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//===--------- LoopSimplifyCFG.cpp - Loop 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 the Loop SimplifyCFG Pass. This pass is responsible for
// basic loop CFG cleanup, primarily to assist other loop passes. If you
// encounter a noncanonical CFG construct that causes another loop pass to
// perform suboptimally, this is the place to fix it up.
#include "llvm/Transforms/Scalar/LoopSimplifyCFG.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/DependenceAnalysis.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/DomTreeUpdater.h"
#include "llvm/IR/Dominators.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/LoopPassManager.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
using namespace llvm;
#define DEBUG_TYPE "loop-simplifycfg"
static cl::opt<bool> EnableTermFolding("enable-loop-simplifycfg-term-folding",
"Number of terminators folded to unconditional branches");
"Number of loop blocks deleted");
"Number of loop exiting edges deleted");
/// If \p BB is a switch or a conditional branch, but only one of its successors
/// can be reached from this block in runtime, return this successor. Otherwise,
/// return nullptr.
static BasicBlock *getOnlyLiveSuccessor(BasicBlock *BB) {
Instruction *TI = BB->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (BI->isUnconditional())
return nullptr;
if (BI->getSuccessor(0) == BI->getSuccessor(1))
return BI->getSuccessor(0);
ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
if (!Cond)
return nullptr;
return Cond->isZero() ? BI->getSuccessor(1) : BI->getSuccessor(0);
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
auto *CI = dyn_cast<ConstantInt>(SI->getCondition());
if (!CI)
return nullptr;
for (auto Case : SI->cases())
if (Case.getCaseValue() == CI)
return Case.getCaseSuccessor();
return SI->getDefaultDest();
return nullptr;
/// Helper class that can turn branches and switches with constant conditions
/// into unconditional branches.
class ConstantTerminatorFoldingImpl {
Loop &L;
LoopInfo &LI;
DominatorTree &DT;
ScalarEvolution &SE;
MemorySSAUpdater *MSSAU;
// Whether or not the current loop has irreducible CFG.
bool HasIrreducibleCFG = false;
// Whether or not the current loop will still exist after terminator constant
// folding will be done. In theory, there are two ways how it can happen:
// 1. Loop's latch(es) become unreachable from loop header;
// 2. Loop's header becomes unreachable from method entry.
// In practice, the second situation is impossible because we only modify the
// current loop and its preheader and do not affect preheader's reachibility
// from any other block. So this variable set to true means that loop's latch
// has become unreachable from loop header.
bool DeleteCurrentLoop = false;
// The blocks of the original loop that will still be reachable from entry
// after the constant folding.
SmallPtrSet<BasicBlock *, 8> LiveLoopBlocks;
// The blocks of the original loop that will become unreachable from entry
// after the constant folding.
SmallVector<BasicBlock *, 8> DeadLoopBlocks;
// The exits of the original loop that will still be reachable from entry
// after the constant folding.
SmallPtrSet<BasicBlock *, 8> LiveExitBlocks;
// The exits of the original loop that will become unreachable from entry
// after the constant folding.
SmallVector<BasicBlock *, 8> DeadExitBlocks;
// The blocks that will still be a part of the current loop after folding.
SmallPtrSet<BasicBlock *, 8> BlocksInLoopAfterFolding;
// The blocks that have terminators with constant condition that can be
// folded. Note: fold candidates should be in L but not in any of its
// subloops to avoid complex LI updates.
SmallVector<BasicBlock *, 8> FoldCandidates;
void dump() const {
dbgs() << "Constant terminator folding for loop " << L << "\n";
dbgs() << "After terminator constant-folding, the loop will";
if (!DeleteCurrentLoop)
dbgs() << " not";
dbgs() << " be destroyed\n";
auto PrintOutVector = [&](const char *Message,
const SmallVectorImpl<BasicBlock *> &S) {
dbgs() << Message << "\n";
for (const BasicBlock *BB : S)
dbgs() << "\t" << BB->getName() << "\n";
auto PrintOutSet = [&](const char *Message,
const SmallPtrSetImpl<BasicBlock *> &S) {
dbgs() << Message << "\n";
for (const BasicBlock *BB : S)
dbgs() << "\t" << BB->getName() << "\n";
PrintOutVector("Blocks in which we can constant-fold terminator:",
PrintOutSet("Live blocks from the original loop:", LiveLoopBlocks);
PrintOutVector("Dead blocks from the original loop:", DeadLoopBlocks);
PrintOutSet("Live exit blocks:", LiveExitBlocks);
PrintOutVector("Dead exit blocks:", DeadExitBlocks);
if (!DeleteCurrentLoop)
PrintOutSet("The following blocks will still be part of the loop:",
/// Whether or not the current loop has irreducible CFG.
bool hasIrreducibleCFG(LoopBlocksDFS &DFS) {
assert(DFS.isComplete() && "DFS is expected to be finished");
// Index of a basic block in RPO traversal.
DenseMap<const BasicBlock *, unsigned> RPO;
unsigned Current = 0;
for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I)
RPO[*I] = Current++;
for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
BasicBlock *BB = *I;
for (auto *Succ : successors(BB))
if (L.contains(Succ) && !LI.isLoopHeader(Succ) && RPO[BB] > RPO[Succ])
// If an edge goes from a block with greater order number into a block
// with lesses number, and it is not a loop backedge, then it can only
// be a part of irreducible non-loop cycle.
return true;
return false;
/// Fill all information about status of blocks and exits of the current loop
/// if constant folding of all branches will be done.
void analyze() {
LoopBlocksDFS DFS(&L);
assert(DFS.isComplete() && "DFS is expected to be finished");
// TODO: The algorithm below relies on both RPO and Postorder traversals.
// When the loop has only reducible CFG inside, then the invariant "all
// predecessors of X are processed before X in RPO" is preserved. However
// an irreducible loop can break this invariant (e.g. latch does not have to
// be the last block in the traversal in this case, and the algorithm relies
// on this). We can later decide to support such cases by altering the
// algorithms, but so far we just give up analyzing them.
if (hasIrreducibleCFG(DFS)) {
HasIrreducibleCFG = true;
// Collect live and dead loop blocks and exits.
for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
BasicBlock *BB = *I;
// If a loop block wasn't marked as live so far, then it's dead.
if (!LiveLoopBlocks.count(BB)) {
BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(BB);
// If a block has only one live successor, it's a candidate on constant
// folding. Only handle blocks from current loop: branches in child loops
// are skipped because if they can be folded, they should be folded during
// the processing of child loops.
if (TheOnlySucc && LI.getLoopFor(BB) == &L)
// Handle successors.
for (BasicBlock *Succ : successors(BB))
if (!TheOnlySucc || TheOnlySucc == Succ) {
if (L.contains(Succ))
// Sanity check: amount of dead and live loop blocks should match the total
// number of blocks in loop.
assert(L.getNumBlocks() == LiveLoopBlocks.size() + DeadLoopBlocks.size() &&
"Malformed block sets?");
// Now, all exit blocks that are not marked as live are dead.
SmallVector<BasicBlock *, 8> ExitBlocks;
for (auto *ExitBlock : ExitBlocks)
if (!LiveExitBlocks.count(ExitBlock))
// Whether or not the edge From->To will still be present in graph after the
// folding.
auto IsEdgeLive = [&](BasicBlock *From, BasicBlock *To) {
if (!LiveLoopBlocks.count(From))
return false;
BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(From);
return !TheOnlySucc || TheOnlySucc == To;
// The loop will not be destroyed if its latch is live.
DeleteCurrentLoop = !IsEdgeLive(L.getLoopLatch(), L.getHeader());
// If we are going to delete the current loop completely, no extra analysis
// is needed.
if (DeleteCurrentLoop)
// Otherwise, we should check which blocks will still be a part of the
// current loop after the transform.
// If the loop is live, then we should compute what blocks are still in
// loop after all branch folding has been done. A block is in loop if
// it has a live edge to another block that is in the loop; by definition,
// latch is in the loop.
auto BlockIsInLoop = [&](BasicBlock *BB) {
return any_of(successors(BB), [&](BasicBlock *Succ) {
return BlocksInLoopAfterFolding.count(Succ) && IsEdgeLive(BB, Succ);
for (auto I = DFS.beginPostorder(), E = DFS.endPostorder(); I != E; ++I) {
BasicBlock *BB = *I;
if (BlockIsInLoop(BB))
// Sanity check: header must be in loop.
assert(BlocksInLoopAfterFolding.count(L.getHeader()) &&
"Header not in loop?");
assert(BlocksInLoopAfterFolding.size() <= LiveLoopBlocks.size() &&
"All blocks that stay in loop should be live!");
/// We need to preserve static reachibility of all loop exit blocks (this is)
/// required by loop pass manager. In order to do it, we make the following
/// trick:
/// preheader:
/// <preheader code>
/// br label %loop_header
/// loop_header:
/// ...
/// br i1 false, label %dead_exit, label %loop_block
/// ...
/// We cannot simply remove edge from the loop to dead exit because in this
/// case dead_exit (and its successors) may become unreachable. To avoid that,
/// we insert the following fictive preheader:
/// preheader:
/// <preheader code>
/// switch i32 0, label %preheader-split,
/// [i32 1, label %dead_exit_1],
/// [i32 2, label %dead_exit_2],
/// ...
/// [i32 N, label %dead_exit_N],
/// preheader-split:
/// br label %loop_header
/// loop_header:
/// ...
/// br i1 false, label %dead_exit_N, label %loop_block
/// ...
/// Doing so, we preserve static reachibility of all dead exits and can later
/// remove edges from the loop to these blocks.
void handleDeadExits() {
// If no dead exits, nothing to do.
if (DeadExitBlocks.empty())
// Construct split preheader and the dummy switch to thread edges from it to
// dead exits.
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
BasicBlock *Preheader = L.getLoopPreheader();
BasicBlock *NewPreheader = Preheader->splitBasicBlock(
DTU.deleteEdge(Preheader, L.getHeader());
DTU.insertEdge(NewPreheader, L.getHeader());
DTU.insertEdge(Preheader, NewPreheader);
IRBuilder<> Builder(Preheader->getTerminator());
SwitchInst *DummySwitch =
Builder.CreateSwitch(Builder.getInt32(0), NewPreheader);
unsigned DummyIdx = 1;
for (BasicBlock *BB : DeadExitBlocks) {
SmallVector<Instruction *, 4> DeadPhis;
for (auto &PN : BB->phis())
// Eliminate all Phis from dead exits.
for (Instruction *PN : DeadPhis) {
assert(DummyIdx != 0 && "Too many dead exits!");
DummySwitch->addCase(Builder.getInt32(DummyIdx++), BB);
DTU.insertEdge(Preheader, BB);
assert(L.getLoopPreheader() == NewPreheader && "Malformed CFG?");
if (Loop *OuterLoop = LI.getLoopFor(Preheader)) {
OuterLoop->addBasicBlockToLoop(NewPreheader, LI);
// When we break dead edges, the outer loop may become unreachable from
// the current loop. We need to fix loop info accordingly. For this, we
// find the most nested loop that still contains L and remove L from all
// loops that are inside of it.
Loop *StillReachable = nullptr;
for (BasicBlock *BB : LiveExitBlocks) {
Loop *BBL = LI.getLoopFor(BB);
if (BBL && BBL->contains(L.getHeader()))
if (!StillReachable ||
BBL->getLoopDepth() > StillReachable->getLoopDepth())
StillReachable = BBL;
// Okay, our loop is no longer in the outer loop (and maybe not in some of
// its parents as well). Make the fixup.
if (StillReachable != OuterLoop) {
LI.changeLoopFor(NewPreheader, StillReachable);
for (Loop *NotContaining = OuterLoop; NotContaining != StillReachable;
NotContaining = NotContaining->getParentLoop()) {
for (auto *BB : L.blocks())
if (StillReachable)
/// Delete loop blocks that have become unreachable after folding. Make all
/// relevant updates to DT and LI.
void deleteDeadLoopBlocks() {
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
if (MSSAU) {
SmallPtrSet<BasicBlock *, 8> DeadLoopBlocksSet(DeadLoopBlocks.begin(),
for (auto *BB : DeadLoopBlocks) {
assert(BB != L.getHeader() &&
"Header of the current loop cannot be dead!");
LLVM_DEBUG(dbgs() << "Deleting dead loop block " << BB->getName()
<< "\n");
if (LI.isLoopHeader(BB)) {
assert(LI.getLoopFor(BB) != &L && "Attempt to remove current loop!");
DeleteDeadBlock(BB, &DTU);
/// Constant-fold terminators of blocks acculumated in FoldCandidates into the
/// unconditional branches.
void foldTerminators() {
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
for (BasicBlock *BB : FoldCandidates) {
assert(LI.getLoopFor(BB) == &L && "Should be a loop block!");
BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(BB);
assert(TheOnlySucc && "Should have one live successor!");
LLVM_DEBUG(dbgs() << "Replacing terminator of " << BB->getName()
<< " with an unconditional branch to the block "
<< TheOnlySucc->getName() << "\n");
SmallPtrSet<BasicBlock *, 2> DeadSuccessors;
// Remove all BB's successors except for the live one.
unsigned TheOnlySuccDuplicates = 0;
for (auto *Succ : successors(BB))
if (Succ != TheOnlySucc) {
// If our successor lies in a different loop, we don't want to remove
// the one-input Phi because it is a LCSSA Phi.
bool PreserveLCSSAPhi = !L.contains(Succ);
Succ->removePredecessor(BB, PreserveLCSSAPhi);
if (MSSAU)
MSSAU->removeEdge(BB, Succ);
} else
assert(TheOnlySuccDuplicates > 0 && "Should be!");
// If TheOnlySucc was BB's successor more than once, after transform it
// will be its successor only once. Remove redundant inputs from
// TheOnlySucc's Phis.
bool PreserveLCSSAPhi = !L.contains(TheOnlySucc);
for (unsigned Dup = 1; Dup < TheOnlySuccDuplicates; ++Dup)
TheOnlySucc->removePredecessor(BB, PreserveLCSSAPhi);
if (MSSAU && TheOnlySuccDuplicates > 1)
MSSAU->removeDuplicatePhiEdgesBetween(BB, TheOnlySucc);
IRBuilder<> Builder(BB->getContext());
Instruction *Term = BB->getTerminator();
for (auto *DeadSucc : DeadSuccessors)
DTU.deleteEdge(BB, DeadSucc);
ConstantTerminatorFoldingImpl(Loop &L, LoopInfo &LI, DominatorTree &DT,
ScalarEvolution &SE,
MemorySSAUpdater *MSSAU)
bool run() {
assert(L.getLoopLatch() && "Should be single latch!");
// Collect all available information about status of blocks after constant
// folding.
LLVM_DEBUG(dbgs() << "In function " << L.getHeader()->getParent()->getName()
<< ": ");
if (HasIrreducibleCFG) {
LLVM_DEBUG(dbgs() << "Loops with irreducible CFG are not supported!\n");
return false;
// Nothing to constant-fold.
if (FoldCandidates.empty()) {
dbgs() << "No constant terminator folding candidates found in loop "
<< L.getHeader()->getName() << "\n");
return false;
// TODO: Support deletion of the current loop.
if (DeleteCurrentLoop) {
<< "Give up constant terminator folding in loop "
<< L.getHeader()->getName()
<< ": we don't currently support deletion of the current loop.\n");
return false;
// TODO: Support blocks that are not dead, but also not in loop after the
// folding.
if (BlocksInLoopAfterFolding.size() + DeadLoopBlocks.size() !=
L.getNumBlocks()) {
dbgs() << "Give up constant terminator folding in loop "
<< L.getHeader()->getName()
<< ": we don't currently"
" support blocks that are not dead, but will stop "
"being a part of the loop after constant-folding.\n");
return false;
// Dump analysis results.
LLVM_DEBUG(dbgs() << "Constant-folding " << FoldCandidates.size()
<< " terminators in loop " << L.getHeader()->getName()
<< "\n");
// Make the actual transforms.
if (!DeadLoopBlocks.empty()) {
LLVM_DEBUG(dbgs() << "Deleting " << DeadLoopBlocks.size()
<< " dead blocks in loop " << L.getHeader()->getName()
<< "\n");
#ifndef NDEBUG
// Make sure that we have preserved all data structures after the transform.
return true;
/// Turn branches and switches with known constant conditions into unconditional
/// branches.
static bool constantFoldTerminators(Loop &L, DominatorTree &DT, LoopInfo &LI,
ScalarEvolution &SE,
MemorySSAUpdater *MSSAU) {
if (!EnableTermFolding)
return false;
// To keep things simple, only process loops with single latch. We
// canonicalize most loops to this form. We can support multi-latch if needed.
if (!L.getLoopLatch())
return false;
ConstantTerminatorFoldingImpl BranchFolder(L, LI, DT, SE, MSSAU);
static bool mergeBlocksIntoPredecessors(Loop &L, DominatorTree &DT,
LoopInfo &LI, MemorySSAUpdater *MSSAU) {
bool Changed = false;
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
// Copy blocks into a temporary array to avoid iterator invalidation issues
// as we remove them.
SmallVector<WeakTrackingVH, 16> Blocks(L.blocks());
for (auto &Block : Blocks) {
// Attempt to merge blocks in the trivial case. Don't modify blocks which
// belong to other loops.
BasicBlock *Succ = cast_or_null<BasicBlock>(Block);
if (!Succ)
BasicBlock *Pred = Succ->getSinglePredecessor();
if (!Pred || !Pred->getSingleSuccessor() || LI.getLoopFor(Pred) != &L)
// Merge Succ into Pred and delete it.
MergeBlockIntoPredecessor(Succ, &DTU, &LI, MSSAU);
Changed = true;
return Changed;
static bool simplifyLoopCFG(Loop &L, DominatorTree &DT, LoopInfo &LI,
ScalarEvolution &SE, MemorySSAUpdater *MSSAU) {
bool Changed = false;
// Constant-fold terminators with known constant conditions.
Changed |= constantFoldTerminators(L, DT, LI, SE, MSSAU);
// Eliminate unconditional branches by merging blocks into their predecessors.
Changed |= mergeBlocksIntoPredecessors(L, DT, LI, MSSAU);
if (Changed)
return Changed;
PreservedAnalyses LoopSimplifyCFGPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &) {
Optional<MemorySSAUpdater> MSSAU;
if (EnableMSSALoopDependency && AR.MSSA)
MSSAU = MemorySSAUpdater(AR.MSSA);
if (!simplifyLoopCFG(L, AR.DT, AR.LI, AR.SE,
MSSAU.hasValue() ? MSSAU.getPointer() : nullptr))
return PreservedAnalyses::all();
return getLoopPassPreservedAnalyses();
namespace {
class LoopSimplifyCFGLegacyPass : public LoopPass {
static char ID; // Pass ID, replacement for typeid
LoopSimplifyCFGLegacyPass() : LoopPass(ID) {
bool runOnLoop(Loop *L, LPPassManager &) override {
if (skipLoop(L))
return false;
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
Optional<MemorySSAUpdater> MSSAU;
if (EnableMSSALoopDependency) {
MemorySSA *MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
MSSAU = MemorySSAUpdater(MSSA);
if (VerifyMemorySSA)
return simplifyLoopCFG(*L, DT, LI, SE,
MSSAU.hasValue() ? MSSAU.getPointer() : nullptr);
void getAnalysisUsage(AnalysisUsage &AU) const override {
if (EnableMSSALoopDependency) {
char LoopSimplifyCFGLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
"Simplify loop CFG", false, false)
INITIALIZE_PASS_END(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
"Simplify loop CFG", false, false)
Pass *llvm::createLoopSimplifyCFGPass() {
return new LoopSimplifyCFGLegacyPass();