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Make tail merging handle many more cases (all it can, I think). 

llvm-svn: 36966
This commit is contained in:
Dale Johannesen 2007-05-10 01:01:49 +00:00
parent 623c738fe9
commit 6e16d09252
1 changed files with 91 additions and 32 deletions
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123
llvm/lib/CodeGen/BranchFolding.cpp
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@ -110,7 +110,7 @@ bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
MMI = getAnalysisToUpdate<MachineModuleInfo>();
bool EverMadeChange = false;
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
@ -336,7 +336,16 @@ static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1,
MachineBasicBlock::iterator MBB1I,
MachineBasicBlock *MBB2,
MachineBasicBlock::iterator MBB2I,
const TargetInstrInfo *TII) {
const TargetInstrInfo *TII,
MachineBasicBlock *PredBB) {
// If one block falls through into the common successor, choose that
// one to split; it is one instruction less to do that.
if (PredBB) {
if (MBB1 == PredBB)
return true;
else if (MBB2 == PredBB)
return false;
}
// TODO: if we had some notion of which block was hotter, we could split
// the hot block, so it is the fall-through. Since we don't have profile info
// make a decision based on which will hurt most to split.
@ -349,6 +358,31 @@ static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1,
return MBB1Time < MBB2Time;
}
// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
// 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,
const TargetInstrInfo *TII) {
MachineFunction *MF = CurMBB->getParent();
MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB));
MachineBasicBlock *TBB = 0, *FBB = 0;
std::vector<MachineOperand> Cond;
if (I != MF->end() &&
!TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) {
MachineBasicBlock *NextBB = I;
if (TBB == NextBB && Cond.size() && !FBB) {
if (!TII->ReverseBranchCondition(Cond)) {
TII->RemoveBranch(*CurMBB);
TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
return;
}
}
}
TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>());
}
// See if any of the blocks in MergePotentials (which all have a common single
// successor, or all have no successor) can be tail-merged. If there is a
// successor, any blocks in MergePotentials that are not tail-merged and
@ -375,7 +409,7 @@ bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
// give up.
if (CurHash != PrevHash) {
if (SuccBB && CurMBB != PredBB)
TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>());
FixTail(CurMBB, SuccBB, TII);
MergePotentials.pop_back();
continue;
}
@ -390,6 +424,8 @@ bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
// If the tails don't have at least two instructions in common, see if there
// is anything else in the equivalence class that does match.
// Since instructions may get combined later (e.g. single stores into
// store multiple) this measure is not particularly accurate.
if (CommonTailLen < 2) {
unsigned FoundMatch = ~0U;
for (int i = MergePotentials.size()-2;
@ -408,7 +444,7 @@ bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
if (FoundMatch == ~0U) {
// Put the unconditional branch back, if we need one.
if (SuccBB && CurMBB != PredBB)
TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>());
FixTail(CurMBB, SuccBB, TII);
MergePotentials.pop_back();
continue;
}
@ -428,7 +464,7 @@ bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
}
// Decide whether we want to split CurMBB or MBB2.
if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII)) {
if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII, PredBB)) {
CurMBB = SplitMBBAt(*CurMBB, BBI1);
BBI1 = CurMBB->begin();
MergePotentials.back().second = CurMBB;
@ -457,49 +493,69 @@ bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
}
bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
MadeChange = false;
if (!EnableTailMerge) return false;
MadeChange = false;
// First find blocks with no successors.
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), I));
}
// See if we can do any crossjumping on those.
// See if we can do any tail merging on those.
MadeChange |= TryMergeBlocks(NULL, NULL);
MergePotentials.clear();
// Look at blocks with two predecessors, where each predecessor has either:
// - a single successor, or
// - two successors, where successor I is reached either by ubr or fallthrough.
// The two-successor case where successor I is reached by cbr
// from both blocks is handled by the preceding case (when we consider the
// other, fallthough block).
// FIXME: The two-successor case where I is reached by cbr
// from one block, and by fallthrough/ubr from the other, is not handled yet.
// Beware that sometimes blocks are in the predecessor list, but can't really
// jump to the "successor" we're looking at. Tolerate this.
// Look at blocks (IBB) with multiple predecessors (PBB).
// We change each predecessor to a canonical form, by
// (1) temporarily removing any unconditional branch from the predecessor
// to IBB, and
// (2) alter conditional branches so they branch to the other block
// not IBB; this may require adding back an unconditional branch to IBB
// later, where there wasn't one coming in. E.g.
// Bcc IBB
// fallthrough to QBB
// here becomes
// Bncc QBB
// with a conceptual B to IBB after that, which never actually exists.
// With those changes, we see whether the predecessors' tails match,
// and merge them if so. We change things out of canonical form and
// back to the way they were later in the process. (OptimizeBranches
// would undo some of this, but we can't use it, because we'd get into
// a compile-time infinite loop repeatedly doing and undoing the same
// transformations.)
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
if (!I->succ_empty() && I->pred_size() >= 2) {
MachineBasicBlock *IBB = I;
MachineBasicBlock *PredBB = prior(I);
MergePotentials.clear();
for (MachineBasicBlock::pred_iterator P = I->pred_begin(), E2 = I->pred_end();
P != E2; ++P) {
MachineBasicBlock* PBB = *P;
// Skip blocks that loop to themselves, can't tail merge these.
if (PBB==IBB)
continue;
MachineBasicBlock *TBB = 0, *FBB = 0;
std::vector<MachineOperand> Cond;
// Remove the unconditional branch at the end, if any.
if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond) &&
((!FBB && Cond.size()==0) || // single successor
(!FBB && TBB!=IBB) || // cbr elsewhere, fallthrough to I
(FBB && FBB==IBB))) { // cbr then branch to I
if (TBB) {
if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) {
// Failing case: IBB is the target of a cbr, and
// we cannot reverse the branch.
std::vector<MachineOperand> NewCond(Cond);
if (Cond.size() && TBB==IBB) {
if (TII->ReverseBranchCondition(NewCond))
continue;
// This is the QBB case described above
if (!FBB)
FBB = next(MachineFunction::iterator(PBB));
}
// Remove the unconditional branch at the end, if any.
if (TBB && (Cond.size()==0 || FBB)) {
TII->RemoveBranch(*PBB);
if (TBB!=IBB)
if (Cond.size())
// reinsert conditional branch only, for now
TII->InsertBranch(*PBB, TBB, 0, Cond);
TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond);
}
MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB), *P));
}
@ -511,12 +567,9 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
// of removing blocks in TryMergeBlocks.
if (MergePotentials.size()==1 &&
(MergePotentials.begin())->second != PredBB)
TII->InsertBranch(*((MergePotentials.begin())->second), I, NULL,
std::vector<MachineOperand>());
MergePotentials.clear();
FixTail((MergePotentials.begin())->second, I, TII);
}
}
return MadeChange;
}
@ -962,12 +1015,18 @@ void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
MachineBasicBlock *PredBB = *PI;
MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
if (PredBB != MBB && !CanFallThrough(PredBB)
&& (!CurFallsThru || !CurTBB || !CurFBB)
&& (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
// If the current block doesn't fall through, just move it.
// If the current block can fall through and does not end with a
// conditional branch, we need to append an unconditional jump to
// the (current) next block. To avoid a possible compile-time
// infinite loop, move blocks only backward in this case.
// Also, if there are already 2 branches here, we cannot add a third;
// this means we have the case
// Bcc next
// B elsewhere
// next:
if (CurFallsThru) {
MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
CurCond.clear();