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CodeGen: Remove dead code in TailDuplicate

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I noticed that this isn't covered by our existing tests and spent some
time trying to come up with an example it actually hits. I tried hand
rolling something based on the explanation in the comment, but couldn't
get anything that didn't abort tail duplication earlier for one reason
or another.

Then, I tried cranking tail-dup-size cranked up so this would fire
more and ran a bootstrap of clang and the nightly test suite - those
don't hit this either.

This reverts r132816 and replaces it with an assert.

llvm-svn: 265347
This commit is contained in:
Justin Bogner 2016-04-04 21:11:40 +00:00
parent 60e529fa0a
commit 2470e42897
1 changed files with 14 additions and 58 deletions
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72
lib/CodeGen/TailDuplication.cpp
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@ -92,8 +92,7 @@ namespace {
MachineBasicBlock *PredBB,
DenseMap<unsigned, unsigned> &LocalVRMap,
SmallVectorImpl<std::pair<unsigned,unsigned> > &Copies,
const DenseSet<unsigned> &UsedByPhi,
bool Remove);
const DenseSet<unsigned> &UsedByPhi);
void DuplicateInstruction(MachineInstr *MI,
MachineBasicBlock *TailBB,
MachineBasicBlock *PredBB,
@ -395,7 +394,7 @@ void TailDuplicatePass::ProcessPHI(
MachineInstr *MI, MachineBasicBlock *TailBB, MachineBasicBlock *PredBB,
DenseMap<unsigned, unsigned> &LocalVRMap,
SmallVectorImpl<std::pair<unsigned, unsigned> > &Copies,
const DenseSet<unsigned> &RegsUsedByPhi, bool Remove) {
const DenseSet<unsigned> &RegsUsedByPhi) {
unsigned DefReg = MI->getOperand(0).getReg();
unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB);
assert(SrcOpIdx && "Unable to find matching PHI source?");
@ -410,9 +409,6 @@ void TailDuplicatePass::ProcessPHI(
if (isDefLiveOut(DefReg, TailBB, MRI) || RegsUsedByPhi.count(DefReg))
AddSSAUpdateEntry(DefReg, NewDef, PredBB);
if (!Remove)
return;
// Remove PredBB from the PHI node.
MI->RemoveOperand(SrcOpIdx+1);
MI->RemoveOperand(SrcOpIdx);
@ -840,7 +836,7 @@ TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB,
if (MI->isPHI()) {
// Replace the uses of the def of the PHI with the register coming
// from PredBB.
ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, true);
ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi);
} else {
// Replace def of virtual registers with new registers, and update
// uses with PHI source register or the new registers.
@ -894,7 +890,7 @@ TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB,
// Replace the uses of the def of the PHI with the register coming
// from PredBB.
MachineInstr *MI = &*I++;
ProcessPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos, UsedByPhi, true);
ProcessPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos, UsedByPhi);
if (MI->getParent())
MI->eraseFromParent();
}
@ -926,56 +922,16 @@ TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB,
Changed = true;
}
// If this is after register allocation, there are no phis to fix.
if (!PreRegAlloc)
return Changed;
// If we made no changes so far, we are safe.
if (!Changed)
return Changed;
// Handle the nasty case in that we duplicated a block that is part of a loop
// into some but not all of its predecessors. For example:
// 1 -> 2 <-> 3 |
// \ |
// \---> rest |
// if we duplicate 2 into 1 but not into 3, we end up with
// 12 -> 3 <-> 2 -> rest |
// \ / |
// \----->-----/ |
// If there was a "var = phi(1, 3)" in 2, it has to be ultimately replaced
// with a phi in 3 (which now dominates 2).
// What we do here is introduce a copy in 3 of the register defined by the
// phi, just like when we are duplicating 2 into 3, but we don't copy any
// real instructions or remove the 3 -> 2 edge from the phi in 2.
for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
PE = Preds.end(); PI != PE; ++PI) {
MachineBasicBlock *PredBB = *PI;
if (std::find(TDBBs.begin(), TDBBs.end(), PredBB) != TDBBs.end())
continue;
// EH edges
if (PredBB->succ_size() != 1)
continue;
DenseMap<unsigned, unsigned> LocalVRMap;
SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
MachineBasicBlock::iterator I = TailBB->begin();
// Process PHI instructions first.
while (I != TailBB->end() && I->isPHI()) {
// Replace the uses of the def of the PHI with the register coming
// from PredBB.
MachineInstr *MI = &*I++;
ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, false);
}
MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
TII->get(TargetOpcode::COPY),
CopyInfos[i].first).addReg(CopyInfos[i].second));
}
}
// TODO: We shouldn't need this assert. It's here to be defensive about
// removing the logic from r132816, which handled an unreachable case.
assert((!PreRegAlloc || !Changed ||
std::all_of(Preds.begin(), Preds.end(),
[&TDBBs](MachineBasicBlock *PredBB) {
return (std::find(TDBBs.begin(), TDBBs.end(), PredBB) !=
TDBBs.end()) ||
PredBB->succ_size() != 1;
})) &&
"loop block duplicated into some but not all predecessors");
return Changed;
}