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//===- BreakCriticalEdges.cpp - Critical Edge Elimination 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
// BreakCriticalEdges pass - Break all of the critical edges in the CFG by
// inserting a dummy basic block. This pass may be "required" by passes that
// cannot deal with critical edges. For this usage, the structure type is
// forward declared. This pass obviously invalidates the CFG, but can update
// dominator trees.
#include "llvm/Transforms/Utils/BreakCriticalEdges.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Type.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
using namespace llvm;
#define DEBUG_TYPE "break-crit-edges"
STATISTIC(NumBroken, "Number of blocks inserted");
namespace {
struct BreakCriticalEdges : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
BreakCriticalEdges() : FunctionPass(ID) {
bool runOnFunction(Function &F) override {
auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
auto *PDTWP = getAnalysisIfAvailable<PostDominatorTreeWrapperPass>();
auto *PDT = PDTWP ? &PDTWP->getPostDomTree() : nullptr;
auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
unsigned N =
SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI, nullptr, PDT));
NumBroken += N;
return N > 0;
void getAnalysisUsage(AnalysisUsage &AU) const override {
// No loop canonicalization guarantees are broken by this pass.
char BreakCriticalEdges::ID = 0;
INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
"Break critical edges in CFG", false, false)
// Publicly exposed interface to pass...
char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID;
FunctionPass *llvm::createBreakCriticalEdgesPass() {
return new BreakCriticalEdges();
PreservedAnalyses BreakCriticalEdgesPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
auto *LI = AM.getCachedResult<LoopAnalysis>(F);
unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
NumBroken += N;
if (N == 0)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
// Implementation of the external critical edge manipulation functions
/// When a loop exit edge is split, LCSSA form may require new PHIs in the new
/// exit block. This function inserts the new PHIs, as needed. Preds is a list
/// of preds inside the loop, SplitBB is the new loop exit block, and DestBB is
/// the old loop exit, now the successor of SplitBB.
static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds,
BasicBlock *SplitBB,
BasicBlock *DestBB) {
// SplitBB shouldn't have anything non-trivial in it yet.
assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() ||
SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!");
// For each PHI in the destination block.
for (PHINode &PN : DestBB->phis()) {
unsigned Idx = PN.getBasicBlockIndex(SplitBB);
Value *V = PN.getIncomingValue(Idx);
// If the input is a PHI which already satisfies LCSSA, don't create
// a new one.
if (const PHINode *VP = dyn_cast<PHINode>(V))
if (VP->getParent() == SplitBB)
// Otherwise a new PHI is needed. Create one and populate it.
PHINode *NewPN = PHINode::Create(
PN.getType(), Preds.size(), "split",
SplitBB->isLandingPad() ? &SplitBB->front() : SplitBB->getTerminator());
for (unsigned i = 0, e = Preds.size(); i != e; ++i)
NewPN->addIncoming(V, Preds[i]);
// Update the original PHI.
PN.setIncomingValue(Idx, NewPN);
BasicBlock *llvm::SplitCriticalEdge(Instruction *TI, unsigned SuccNum,
const CriticalEdgeSplittingOptions &Options,
const Twine &BBName) {
if (!isCriticalEdge(TI, SuccNum, Options.MergeIdenticalEdges))
return nullptr;
assert(!isa<IndirectBrInst>(TI) &&
"Cannot split critical edge from IndirectBrInst");
BasicBlock *TIBB = TI->getParent();
BasicBlock *DestBB = TI->getSuccessor(SuccNum);
// Splitting the critical edge to a pad block is non-trivial. Don't do
// it in this generic function.
if (DestBB->isEHPad()) return nullptr;
if (Options.IgnoreUnreachableDests &&
return nullptr;
auto *LI = Options.LI;
SmallVector<BasicBlock *, 4> LoopPreds;
// Check if extra modifications will be required to preserve loop-simplify
// form after splitting. If it would require splitting blocks with IndirectBr
// or CallBr terminators, bail out if preserving loop-simplify form is
// requested.
if (LI) {
if (Loop *TIL = LI->getLoopFor(TIBB)) {
// The only way that we can break LoopSimplify form by splitting a
// critical edge is if after the split there exists some edge from TIL to
// DestBB *and* the only edge into DestBB from outside of TIL is that of
// NewBB. If the first isn't true, then LoopSimplify still holds, NewBB
// is the new exit block and it has no non-loop predecessors. If the
// second isn't true, then DestBB was not in LoopSimplify form prior to
// the split as it had a non-loop predecessor. In both of these cases,
// the predecessor must be directly in TIL, not in a subloop, or again
// LoopSimplify doesn't hold.
for (BasicBlock *P : predecessors(DestBB)) {
if (P == TIBB)
continue; // The new block is known.
if (LI->getLoopFor(P) != TIL) {
// No need to re-simplify, it wasn't to start with.
// Loop-simplify form can be preserved, if we can split all in-loop
// predecessors.
if (any_of(LoopPreds, [](BasicBlock *Pred) {
const Instruction *T = Pred->getTerminator();
if (const auto *CBR = dyn_cast<CallBrInst>(T))
return CBR->getDefaultDest() != Pred;
return isa<IndirectBrInst>(T);
})) {
if (Options.PreserveLoopSimplify)
return nullptr;
// Create a new basic block, linking it into the CFG.
BasicBlock *NewBB = nullptr;
if (BBName.str() != "")
NewBB = BasicBlock::Create(TI->getContext(), BBName);
NewBB = BasicBlock::Create(TI->getContext(), TIBB->getName() + "." +
DestBB->getName() +
// Create our unconditional branch.
BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
// Insert the block into the function... right after the block TI lives in.
Function &F = *TIBB->getParent();
Function::iterator FBBI = TIBB->getIterator();
F.getBasicBlockList().insert(++FBBI, NewBB);
// Branch to the new block, breaking the edge.
TI->setSuccessor(SuccNum, NewBB);
// If there are any PHI nodes in DestBB, we need to update them so that they
// merge incoming values from NewBB instead of from TIBB.
unsigned BBIdx = 0;
for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
// We no longer enter through TIBB, now we come in through NewBB.
// Revector exactly one entry in the PHI node that used to come from
// TIBB to come from NewBB.
PHINode *PN = cast<PHINode>(I);
// Reuse the previous value of BBIdx if it lines up. In cases where we
// have multiple phi nodes with *lots* of predecessors, this is a speed
// win because we don't have to scan the PHI looking for TIBB. This
// happens because the BB list of PHI nodes are usually in the same
// order.
if (PN->getIncomingBlock(BBIdx) != TIBB)
BBIdx = PN->getBasicBlockIndex(TIBB);
PN->setIncomingBlock(BBIdx, NewBB);
// If there are any other edges from TIBB to DestBB, update those to go
// through the split block, making those edges non-critical as well (and
// reducing the number of phi entries in the DestBB if relevant).
if (Options.MergeIdenticalEdges) {
for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
if (TI->getSuccessor(i) != DestBB) continue;
// Remove an entry for TIBB from DestBB phi nodes.
DestBB->removePredecessor(TIBB, Options.KeepOneInputPHIs);
// We found another edge to DestBB, go to NewBB instead.
TI->setSuccessor(i, NewBB);
// If we have nothing to update, just return.
auto *DT = Options.DT;
auto *PDT = Options.PDT;
auto *MSSAU = Options.MSSAU;
if (MSSAU)
DestBB, NewBB, {TIBB}, Options.MergeIdenticalEdges);
if (!DT && !PDT && !LI)
return NewBB;
if (DT || PDT) {
// Update the DominatorTree.
// ---> NewBB -----\
// / V
// TIBB -------\\------> DestBB
// First, inform the DT about the new path from TIBB to DestBB via NewBB,
// then delete the old edge from TIBB to DestBB. By doing this in that order
// DestBB stays reachable in the DT the whole time and its subtree doesn't
// get disconnected.
SmallVector<DominatorTree::UpdateType, 3> Updates;
Updates.push_back({DominatorTree::Insert, TIBB, NewBB});
Updates.push_back({DominatorTree::Insert, NewBB, DestBB});
if (!llvm::is_contained(successors(TIBB), DestBB))
Updates.push_back({DominatorTree::Delete, TIBB, DestBB});
if (DT)
if (PDT)
// Update LoopInfo if it is around.
if (LI) {
if (Loop *TIL = LI->getLoopFor(TIBB)) {
// If one or the other blocks were not in a loop, the new block is not
// either, and thus LI doesn't need to be updated.
if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
if (TIL == DestLoop) {
// Both in the same loop, the NewBB joins loop.
DestLoop->addBasicBlockToLoop(NewBB, *LI);
} else if (TIL->contains(DestLoop)) {
// Edge from an outer loop to an inner loop. Add to the outer loop.
TIL->addBasicBlockToLoop(NewBB, *LI);
} else if (DestLoop->contains(TIL)) {
// Edge from an inner loop to an outer loop. Add to the outer loop.
DestLoop->addBasicBlockToLoop(NewBB, *LI);
} else {
// Edge from two loops with no containment relation. Because these
// are natural loops, we know that the destination block must be the
// header of its loop (adding a branch into a loop elsewhere would
// create an irreducible loop).
assert(DestLoop->getHeader() == DestBB &&
"Should not create irreducible loops!");
if (Loop *P = DestLoop->getParentLoop())
P->addBasicBlockToLoop(NewBB, *LI);
// If TIBB is in a loop and DestBB is outside of that loop, we may need
// to update LoopSimplify form and LCSSA form.
if (!TIL->contains(DestBB)) {
assert(!TIL->contains(NewBB) &&
"Split point for loop exit is contained in loop!");
// Update LCSSA form in the newly created exit block.
if (Options.PreserveLCSSA) {
createPHIsForSplitLoopExit(TIBB, NewBB, DestBB);
if (!LoopPreds.empty()) {
assert(!DestBB->isEHPad() && "We don't split edges to EH pads!");
BasicBlock *NewExitBB = SplitBlockPredecessors(
DestBB, LoopPreds, "split", DT, LI, MSSAU, Options.PreserveLCSSA);
if (Options.PreserveLCSSA)
createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB);
return NewBB;
// Return the unique indirectbr predecessor of a block. This may return null
// even if such a predecessor exists, if it's not useful for splitting.
// If a predecessor is found, OtherPreds will contain all other (non-indirectbr)
// predecessors of BB.
static BasicBlock *
findIBRPredecessor(BasicBlock *BB, SmallVectorImpl<BasicBlock *> &OtherPreds) {
// If the block doesn't have any PHIs, we don't care about it, since there's
// no point in splitting it.
PHINode *PN = dyn_cast<PHINode>(BB->begin());
if (!PN)
return nullptr;
// Verify we have exactly one IBR predecessor.
// Conservatively bail out if one of the other predecessors is not a "regular"
// terminator (that is, not a switch or a br).
BasicBlock *IBB = nullptr;
for (unsigned Pred = 0, E = PN->getNumIncomingValues(); Pred != E; ++Pred) {
BasicBlock *PredBB = PN->getIncomingBlock(Pred);
Instruction *PredTerm = PredBB->getTerminator();
switch (PredTerm->getOpcode()) {
case Instruction::IndirectBr:
if (IBB)
return nullptr;
IBB = PredBB;
case Instruction::Br:
case Instruction::Switch:
return nullptr;
return IBB;
bool llvm::SplitIndirectBrCriticalEdges(Function &F,
BranchProbabilityInfo *BPI,
BlockFrequencyInfo *BFI) {
// Check whether the function has any indirectbrs, and collect which blocks
// they may jump to. Since most functions don't have indirect branches,
// this lowers the common case's overhead to O(Blocks) instead of O(Edges).
SmallSetVector<BasicBlock *, 16> Targets;
for (auto &BB : F) {
auto *IBI = dyn_cast<IndirectBrInst>(BB.getTerminator());
if (!IBI)
for (unsigned Succ = 0, E = IBI->getNumSuccessors(); Succ != E; ++Succ)
if (Targets.empty())
return false;
bool ShouldUpdateAnalysis = BPI && BFI;
bool Changed = false;
for (BasicBlock *Target : Targets) {
SmallVector<BasicBlock *, 16> OtherPreds;
BasicBlock *IBRPred = findIBRPredecessor(Target, OtherPreds);
// If we did not found an indirectbr, or the indirectbr is the only
// incoming edge, this isn't the kind of edge we're looking for.
if (!IBRPred || OtherPreds.empty())
// Don't even think about ehpads/landingpads.
Instruction *FirstNonPHI = Target->getFirstNonPHI();
if (FirstNonPHI->isEHPad() || Target->isLandingPad())
// Remember edge probabilities if needed.
SmallVector<BranchProbability, 4> EdgeProbabilities;
if (ShouldUpdateAnalysis) {
for (unsigned I = 0, E = Target->getTerminator()->getNumSuccessors();
I < E; ++I)
EdgeProbabilities.emplace_back(BPI->getEdgeProbability(Target, I));
BasicBlock *BodyBlock = Target->splitBasicBlock(FirstNonPHI, ".split");
if (ShouldUpdateAnalysis) {
// Copy the BFI/BPI from Target to BodyBlock.
BPI->setEdgeProbability(BodyBlock, EdgeProbabilities);
BFI->setBlockFreq(BodyBlock, BFI->getBlockFreq(Target).getFrequency());
// It's possible Target was its own successor through an indirectbr.
// In this case, the indirectbr now comes from BodyBlock.
if (IBRPred == Target)
IBRPred = BodyBlock;
// At this point Target only has PHIs, and BodyBlock has the rest of the
// block's body. Create a copy of Target that will be used by the "direct"
// preds.
ValueToValueMapTy VMap;
BasicBlock *DirectSucc = CloneBasicBlock(Target, VMap, ".clone", &F);
BlockFrequency BlockFreqForDirectSucc;
for (BasicBlock *Pred : OtherPreds) {
// If the target is a loop to itself, then the terminator of the split
// block (BodyBlock) needs to be updated.
BasicBlock *Src = Pred != Target ? Pred : BodyBlock;
Src->getTerminator()->replaceUsesOfWith(Target, DirectSucc);
if (ShouldUpdateAnalysis)
BlockFreqForDirectSucc += BFI->getBlockFreq(Src) *
BPI->getEdgeProbability(Src, DirectSucc);
if (ShouldUpdateAnalysis) {
BFI->setBlockFreq(DirectSucc, BlockFreqForDirectSucc.getFrequency());
BlockFrequency NewBlockFreqForTarget =
BFI->getBlockFreq(Target) - BlockFreqForDirectSucc;
BFI->setBlockFreq(Target, NewBlockFreqForTarget.getFrequency());
// Ok, now fix up the PHIs. We know the two blocks only have PHIs, and that
// they are clones, so the number of PHIs are the same.
// (a) Remove the edge coming from IBRPred from the "Direct" PHI
// (b) Leave that as the only edge in the "Indirect" PHI.
// (c) Merge the two in the body block.
BasicBlock::iterator Indirect = Target->begin(),
End = Target->getFirstNonPHI()->getIterator();
BasicBlock::iterator Direct = DirectSucc->begin();
BasicBlock::iterator MergeInsert = BodyBlock->getFirstInsertionPt();
assert(&*End == Target->getTerminator() &&
"Block was expected to only contain PHIs");
while (Indirect != End) {
PHINode *DirPHI = cast<PHINode>(Direct);
PHINode *IndPHI = cast<PHINode>(Indirect);
// Now, clean up - the direct block shouldn't get the indirect value,
// and vice versa.
// Advance the pointer here, to avoid invalidation issues when the old
// PHI is erased.
PHINode *NewIndPHI = PHINode::Create(IndPHI->getType(), 1, "ind", IndPHI);
// Create a PHI in the body block, to merge the direct and indirect
// predecessors.
PHINode *MergePHI =
PHINode::Create(IndPHI->getType(), 2, "merge", &*MergeInsert);
MergePHI->addIncoming(NewIndPHI, Target);
MergePHI->addIncoming(DirPHI, DirectSucc);
Changed = true;
return Changed;