#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>
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",
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);
+
+namespace {
+ /// BranchFolderPass - Wrap branch folder in a machine function pass.
+ class BranchFolderPass : public MachineFunctionPass,
+ public BranchFolder {
+ public:
+ static char ID;
+ explicit BranchFolderPass(bool defaultEnableTailMerge)
+ : MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {}
+
+ virtual bool runOnMachineFunction(MachineFunction &MF);
+ virtual const char *getPassName() const { return "Control Flow Optimizer"; }
+ };
+}
char BranchFolderPass::ID = 0;
MadeChange |= OptimizeImpDefsBlock(MBB);
}
-
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
MadeChangeThisIteration = false;
RS->forward(prior(CurMBB.end()));
BitVector RegsLiveAtExit(TRI->getNumRegs());
RS->getRegsUsed(RegsLiveAtExit, false);
- for (unsigned int i=0, e=TRI->getNumRegs(); i!=e; i++)
+ for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
if (RegsLiveAtExit[i])
NewMBB->addLiveIn(i);
}
// 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));
+ MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB));
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (I != MF->end() &&
TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>());
}
-static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p,
- const std::pair<unsigned,MachineBasicBlock*> &q) {
- if (p.first < q.first)
- return true;
- else if (p.first > q.first)
- return false;
- else if (p.second->getNumber() < q.second->getNumber())
- return true;
- else if (p.second->getNumber() > q.second->getNumber())
- return false;
- else {
- // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
- // an object with itself.
+bool
+BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
+ if (getHash() < o.getHash())
+ return true;
+ else if (getHash() > o.getHash())
+ return false;
+ else if (getBlock()->getNumber() < o.getBlock()->getNumber())
+ return true;
+ else if (getBlock()->getNumber() > o.getBlock()->getNumber())
+ return false;
+ else {
+ // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
+ // an object with itself.
#ifndef _GLIBCXX_DEBUG
- llvm_unreachable("Predecessor appears twice");
+ llvm_unreachable("Predecessor appears twice");
#endif
- return false;
+ return false;
+ }
+}
+
+/// 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
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;
- // 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
- // even with falling through into the duplicated common tail.
- if (MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
+ // 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 one of the blocks can be completely merged and happens to be in
+ // a position where the other could fall through into it, merge any number
+ // of instructions, because it can be done without a branch.
+ // TODO: If the blocks are not adjacent, move one of them so that they are?
+ if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
+ return true;
+ if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
+ return true;
+
+ // If both blocks have an unconditional branch temporarily stripped out,
+ // count that as an additional common instruction for the following
+ // heuristics.
+ unsigned EffectiveTailLen = CommonTailLen;
+ if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
+ !MBB1->back().getDesc().isBarrier() &&
+ !MBB2->back().getDesc().isBarrier())
+ ++EffectiveTailLen;
+
+ // Check if the common tail is long enough to be worthwhile.
+ if (EffectiveTailLen >= minCommonTailLength)
+ return true;
+
+ // If we are optimizing for code size, 2 instructions in common is enough if
+ // we don't have to split a block. At worst we will be introducing 1 new
+ // branch instruction, which is likely to be smaller than the 2
+ // instructions that would be deleted in the merge.
+ if (EffectiveTailLen >= 2 &&
+ MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
(I1 == MBB1->begin() || I2 == MBB2->begin()))
return true;
/// 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;
MPIterator HighestMPIter = prior(MergePotentials.end());
for (MPIterator CurMPIter = prior(MergePotentials.end()),
B = MergePotentials.begin();
- CurMPIter!=B && CurMPIter->first == CurHash;
+ CurMPIter != B && CurMPIter->getHash() == CurHash;
--CurMPIter) {
- for (MPIterator I = prior(CurMPIter); I->first == CurHash ; --I) {
+ for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) {
unsigned CommonTailLen;
- if (ProfitableToMerge(CurMPIter->second, I->second, minCommonTailLength,
- CommonTailLen, TrialBBI1, TrialBBI2)) {
+ if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
+ minCommonTailLength,
+ CommonTailLen, TrialBBI1, TrialBBI2,
+ SuccBB, PredBB)) {
if (CommonTailLen > maxCommonTailLength) {
SameTails.clear();
maxCommonTailLength = CommonTailLen;
HighestMPIter = CurMPIter;
- SameTails.push_back(std::make_pair(CurMPIter, TrialBBI1));
+ SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
}
if (HighestMPIter == CurMPIter &&
CommonTailLen == maxCommonTailLength)
- SameTails.push_back(std::make_pair(I, TrialBBI2));
+ SameTails.push_back(SameTailElt(I, TrialBBI2));
}
if (I == B)
break;
/// 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->first == CurHash;
+ CurMPIter->getHash() == CurHash;
--CurMPIter) {
// Put the unconditional branch back, if we need one.
- MachineBasicBlock *CurMBB = CurMPIter->second;
+ MachineBasicBlock *CurMBB = CurMPIter->getBlock();
if (SuccBB && CurMBB != PredBB)
FixTail(CurMBB, SuccBB, TII);
if (CurMPIter == B)
break;
}
- if (CurMPIter->first!=CurHash)
+ if (CurMPIter->getHash() != CurHash)
CurMPIter++;
MergePotentials.erase(CurMPIter, MergePotentials.end());
}
/// only of the common tail. Create a block that does by splitting one.
unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
unsigned maxCommonTailLength) {
- unsigned i, commonTailIndex;
+ unsigned commonTailIndex = 0;
unsigned TimeEstimate = ~0U;
- for (i=0, commonTailIndex=0; i<SameTails.size(); i++) {
+ for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
// Use PredBB if possible; that doesn't require a new branch.
- if (SameTails[i].first->second == PredBB) {
+ if (SameTails[i].getBlock() == PredBB) {
commonTailIndex = i;
break;
}
// Otherwise, make a (fairly bogus) choice based on estimate of
// how long it will take the various blocks to execute.
- unsigned t = EstimateRuntime(SameTails[i].first->second->begin(),
- SameTails[i].second);
+ unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
+ SameTails[i].getTailStartPos());
if (t <= TimeEstimate) {
TimeEstimate = t;
commonTailIndex = i;
}
}
- MachineBasicBlock::iterator BBI = SameTails[commonTailIndex].second;
- MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second;
+ MachineBasicBlock::iterator BBI =
+ SameTails[commonTailIndex].getTailStartPos();
+ MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
DEBUG(errs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
<< maxCommonTailLength);
MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
- SameTails[commonTailIndex].first->second = newMBB;
- SameTails[commonTailIndex].second = newMBB->begin();
+ SameTails[commonTailIndex].setBlock(newMBB);
+ SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
// If we split PredBB, newMBB is the new predecessor.
if (PredBB == MBB)
// 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;
-
- DEBUG(errs() << "\nTryMergeBlocks " << MergePotentials.size() << '\n');
+ // Except for the special cases below, tail-merge if there are at least
+ // this many instructions in common.
+ unsigned minCommonTailLength = TailMergeSize;
+
+ DEBUG(errs() << "\nTryTailMergeBlocks: ";
+ for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
+ errs() << "BB#" << MergePotentials[i].getBlock()->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.
- std::stable_sort(MergePotentials.begin(), MergePotentials.end(),MergeCompare);
+ std::stable_sort(MergePotentials.begin(), MergePotentials.end());
// Walk through equivalence sets looking for actual exact matches.
while (MergePotentials.size() > 1) {
- unsigned CurHash = MergePotentials.back().first;
+ unsigned CurHash = MergePotentials.back().getHash();
// 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.
// block, which we can't jump to), we can treat all blocks with this same
// tail at once. Use PredBB if that is one of the possibilities, as that
// will not introduce any extra branches.
- MachineBasicBlock *EntryBB = MergePotentials.begin()->second->
- getParent()->begin();
- 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) {
- commonTailIndex = i;
- if (MBB == PredBB)
+ MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
+ getParent()->begin();
+ unsigned commonTailIndex = SameTails.size();
+ // If there are two blocks, check to see if one can be made to fall through
+ // into the other.
+ if (SameTails.size() == 2 &&
+ SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
+ SameTails[1].tailIsWholeBlock())
+ commonTailIndex = 1;
+ else if (SameTails.size() == 2 &&
+ SameTails[1].getBlock()->isLayoutSuccessor(
+ SameTails[0].getBlock()) &&
+ SameTails[0].tailIsWholeBlock())
+ commonTailIndex = 0;
+ else {
+ // Otherwise just pick one, favoring the fall-through predecessor if
+ // there is one.
+ for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
+ MachineBasicBlock *MBB = SameTails[i].getBlock();
+ if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
+ continue;
+ if (MBB == PredBB) {
+ commonTailIndex = i;
break;
+ }
+ if (SameTails[i].tailIsWholeBlock())
+ commonTailIndex = i;
}
}
- if (commonTailIndex == SameTails.size()) {
+ if (commonTailIndex == SameTails.size() ||
+ (SameTails[commonTailIndex].getBlock() == PredBB &&
+ !SameTails[commonTailIndex].tailIsWholeBlock())) {
// 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;
+ 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 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].getBlock()->getNumber()
+ << (i == e-1 ? "" : ", "));
// Hack the end off BB i, making it jump to BB commonTailIndex instead.
- ReplaceTailWithBranchTo(SameTails[i].second, MBB);
+ ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
// BB i is no longer a predecessor of SuccBB; remove it from the worklist.
- MergePotentials.erase(SameTails[i].first);
+ MergePotentials.erase(SameTails[i].getMPIter());
}
DEBUG(errs() << "\n");
// We leave commonTailIndex in the worklist in case there are other blocks
MergePotentials.clear();
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
if (I->succ_empty())
- MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I));
+ MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I));
}
// 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
// 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 = llvm::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;
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;
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
// 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;
- if (IP!=MF.end())
+ MachineBasicBlock *PredNextBB = NULL;
+ if (IP != MF.end())
PredNextBB = IP;
if (TBB == NULL) {
- if (IBB!=PredNextBB) // fallthrough
+ if (IBB != PredNextBB) // fallthrough
continue;
} else if (FBB) {
- if (TBB!=IBB && FBB!=IBB) // cbr then ubr
+ if (TBB != IBB && FBB != IBB) // cbr then ubr
continue;
} else if (Cond.empty()) {
- if (TBB!=IBB) // ubr
+ if (TBB != IBB) // ubr
continue;
} else {
- if (TBB!=IBB && IBB!=PredNextBB) // cbr
+ if (TBB != IBB && IBB != PredNextBB) // cbr
continue;
}
}
// reinsert conditional branch only, for now
TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond);
}
- MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P));
+ MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U),
+ *P));
}
}
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);
+ MergePotentials.begin()->getBlock() != PredBB)
+ FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
}
}
return MadeChange;
}
-/// 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, assume it could fall through.
- if (BranchUnAnalyzable) return true;
-
- // 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
bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
bool MadeChange = false;
MachineFunction &MF = *MBB->getParent();
+ReoptimizeBlock:
MachineFunction::iterator FallThrough = MBB;
++FallThrough;
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 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 &&
+ 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
TII->RemoveBranch(PrevBB);
MadeChange = true;
++NumBranchOpts;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
// If the prior block branches somewhere else on the condition and here if
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
TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
MadeChange = true;
++NumBranchOpts;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
}
// 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;
TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
MadeChange = true;
++NumBranchOpts;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
}
-
// 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 &&
TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond);
MadeChange = true;
++NumBranchOpts;
- PMBB->CorrectExtraCFGEdges(NewCurTBB, NewCurFBB, false);
+ PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
}
}
}
// 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;
- if (PredBB != MBB && !CanFallThrough(PredBB)
+ MachineBasicBlock *PredTBB, *PredFBB;
+ SmallVector<MachineOperand, 4> PredCond;
+ if (PredBB != MBB && !PredBB->canFallThrough() &&
+ !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.
// 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);
}
MBB->moveAfter(PredBB);
MadeChange = true;
- return OptimizeBlock(MBB);
+ goto ReoptimizeBlock;
}
}
}
// 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 &&
+ !SuccPrev->canFallThrough() && !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;