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Enable early duplication of small blocks. There are still improvements to

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be made, but this is already a win.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133240 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Rafael Espindola 2011-06-17 05:54:50 +00:00
parent b712ef9569
commit ec324e5ae4
1 changed files with 33 additions and 30 deletions
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63
lib/CodeGen/TailDuplication.cpp
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@ -490,6 +490,10 @@ TailDuplicatePass::shouldTailDuplicate(const MachineFunction &MF,
if (TailBB.canFallThrough())
return false;
// Don't try to tail-duplicate single-block loops.
if (TailBB.isSuccessor(&TailBB))
return false;
// Set the limit on the cost to duplicate. When optimizing for size,
// duplicate only one, because one branch instruction can be eliminated to
// compensate for the duplication.
@ -500,50 +504,45 @@ TailDuplicatePass::shouldTailDuplicate(const MachineFunction &MF,
else
MaxDuplicateCount = TailDuplicateSize;
if (PreRegAlloc) {
if (TailBB.empty())
return false;
const TargetInstrDesc &TID = TailBB.back().getDesc();
// Pre-regalloc tail duplication hurts compile time and doesn't help
// much except for indirect branches.
if (!TID.isIndirectBranch())
return false;
// If the target has hardware branch prediction that can handle indirect
// branches, duplicating them can often make them predictable when there
// are common paths through the code. The limit needs to be high enough
// to allow undoing the effects of tail merging and other optimizations
// that rearrange the predecessors of the indirect branch.
MaxDuplicateCount = 20;
}
// If the target has hardware branch prediction that can handle indirect
// branches, duplicating them can often make them predictable when there
// are common paths through the code. The limit needs to be high enough
// to allow undoing the effects of tail merging and other optimizations
// that rearrange the predecessors of the indirect branch.
// Don't try to tail-duplicate single-block loops.
if (TailBB.isSuccessor(&TailBB))
return false;
if (PreRegAlloc && !TailBB.empty()) {
const TargetInstrDesc &TID = TailBB.back().getDesc();
if (TID.isIndirectBranch())
MaxDuplicateCount = 20;
}
// Check the instructions in the block to determine whether tail-duplication
// is invalid or unlikely to be profitable.
unsigned InstrCount = 0;
bool HasCall = false;
for (MachineBasicBlock::const_iterator I = TailBB.begin(); I != TailBB.end();
++I) {
// Non-duplicable things shouldn't be tail-duplicated.
if (I->getDesc().isNotDuplicable()) return false;
if (I->getDesc().isNotDuplicable())
return false;
// Do not duplicate 'return' instructions if this is a pre-regalloc run.
// A return may expand into a lot more instructions (e.g. reload of callee
// saved registers) after PEI.
if (PreRegAlloc && I->getDesc().isReturn()) return false;
// Don't duplicate more than the threshold.
if (InstrCount == MaxDuplicateCount) return false;
// Remember if we saw a call.
if (I->getDesc().isCall()) HasCall = true;
if (PreRegAlloc && I->getDesc().isReturn())
return false;
// Avoid duplicating calls before register allocation. Calls presents a
// barrier to register allocation so duplicating them may end up increasing
// spills.
if (PreRegAlloc && I->getDesc().isCall())
return false;
if (!I->isPHI() && !I->isDebugValue())
InstrCount += 1;
if (InstrCount > MaxDuplicateCount)
return false;
}
// Don't tail-duplicate calls before register allocation. Calls presents a
// barrier to register allocation so duplicating them may end up increasing
// spills.
if (InstrCount > 1 && (PreRegAlloc && HasCall))
return false;
return true;
}
@ -618,6 +617,10 @@ TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
TII->get(TargetOpcode::COPY),
CopyInfos[i].first).addReg(CopyInfos[i].second));
}
// Simplify
TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true);
NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
// Update the CFG.