blob: 4544975a4887b84c7b5f79b9e98dcf9beb0d6051 [file] [log] [blame]
//===- 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/GlobalsModRef.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Transforms/Utils/Local.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/Scalar.h"
#include "llvm/Transforms/Scalar/SimplifyCFG.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> 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) {
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,
const SimplifyCFGOptions &Options) {
bool Changed = false;
bool LocalChange = true;
SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
FindFunctionBackedges(F, Edges);
SmallPtrSet<BasicBlock *, 16> LoopHeaders;
for (unsigned i = 0, e = Edges.size(); i != e; ++i)
LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
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, Options, &LoopHeaders)) {
LocalChange = true;
Changed |= LocalChange;
return Changed;
static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
const SimplifyCFGOptions &Options) {
bool EverChanged = removeUnreachableBlocks(F);
EverChanged |= mergeEmptyReturnBlocks(F);
EverChanged |= iterativelySimplifyCFG(F, TTI, 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))
return true;
do {
EverChanged = iterativelySimplifyCFG(F, TTI, Options);
EverChanged |= removeUnreachableBlocks(F);
} while (EverChanged);
return true;
// Command-line settings override compile-time settings.
SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts) {
Options.BonusInstThreshold = UserBonusInstThreshold.getNumOccurrences()
? UserBonusInstThreshold
: Opts.BonusInstThreshold;
Options.ForwardSwitchCondToPhi = UserForwardSwitchCond.getNumOccurrences()
? UserForwardSwitchCond
: Opts.ForwardSwitchCondToPhi;
Options.ConvertSwitchToLookupTable = UserSwitchToLookup.getNumOccurrences()
? UserSwitchToLookup
: Opts.ConvertSwitchToLookupTable;
Options.NeedCanonicalLoop = UserKeepLoops.getNumOccurrences()
? UserKeepLoops
: Opts.NeedCanonicalLoop;
Options.SinkCommonInsts = UserSinkCommonInsts.getNumOccurrences()
? UserSinkCommonInsts
: Opts.SinkCommonInsts;
PreservedAnalyses SimplifyCFGPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &TTI = AM.getResult<TargetIRAnalysis>(F);
Options.AC = &AM.getResult<AssumptionAnalysis>(F);
if (!simplifyFunctionCFG(F, TTI, Options))
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
namespace {
struct CFGSimplifyPass : public FunctionPass {
static char ID;
SimplifyCFGOptions Options;
std::function<bool(const Function &)> PredicateFtor;
CFGSimplifyPass(unsigned Threshold = 1, bool ForwardSwitchCond = false,
bool ConvertSwitch = false, bool KeepLoops = true,
bool SinkCommon = false,
std::function<bool(const Function &)> Ftor = nullptr)
: FunctionPass(ID), PredicateFtor(std::move(Ftor)) {
// Check for command-line overrides of options for debug/customization.
Options.BonusInstThreshold = UserBonusInstThreshold.getNumOccurrences()
? UserBonusInstThreshold
: Threshold;
Options.ForwardSwitchCondToPhi = UserForwardSwitchCond.getNumOccurrences()
? UserForwardSwitchCond
: ForwardSwitchCond;
Options.ConvertSwitchToLookupTable = UserSwitchToLookup.getNumOccurrences()
? UserSwitchToLookup
: ConvertSwitch;
Options.NeedCanonicalLoop =
UserKeepLoops.getNumOccurrences() ? UserKeepLoops : KeepLoops;
Options.SinkCommonInsts = UserSinkCommonInsts.getNumOccurrences()
? UserSinkCommonInsts
: SinkCommon;
bool runOnFunction(Function &F) override {
if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
return false;
Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
return simplifyFunctionCFG(F, TTI, Options);
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(unsigned Threshold, bool ForwardSwitchCond,
bool ConvertSwitch, bool KeepLoops,
bool SinkCommon,
std::function<bool(const Function &)> Ftor) {
return new CFGSimplifyPass(Threshold, ForwardSwitchCond, ConvertSwitch,
KeepLoops, SinkCommon, std::move(Ftor));