STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
STATISTIC(NumBranchOpts, "Number of branches optimized");
STATISTIC(NumTailMerge , "Number of block tails merged");
+
static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
cl::init(cl::BOU_UNSET), cl::Hidden);
+
// Throttle for huge numbers of predecessors (compile speed problems)
static cl::opt<unsigned>
TailMergeThreshold("tail-merge-threshold",
/// function, updating the CFG.
void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
assert(MBB->pred_empty() && "MBB must be dead!");
- DEBUG(errs() << "\nRemoving MBB: " << *MBB);
+ DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
MachineFunction *MF = MBB->getParent();
// drop all successors.
MadeChange |= OptimizeImpDefsBlock(MBB);
}
-
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
MadeChangeThisIteration = false;
MadeChange |= MadeChangeThisIteration;
}
- // Do tail duplication once after tail merging is done. Otherwise it is
- // tough to avoid situations where tail duplication and tail merging undo
- // each other's transformations ad infinitum.
- MadeChange |= TailDuplicateBlocks(MF);
-
// See if any jump tables have become mergable or dead as the code generator
// did its thing.
MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
const TargetInstrInfo *TII) {
MachineFunction *MF = CurMBB->getParent();
- MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB));
+ MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB));
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (I != MF->end() &&
SameTails[commonTailIndex].getTailStartPos();
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
- DEBUG(errs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
+ DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
<< maxCommonTailLength);
MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
// this many instructions in common.
unsigned minCommonTailLength = TailMergeSize;
- DEBUG(errs() << "\nTryTailMergeBlocks: ";
+ DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
- errs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
+ dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
<< (i == e-1 ? "" : ", ");
- errs() << "\n";
+ dbgs() << "\n";
if (SuccBB) {
- errs() << " with successor BB#" << SuccBB->getNumber() << '\n';
+ dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
if (PredBB)
- errs() << " which has fall-through from BB#"
+ dbgs() << " which has fall-through from BB#"
<< PredBB->getNumber() << "\n";
}
- errs() << "Looking for common tails of at least "
+ dbgs() << "Looking for common tails of at least "
<< minCommonTailLength << " instruction"
<< (minCommonTailLength == 1 ? "" : "s") << '\n';
);
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
// 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 in BB#" << MBB->getNumber()
+ DEBUG(dbgs() << "\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].getBlock()->getNumber()
+ DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
<< (i == e-1 ? "" : ", "));
// Hack the end off BB i, making it jump to BB commonTailIndex instead.
ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
// BB i is no longer a predecessor of SuccBB; remove it from the worklist.
MergePotentials.erase(SameTails[i].getMPIter());
}
- DEBUG(errs() << "\n");
+ DEBUG(dbgs() << "\n");
// We leave commonTailIndex in the worklist in case there are other blocks
// that match it with a smaller number of instructions.
MadeChange = true;
// a compile-time infinite loop repeatedly doing and undoing the same
// transformations.)
- for (MachineFunction::iterator I = next(MF.begin()), E = MF.end();
+ for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
I != E; ++I) {
if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
continue;
// This is the QBB case described above
if (!FBB)
- FBB = next(MachineFunction::iterator(PBB));
+ FBB = llvm::next(MachineFunction::iterator(PBB));
}
// Failing case: the only way IBB can be reached from PBB is via
// exception handling. Happens for landing pads. Would be nice
}
-/// CanFallThrough - Return true if the specified block (with the specified
-/// branch condition) can implicitly transfer control to the block after it by
-/// falling off the end of it. This should return false if it can reach the
-/// block after it, but it uses an explicit branch to do so (e.g. a table jump).
-///
-/// True is a conservative answer.
-///
-bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB,
- bool BranchUnAnalyzable,
- MachineBasicBlock *TBB,
- MachineBasicBlock *FBB,
- const SmallVectorImpl<MachineOperand> &Cond) {
- MachineFunction::iterator Fallthrough = CurBB;
- ++Fallthrough;
- // If FallthroughBlock is off the end of the function, it can't fall through.
- if (Fallthrough == CurBB->getParent()->end())
- return false;
-
- // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible.
- if (!CurBB->isSuccessor(Fallthrough))
- return false;
-
- // 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;
-
- // If there is some explicit branch to the fallthrough block, it can obviously
- // reach, even though the branch should get folded to fall through implicitly.
- if (MachineFunction::iterator(TBB) == Fallthrough ||
- MachineFunction::iterator(FBB) == Fallthrough)
- return true;
-
- // If it's an unconditional branch to some block not the fall through, it
- // doesn't fall through.
- if (Cond.empty()) return false;
-
- // Otherwise, if it is conditional and has no explicit false block, it falls
- // through.
- return FBB == 0;
-}
-
-/// CanFallThrough - Return true if the specified can implicitly transfer
-/// control to the block after it by falling off the end of it. This should
-/// return false if it can reach the block after it, but it uses an explicit
-/// branch to do so (e.g. a table jump).
-///
-/// True is a conservative answer.
-///
-bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) {
- MachineBasicBlock *TBB = 0, *FBB = 0;
- SmallVector<MachineOperand, 4> Cond;
- bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true);
- return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond);
-}
-
/// IsBetterFallthrough - Return true if it would be clearly better to
/// fall-through to MBB1 than to fall through into MBB2. This has to return
/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
}
-/// TailDuplicateBlocks - Look for small blocks that are unconditionally
-/// branched to and do not fall through. Tail-duplicate their instructions
-/// into their predecessors to eliminate (dynamic) branches.
-bool BranchFolder::TailDuplicateBlocks(MachineFunction &MF) {
- bool MadeChange = false;
-
- // Make sure blocks are numbered in order
- MF.RenumberBlocks();
-
- for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
- MachineBasicBlock *MBB = I++;
-
- // Only duplicate blocks that end with unconditional branches.
- if (CanFallThrough(MBB))
- continue;
-
- MadeChange |= TailDuplicate(MBB, MF);
-
- // If it is dead, remove it.
- if (MBB->pred_empty()) {
- RemoveDeadBlock(MBB);
- MadeChange = true;
- ++NumDeadBlocks;
- }
- }
- return MadeChange;
-}
-
-/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
-/// of its predecessors.
-bool BranchFolder::TailDuplicate(MachineBasicBlock *TailBB,
- MachineFunction &MF) {
- // Don't try to tail-duplicate single-block loops.
- if (TailBB->isSuccessor(TailBB))
- return false;
-
- // Set the limit on the number of instructions to duplicate, with a default
- // of one less than the tail-merge threshold. 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 : TII->TailDuplicationLimit(*TailBB, TailMergeSize - 1);
-
- // Check the instructions in the block to determine whether tail-duplication
- // is invalid or unlikely to be profitable.
- 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 (at least for now).
- if (PredBB->isLayoutSuccessor(TailBB) && CanFallThrough(PredBB))
- 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;
- }
-
- // If TailBB was duplicated into all its predecessors except for the prior
- // block, which falls through unconditionally, move the contents of this
- // block into the prior block.
- MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(TailBB));
- MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
- SmallVector<MachineOperand, 4> PriorCond;
- bool PriorUnAnalyzable =
- TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
- // This has to check PrevBB->succ_size() because EH edges are ignored by
- // AnalyzeBranch.
- if (!PriorUnAnalyzable && PriorCond.empty() && !PriorTBB &&
- TailBB->pred_size() == 1 && PrevBB.succ_size() == 1 &&
- !TailBB->hasAddressTaken()) {
- DEBUG(errs() << "\nMerging into block: " << PrevBB
- << "From MBB: " << *TailBB);
- PrevBB.splice(PrevBB.end(), TailBB, TailBB->begin(), TailBB->end());
- PrevBB.removeSuccessor(PrevBB.succ_begin());;
- assert(PrevBB.succ_empty());
- PrevBB.transferSuccessors(TailBB);
- 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) {
if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
PrevBB.succ_size() == 1 &&
!MBB->hasAddressTaken()) {
- DEBUG(errs() << "\nMerging into block: " << PrevBB
+ DEBUG(dbgs() << "\nMerging into block: " << PrevBB
<< "From MBB: " << *MBB);
PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
PrevBB.removeSuccessor(PrevBB.succ_begin());;
// the assert condition out of the loop body.
if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
MachineFunction::iterator(PriorTBB) == FallThrough &&
- !CanFallThrough(MBB)) {
+ !MBB->canFallThrough()) {
bool DoTransform = true;
// We have to be careful that the succs of PredBB aren't both no-successor
// In this case, we could actually be moving the return block *into* a
// loop!
if (DoTransform && !MBB->succ_empty() &&
- (!CanFallThrough(PriorTBB) || PriorTBB->empty()))
+ (!PriorTBB->canFallThrough() || PriorTBB->empty()))
DoTransform = false;
// Reverse the branch so we will fall through on the previous true cond.
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
if (!TII->ReverseBranchCondition(NewPriorCond)) {
- DEBUG(errs() << "\nMoving MBB: " << *MBB
+ DEBUG(dbgs() << "\nMoving MBB: " << *MBB
<< "To make fallthrough to: " << *PriorTBB << "\n");
TII->RemoveBranch(PrevBB);
// falls through into MBB and we can't understand the prior block's branch
// condition.
if (MBB->empty()) {
- bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB);
+ bool PredHasNoFallThrough = !PrevBB.canFallThrough();
if (PredHasNoFallThrough || !PriorUnAnalyzable ||
!PrevBB.isSuccessor(MBB)) {
// If the prior block falls through into us, turn it into an
// 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)) {
+ if (!PrevBB.canFallThrough()) {
// 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 CurFallsThru = MBB->canFallThrough();
if (!MBB->isLandingPad()) {
// Check all the predecessors of this block. If one of them has no fall
// Analyze the branch at the end of the pred.
MachineBasicBlock *PredBB = *PI;
MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
- MachineBasicBlock *PredTBB, *PredFBB;
+ MachineBasicBlock *PredTBB = 0, *PredFBB = 0;
SmallVector<MachineOperand, 4> PredCond;
- if (PredBB != MBB && !CanFallThrough(PredBB) &&
+ if (PredBB != MBB && !PredBB->canFallThrough() &&
!TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
&& (!CurFallsThru || !CurTBB || !CurFBB)
&& (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
// B elsewhere
// next:
if (CurFallsThru) {
- MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
+ MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));
CurCond.clear();
TII->InsertBranch(*MBB, NextBB, 0, CurCond);
}
// and if the successor isn't an EH destination, we can arrange for the
// fallthrough to happen.
if (SuccBB != MBB && &*SuccPrev != MBB &&
- !CanFallThrough(SuccPrev) && !CurUnAnalyzable &&
+ !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
!SuccBB->isLandingPad()) {
MBB->moveBefore(SuccBB);
MadeChange = true;
// 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;
+ MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0;
SmallVector<MachineOperand, 4> PrevCond;
if (FallThrough != MF.end() &&
!TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
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