Add support for tail duplication to BranchFolding, and extend
tail merging support to handle more cases.
- Recognize several cases where tail merging is beneficial even when
the tail size is smaller than the generic threshold.
- Make use of MachineInstrDesc::isBarrier to help detect
non-fallthrough blocks.
- Check for and avoid disrupting fall-through edges in more cases.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86871 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index 38f11ab..3fa8c95 100644
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -32,6 +32,7 @@
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
@@ -48,10 +49,16 @@
cl::desc("Max number of predecessors to consider tail merging"),
cl::init(150), cl::Hidden);
+// Heuristic for tail merging (and, inversely, tail duplication).
+// TODO: This should be replaced with a target query.
+static cl::opt<unsigned>
+TailMergeSize("tail-merge-size",
+ cl::desc("Min number of instructions to consider tail merging"),
+ cl::init(3), cl::Hidden);
char BranchFolderPass::ID = 0;
-FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
+Pass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
return new BranchFolderPass(DefaultEnableTailMerge);
}
@@ -442,6 +449,25 @@
}
}
+/// CountTerminators - Count the number of terminators in the given
+/// block and set I to the position of the first non-terminator, if there
+/// is one, or MBB->end() otherwise.
+static unsigned CountTerminators(MachineBasicBlock *MBB,
+ MachineBasicBlock::iterator &I) {
+ I = MBB->end();
+ unsigned NumTerms = 0;
+ for (;;) {
+ if (I == MBB->begin()) {
+ I = MBB->end();
+ break;
+ }
+ --I;
+ if (!I->getDesc().isTerminator()) break;
+ ++NumTerms;
+ }
+ return NumTerms;
+}
+
/// ProfitableToMerge - Check if two machine basic blocks have a common tail
/// and decide if it would be profitable to merge those tails. Return the
/// length of the common tail and iterators to the first common instruction
@@ -451,16 +477,35 @@
unsigned minCommonTailLength,
unsigned &CommonTailLen,
MachineBasicBlock::iterator &I1,
- MachineBasicBlock::iterator &I2) {
+ MachineBasicBlock::iterator &I2,
+ MachineBasicBlock *SuccBB,
+ MachineBasicBlock *PredBB) {
CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
MachineFunction *MF = MBB1->getParent();
- if (CommonTailLen >= minCommonTailLength)
- return true;
-
if (CommonTailLen == 0)
return false;
+ // It's almost always profitable to merge any number of non-terminator
+ // instructions with the block that falls through into the common successor.
+ if (MBB1 == PredBB || MBB2 == PredBB) {
+ MachineBasicBlock::iterator I;
+ unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
+ if (CommonTailLen > NumTerms)
+ return true;
+ }
+
+ // If both blocks have an unconditional branch temporarily stripped out,
+ // treat that as an additional common instruction.
+ if (MBB1 != PredBB && MBB2 != PredBB &&
+ !MBB1->back().getDesc().isBarrier() &&
+ !MBB2->back().getDesc().isBarrier())
+ --minCommonTailLength;
+
+ // Check if the common tail is long enough to be worthwhile.
+ if (CommonTailLen >= minCommonTailLength)
+ return true;
+
// If we are optimizing for code size, 1 instruction in common is enough if
// we don't have to split a block. At worst we will be replacing a
// fallthrough into the common tail with a branch, which at worst breaks
@@ -483,7 +528,9 @@
/// those blocks appear in MergePotentials (where they are not necessarily
/// consecutive).
unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
- unsigned minCommonTailLength) {
+ unsigned minCommonTailLength,
+ MachineBasicBlock *SuccBB,
+ MachineBasicBlock *PredBB) {
unsigned maxCommonTailLength = 0U;
SameTails.clear();
MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
@@ -495,7 +542,8 @@
for (MPIterator I = prior(CurMPIter); I->first == CurHash ; --I) {
unsigned CommonTailLen;
if (ProfitableToMerge(CurMPIter->second, I->second, minCommonTailLength,
- CommonTailLen, TrialBBI1, TrialBBI2)) {
+ CommonTailLen, TrialBBI1, TrialBBI2,
+ SuccBB, PredBB)) {
if (CommonTailLen > maxCommonTailLength) {
SameTails.clear();
maxCommonTailLength = CommonTailLen;
@@ -581,16 +629,62 @@
// The lone predecessor of Succ that falls through into Succ,
// if any, is given in PredBB.
-bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
- MachineBasicBlock* PredBB) {
+bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
+ MachineBasicBlock* PredBB) {
bool MadeChange = false;
- // It doesn't make sense to save a single instruction since tail merging
- // will add a jump.
- // FIXME: Ask the target to provide the threshold?
- unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1;
+ // Except for the special cases below, tail-merge if there are at least
+ // this many instructions in common.
+ unsigned minCommonTailLength = TailMergeSize;
- DEBUG(errs() << "\nTryMergeBlocks " << MergePotentials.size() << '\n');
+ // If there's a successor block, there are some cases which don't require
+ // new branching and as such are very likely to be profitable.
+ if (SuccBB) {
+ if (SuccBB->pred_size() == MergePotentials.size() &&
+ !MergePotentials[0].second->empty()) {
+ // If all the predecessors have at least one tail instruction in common,
+ // merging is very likely to be a win since it won't require an increase
+ // in static branches, and it will decrease the static instruction count.
+ bool AllPredsMatch = true;
+ MachineBasicBlock::iterator FirstNonTerm;
+ unsigned MinNumTerms = CountTerminators(MergePotentials[0].second,
+ FirstNonTerm);
+ if (FirstNonTerm != MergePotentials[0].second->end()) {
+ for (unsigned i = 1, e = MergePotentials.size(); i != e; ++i) {
+ MachineBasicBlock::iterator OtherFirstNonTerm;
+ unsigned NumTerms = CountTerminators(MergePotentials[0].second,
+ OtherFirstNonTerm);
+ if (NumTerms < MinNumTerms)
+ MinNumTerms = NumTerms;
+ if (OtherFirstNonTerm == MergePotentials[i].second->end() ||
+ OtherFirstNonTerm->isIdenticalTo(FirstNonTerm)) {
+ AllPredsMatch = false;
+ break;
+ }
+ }
+
+ // If they all have an instruction in common, do any amount of merging.
+ if (AllPredsMatch)
+ minCommonTailLength = MinNumTerms + 1;
+ }
+ }
+ }
+
+ DEBUG(errs() << "\nTryTailMergeBlocks: ";
+ for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
+ errs() << "BB#" << MergePotentials[i].second->getNumber()
+ << (i == e-1 ? "" : ", ");
+ errs() << "\n";
+ if (SuccBB) {
+ errs() << " with successor BB#" << SuccBB->getNumber() << '\n';
+ if (PredBB)
+ errs() << " which has fall-through from BB#"
+ << PredBB->getNumber() << "\n";
+ }
+ errs() << "Looking for common tails of at least "
+ << minCommonTailLength << " instruction"
+ << (minCommonTailLength == 1 ? "" : "s") << '\n';
+ );
// Sort by hash value so that blocks with identical end sequences sort
// together.
@@ -603,7 +697,8 @@
// Build SameTails, identifying the set of blocks with this hash code
// and with the maximum number of instructions in common.
unsigned maxCommonTailLength = ComputeSameTails(CurHash,
- minCommonTailLength);
+ minCommonTailLength,
+ SuccBB, PredBB);
// If we didn't find any pair that has at least minCommonTailLength
// instructions in common, remove all blocks with this hash code and retry.
@@ -621,27 +716,35 @@
unsigned int commonTailIndex, i;
for (commonTailIndex=SameTails.size(), i=0; i<SameTails.size(); i++) {
MachineBasicBlock *MBB = SameTails[i].first->second;
- if (MBB->begin() == SameTails[i].second && MBB != EntryBB) {
+ if (MBB == EntryBB)
+ continue;
+ if (MBB == PredBB) {
commonTailIndex = i;
- if (MBB == PredBB)
- break;
+ break;
}
+ if (MBB->begin() == SameTails[i].second)
+ commonTailIndex = i;
}
- if (commonTailIndex == SameTails.size()) {
+ if (commonTailIndex == SameTails.size() ||
+ (SameTails[commonTailIndex].first->second == PredBB &&
+ SameTails[commonTailIndex].first->second->begin() !=
+ SameTails[i].second)) {
// None of the blocks consist entirely of the common tail.
// Split a block so that one does.
- commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
+ commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
}
MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second;
// MBB is common tail. Adjust all other BB's to jump to this one.
// Traversal must be forwards so erases work.
- DEBUG(errs() << "\nUsing common tail BB#" << MBB->getNumber() << " for ");
- for (unsigned int i=0; i<SameTails.size(); ++i) {
+ DEBUG(errs() << "\nUsing common tail in BB#" << MBB->getNumber()
+ << " for ");
+ for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
if (commonTailIndex == i)
continue;
- DEBUG(errs() << "BB#" << SameTails[i].first->second->getNumber() << ", ");
+ DEBUG(errs() << "BB#" << SameTails[i].first->second->getNumber()
+ << (i == e-1 ? "" : ", "));
// Hack the end off BB i, making it jump to BB commonTailIndex instead.
ReplaceTailWithBranchTo(SameTails[i].second, MBB);
// BB i is no longer a predecessor of SuccBB; remove it from the worklist.
@@ -671,7 +774,7 @@
// See if we can do any tail merging on those.
if (MergePotentials.size() < TailMergeThreshold &&
MergePotentials.size() >= 2)
- MadeChange |= TryMergeBlocks(NULL, NULL);
+ MadeChange |= TryTailMergeBlocks(NULL, NULL);
// Look at blocks (IBB) with multiple predecessors (PBB).
// We change each predecessor to a canonical form, by
@@ -692,7 +795,8 @@
// a compile-time infinite loop repeatedly doing and undoing the same
// transformations.)
- for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
+ for (MachineFunction::iterator I = next(MF.begin()), E = MF.end();
+ I != E; ++I) {
if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
MachineBasicBlock *IBB = I;
@@ -754,13 +858,13 @@
}
}
if (MergePotentials.size() >= 2)
- MadeChange |= TryMergeBlocks(I, PredBB);
+ MadeChange |= TryTailMergeBlocks(IBB, PredBB);
// Reinsert an unconditional branch if needed.
- // The 1 below can occur as a result of removing blocks in TryMergeBlocks.
- PredBB = prior(I); // this may have been changed in TryMergeBlocks
+ // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks.
+ PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
if (MergePotentials.size() == 1 &&
MergePotentials.begin()->second != PredBB)
- FixTail(MergePotentials.begin()->second, I, TII);
+ FixTail(MergePotentials.begin()->second, IBB, TII);
}
}
return MadeChange;
@@ -813,9 +917,17 @@
if (!CurBB->isSuccessor(Fallthrough))
return false;
- // If we couldn't analyze the branch, assume it could fall through.
- if (BranchUnAnalyzable) return true;
-
+ // If we couldn't analyze the branch, examine the last instruction.
+ // If the block doesn't end in a known control barrier, assume fallthrough
+ // is possible. The isPredicable check is needed because this code can be
+ // called during IfConversion, where an instruction which is normally a
+ // Barrier is predicated and thus no longer an actual control barrier. This
+ // is over-conservative though, because if an instruction isn't actually
+ // predicated we could still treat it like a barrier.
+ if (BranchUnAnalyzable)
+ return CurBB->empty() || !CurBB->back().getDesc().isBarrier() ||
+ CurBB->back().getDesc().isPredicable();
+
// If there is no branch, control always falls through.
if (TBB == 0) return true;
@@ -870,11 +982,108 @@
return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
}
+/// TailDuplicate - MBB unconditionally branches to SuccBB. If it is profitable,
+/// duplicate SuccBB's contents in MBB to eliminate the branch.
+bool BranchFolder::TailDuplicate(MachineBasicBlock *TailBB,
+ bool PrevFallsThrough,
+ MachineFunction &MF) {
+ // Don't try to tail-duplicate single-block loops.
+ if (TailBB->isSuccessor(TailBB))
+ return false;
+
+ // Don't tail-duplicate a block which will soon be folded into its successor.
+ if (TailBB->succ_size() == 1 &&
+ TailBB->succ_begin()[0]->pred_size() == 1)
+ return false;
+
+ // Duplicate up to one less that the tail-merge threshold, so that we don't
+ // get into an infinite loop between duplicating and merging. When optimizing
+ // for size, duplicate only one, because one branch instruction can be
+ // eliminated to compensate for the duplication.
+ unsigned MaxDuplicateCount =
+ MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize) ?
+ 1 : (TailMergeSize - 1);
+
+ // Check the instructions in the block to determine whether tail-duplication
+ // is invalid or unlikely to be unprofitable.
+ unsigned i = 0;
+ bool HasCall = false;
+ for (MachineBasicBlock::iterator I = TailBB->begin();
+ I != TailBB->end(); ++I, ++i) {
+ // Non-duplicable things shouldn't be tail-duplicated.
+ if (I->getDesc().isNotDuplicable()) return false;
+ // Don't duplicate more than the threshold.
+ if (i == MaxDuplicateCount) return false;
+ // Remember if we saw a call.
+ if (I->getDesc().isCall()) HasCall = true;
+ }
+ // Heuristically, don't tail-duplicate calls if it would expand code size,
+ // as it's less likely to be worth the extra cost.
+ if (i > 1 && HasCall)
+ return false;
+
+ // Iterate through all the unique predecessors and tail-duplicate this
+ // block into them, if possible. Copying the list ahead of time also
+ // avoids trouble with the predecessor list reallocating.
+ bool Changed = false;
+ SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
+ TailBB->pred_end());
+ for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
+ PE = Preds.end(); PI != PE; ++PI) {
+ MachineBasicBlock *PredBB = *PI;
+
+ assert(TailBB != PredBB &&
+ "Single-block loop should have been rejected earlier!");
+ if (PredBB->succ_size() > 1) continue;
+
+ MachineBasicBlock *PredTBB, *PredFBB;
+ SmallVector<MachineOperand, 4> PredCond;
+ if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
+ continue;
+ if (!PredCond.empty())
+ continue;
+ // EH edges are ignored by AnalyzeBranch.
+ if (PredBB->succ_size() != 1)
+ continue;
+ // Don't duplicate into a fall-through predecessor unless its the
+ // only predecessor.
+ if (&*next(MachineFunction::iterator(PredBB)) == TailBB &&
+ PrevFallsThrough &&
+ TailBB->pred_size() != 1)
+ continue;
+
+ DEBUG(errs() << "\nTail-duplicating into PredBB: " << *PredBB
+ << "From Succ: " << *TailBB);
+
+ // Remove PredBB's unconditional branch.
+ TII->RemoveBranch(*PredBB);
+ // Clone the contents of TailBB into PredBB.
+ for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end();
+ I != E; ++I) {
+ MachineInstr *NewMI = MF.CloneMachineInstr(I);
+ PredBB->insert(PredBB->end(), NewMI);
+ }
+
+ // Update the CFG.
+ PredBB->removeSuccessor(PredBB->succ_begin());
+ assert(PredBB->succ_empty() &&
+ "TailDuplicate called on block with multiple successors!");
+ for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
+ E = TailBB->succ_end(); I != E; ++I)
+ PredBB->addSuccessor(*I);
+
+ Changed = true;
+ }
+
+ return Changed;
+}
+
/// OptimizeBlock - Analyze and optimize control flow related to the specified
/// block. This is never called on the entry block.
bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
bool MadeChange = false;
MachineFunction &MF = *MBB->getParent();
+ReoptimizeBlock:
MachineFunction::iterator FallThrough = MBB;
++FallThrough;
@@ -927,16 +1136,36 @@
TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
MadeChange = true;
++NumBranchOpts;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
+ // If the previous block unconditionally falls through to this block and
+ // this block has no other predecessors, move the contents of this block
+ // into the prior block. This doesn't usually happen when SimplifyCFG
+ // has been used, but it can happen tail duplication eliminates all the
+ // non-branch predecessors of a block leaving only the fall-through edge.
+ // This has to check PrevBB->succ_size() because EH edges are ignored by
+ // AnalyzeBranch.
+ if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
+ PrevBB.succ_size() == 1 &&
+ !MBB->hasAddressTaken()) {
+ DEBUG(errs() << "\nMerging into block: " << PrevBB
+ << "From MBB: " << *MBB);
+ PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
+ PrevBB.removeSuccessor(PrevBB.succ_begin());;
+ assert(PrevBB.succ_empty());
+ PrevBB.transferSuccessors(MBB);
+ MadeChange = true;
+ return MadeChange;
+ }
+
// If the previous branch *only* branches to *this* block (conditional or
// not) remove the branch.
if (PriorTBB == MBB && PriorFBB == 0) {
TII->RemoveBranch(PrevBB);
MadeChange = true;
++NumBranchOpts;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
// If the prior block branches somewhere else on the condition and here if
@@ -946,7 +1175,7 @@
TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
MadeChange = true;
++NumBranchOpts;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
// If the prior block branches here on true and somewhere else on false, and
@@ -959,7 +1188,7 @@
TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
MadeChange = true;
++NumBranchOpts;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
}
@@ -1041,7 +1270,7 @@
TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
MadeChange = true;
++NumBranchOpts;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
}
@@ -1107,7 +1336,7 @@
TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond);
MadeChange = true;
++NumBranchOpts;
- PMBB->CorrectExtraCFGEdges(NewCurTBB, NewCurFBB, false);
+ PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
}
}
}
@@ -1127,16 +1356,26 @@
}
}
+ // Now we know that there was no fall-through into this block, check to
+ // see if it has a fall-through into its successor.
+ bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
+ CurCond);
+ bool PrevFallsThru = CanFallThrough(&PrevBB, PriorUnAnalyzable,
+ PriorTBB, PriorFBB, PriorCond);
+
+ // If this block is small, unconditionally branched to, and does not
+ // fall through, tail-duplicate its instructions into its predecessors
+ // to eliminate a (dynamic) branch.
+ if (!CurFallsThru)
+ if (TailDuplicate(MBB, PrevFallsThru, MF)) {
+ MadeChange = true;
+ return MadeChange;
+ }
+
// If the prior block doesn't fall through into this block, and if this
// block doesn't fall through into some other block, see if we can find a
// place to move this block where a fall-through will happen.
- if (!CanFallThrough(&PrevBB, PriorUnAnalyzable,
- PriorTBB, PriorFBB, PriorCond)) {
- // Now we know that there was no fall-through into this block, check to
- // see if it has a fall-through into its successor.
- bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
- CurCond);
-
+ if (!PrevFallsThru) {
if (!MBB->isLandingPad()) {
// Check all the predecessors of this block. If one of them has no fall
// throughs, move this block right after it.
@@ -1145,7 +1384,10 @@
// Analyze the branch at the end of the pred.
MachineBasicBlock *PredBB = *PI;
MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
- if (PredBB != MBB && !CanFallThrough(PredBB)
+ MachineBasicBlock *PredTBB, *PredFBB;
+ SmallVector<MachineOperand, 4> PredCond;
+ if (PredBB != MBB && !CanFallThrough(PredBB) &&
+ !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
&& (!CurFallsThru || !CurTBB || !CurFBB)
&& (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
// If the current block doesn't fall through, just move it.
@@ -1165,7 +1407,7 @@
}
MBB->moveAfter(PredBB);
MadeChange = true;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
}
}
@@ -1182,18 +1424,22 @@
// the succ doesn't already have a block that can fall through into it,
// and if the successor isn't an EH destination, we can arrange for the
// fallthrough to happen.
- if (SuccBB != MBB && !CanFallThrough(SuccPrev) &&
+ if (SuccBB != MBB && &*SuccPrev != MBB &&
+ !CanFallThrough(SuccPrev) && !CurUnAnalyzable &&
!SuccBB->isLandingPad()) {
MBB->moveBefore(SuccBB);
MadeChange = true;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
}
// Okay, there is no really great place to put this block. If, however,
// the block before this one would be a fall-through if this block were
// removed, move this block to the end of the function.
+ MachineBasicBlock *PrevTBB, *PrevFBB;
+ SmallVector<MachineOperand, 4> PrevCond;
if (FallThrough != MF.end() &&
+ !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
PrevBB.isSuccessor(FallThrough)) {
MBB->moveAfter(--MF.end());
MadeChange = true;
diff --git a/lib/CodeGen/MachineBasicBlock.cpp b/lib/CodeGen/MachineBasicBlock.cpp
index 7fbdb12..14d6682 100644
--- a/lib/CodeGen/MachineBasicBlock.cpp
+++ b/lib/CodeGen/MachineBasicBlock.cpp
@@ -371,10 +371,7 @@
MachineBasicBlock::succ_iterator SI = succ_begin();
MachineBasicBlock *OrigDestA = DestA, *OrigDestB = DestB;
while (SI != succ_end()) {
- if (*SI == DestA && DestA == DestB) {
- DestA = DestB = 0;
- ++SI;
- } else if (*SI == DestA) {
+ if (*SI == DestA) {
DestA = 0;
++SI;
} else if (*SI == DestB) {