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",
MadeChange |= OptimizeImpDefsBlock(MBB);
}
-
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
MadeChangeThisIteration = false;
// branches temporarily for tail merging). In the case where CurMBB ends
// with a conditional branch to the next block, optimize by reversing the
// test and conditionally branching to SuccMBB instead.
-static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB,
+static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
const TargetInstrInfo *TII) {
MachineFunction *MF = CurMBB->getParent();
MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB));
/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
/// MergePotentials, restoring branches at ends of blocks as appropriate.
void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
- MachineBasicBlock* SuccBB,
- MachineBasicBlock* PredBB) {
+ MachineBasicBlock *SuccBB,
+ MachineBasicBlock *PredBB) {
MPIterator CurMPIter, B;
for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
CurMPIter->getHash() == CurHash;
// if any, is given in PredBB.
bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
- MachineBasicBlock* PredBB) {
+ MachineBasicBlock *PredBB) {
bool MadeChange = false;
// Except for the special cases below, tail-merge if there are at least
// this many instructions in common.
unsigned minCommonTailLength = TailMergeSize;
- // 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].getBlock()->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].getBlock(),
- FirstNonTerm);
- if (FirstNonTerm != MergePotentials[0].getBlock()->end()) {
- for (unsigned i = 1, e = MergePotentials.size(); i != e; ++i) {
- MachineBasicBlock::iterator OtherFirstNonTerm;
- unsigned NumTerms = CountTerminators(MergePotentials[0].getBlock(),
- OtherFirstNonTerm);
- if (NumTerms < MinNumTerms)
- MinNumTerms = NumTerms;
- if (OtherFirstNonTerm == MergePotentials[i].getBlock()->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].getBlock()->getNumber()
for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
E2 = I->pred_end();
P != E2; ++P) {
- MachineBasicBlock* PBB = *P;
+ MachineBasicBlock *PBB = *P;
// Skip blocks that loop to themselves, can't tail merge these.
if (PBB == IBB)
continue;
// to have a bit in the edge so we didn't have to do all this.
if (IBB->isLandingPad()) {
MachineFunction::iterator IP = PBB; IP++;
- MachineBasicBlock* PredNextBB = NULL;
+ MachineBasicBlock *PredNextBB = NULL;
if (IP != MF.end())
PredNextBB = IP;
if (TBB == NULL) {
}
-/// 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();
}
-/// 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 (PredBB->isLayoutSuccessor(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) {
// 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.
+ // has been used, but it can happen if tail merging splits a fall-through
+ // predecessor of a block.
// This has to check PrevBB->succ_size() because EH edges are ignored by
// AnalyzeBranch.
if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
// 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;
}
}
-
// If this branch is the only thing in its block, see if we can forward
// other blocks across it.
if (CurTBB && CurCond.empty() && CurFBB == 0 &&
}
}
- // 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 (!PrevFallsThru) {
+ 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 = MBB->canFallThrough();
+
if (!MBB->isLandingPad()) {
// Check all the predecessors of this block. If one of them has no fall
// throughs, move this block right after it.
MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
MachineBasicBlock *PredTBB, *PredFBB;
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())) {
// 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;
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