CBI->getNumSuccessors(); i != e; ++i)
if (RetBlock == CBI->getSuccessor(i)) {
SkipCallBr = true;
if (SkipCallBr)
// 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)) {
// All predecessors of BB should now branch to RetBlock instead.
if (DTU) {
for (auto *Predecessor : predecessors(&BB)) {
// But, iff Predecessor already branches to RetBlock,
// don't (re-)add DomTree edge, because it already exists.
if (!is_contained(successors(Predecessor), RetBlock))
Updates.push_back({DominatorTree::Insert, Predecessor, RetBlock});
Updates.push_back({DominatorTree::Delete, Predecessor, &BB});
// 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);
if (DTU)
Updates.push_back({DominatorTree::Insert, &BB, RetBlock});
if (DTU) {
for (auto *BB : DeadBlocks)
} else {
for (auto *BB : DeadBlocks)
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,
DomTreeUpdater *DTU,
const SimplifyCFGOptions &Options) {
bool Changed = false;
bool LocalChange = true;
SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
FindFunctionBackedges(F, Edges);
SmallPtrSet<BasicBlock *, 16> UniqueLoopHeaders;
for (unsigned i = 0, e = Edges.size(); i != e; ++i)
UniqueLoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
SmallVector<WeakVH, 16> LoopHeaders(UniqueLoopHeaders.begin(),
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(); ) {
BasicBlock &BB = *BBIt++;
if (DTU) {
!DTU->isBBPendingDeletion(&BB) &&
"Should not end up trying to simplify blocks marked for removal.");
// Make sure that the advanced iterator does not point at the blocks
// that are marked for removal, skip over all such blocks.
while (BBIt != F.end() && DTU->isBBPendingDeletion(&*BBIt))
if (simplifyCFG(&BB, TTI, DTU, Options, LoopHeaders)) {
LocalChange = true;
Changed |= LocalChange;
return Changed;
static bool simplifyFunctionCFGImpl(Function &F, const TargetTransformInfo &TTI,
DominatorTree *DT,
const SimplifyCFGOptions &Options) {
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
bool EverChanged = removeUnreachableBlocks(F, DT ? &DTU : nullptr);
EverChanged |= mergeEmptyReturnBlocks(F, DT ? &DTU : nullptr);
EverChanged |= iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options);
// 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, DT ? &DTU : nullptr))
return true;
do {
EverChanged = iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options);
EverChanged |= removeUnreachableBlocks(F, DT ? &DTU : nullptr);
} while (EverChanged);
return true;
static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
DominatorTree *DT,
const SimplifyCFGOptions &Options) {
assert((!RequireAndPreserveDomTree ||
(DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
"Original domtree is invalid?");
bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options);
assert((!RequireAndPreserveDomTree ||
(DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
"Failed to maintain validity of domtree!");
return Changed;
// Command-line settings override compile-time settings.
static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) {
if (UserBonusInstThreshold.getNumOccurrences())
Options.BonusInstThreshold = UserBonusInstThreshold;
if (UserForwardSwitchCond.getNumOccurrences())
Options.ForwardSwitchCondToPhi = UserForwardSwitchCond;
if (UserSwitchToLookup.getNumOccurrences())
Options.ConvertSwitchToLookupTable = UserSwitchToLookup;
if (UserKeepLoops.getNumOccurrences())
Options.NeedCanonicalLoop = UserKeepLoops;
if (UserHoistCommonInsts.getNumOccurrences())
Options.HoistCommonInsts = UserHoistCommonInsts;
if (UserSinkCommonInsts.getNumOccurrences())
Options.SinkCommonInsts = UserSinkCommonInsts;
SimplifyCFGPass::SimplifyCFGPass() : Options() {
SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts)
: Options(Opts) {
PreservedAnalyses SimplifyCFGPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &TTI = AM.getResult<TargetIRAnalysis>(F);
Options.AC = &AM.getResult<AssumptionAnalysis>(F);
DominatorTree *DT = nullptr;
if (RequireAndPreserveDomTree)
DT = &AM.getResult<DominatorTreeAnalysis>(F);
if (F.hasFnAttribute(Attribute::OptForFuzzing)) {
} else {
if (!simplifyFunctionCFG(F, TTI, DT, Options))
return PreservedAnalyses::all();
PreservedAnalyses PA;
if (RequireAndPreserveDomTree)
return PA;
namespace {
struct CFGSimplifyPass : public FunctionPass {
static char ID;
SimplifyCFGOptions Options;
std::function<bool(const Function &)> PredicateFtor;
CFGSimplifyPass(SimplifyCFGOptions Options_ = SimplifyCFGOptions(),
std::function<bool(const Function &)> Ftor = nullptr)
: FunctionPass(ID), Options(Options_), PredicateFtor(std::move(Ftor)) {
// Check for command-line overrides of options for debug/customization.
bool runOnFunction(Function &F) override {
if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
return false;
Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
DominatorTree *DT = nullptr;
if (RequireAndPreserveDomTree)
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
if (F.hasFnAttribute(Attribute::OptForFuzzing)) {
} else {
auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
return simplifyFunctionCFG(F, TTI, DT, Options);
void getAnalysisUsage(AnalysisUsage &AU) const override {
if (RequireAndPreserveDomTree)
if (RequireAndPreserveDomTree)
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(SimplifyCFGOptions Options,
std::function<bool(const Function &)> Ftor) {
return new CFGSimplifyPass(Options, std::move(Ftor));