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Treat indirect branches specially only during pre-regalloc tail duplication,

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not during the later post-alloc tail duplication.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93600 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bob Wilson 2010-01-16 00:42:25 +00:00
parent 551cdd56ce
commit cb44b28f4d
1 changed files with 14 additions and 14 deletions
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28
lib/CodeGen/TailDuplication.cpp
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@ -433,29 +433,29 @@ bool
TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
SmallVector<MachineBasicBlock*, 8> &TDBBs,
SmallVector<MachineInstr*, 16> &Copies) {
// Pre-regalloc tail duplication hurts compile time and doesn't help
// much except for indirect branches.
bool hasIndirectBranch = (!TailBB->empty() &&
TailBB->back().getDesc().isIndirectBranch());
if (PreRegAlloc && !hasIndirectBranch)
return false;
// Set the limit on the number of instructions to duplicate, with a default
// of one less than the tail-merge threshold. When optimizing for size,
// duplicate only one, because one branch instruction can be eliminated to
// compensate for the duplication.
unsigned MaxDuplicateCount;
if (hasIndirectBranch)
// 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.
MaxDuplicateCount = 20;
else if (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
if (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
MaxDuplicateCount = 1;
else
MaxDuplicateCount = TailDuplicateSize;
if (PreRegAlloc) {
// Pre-regalloc tail duplication hurts compile time and doesn't help
// much except for indirect branches.
if (TailBB->empty() || !TailBB->back().getDesc().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;
}
// Don't try to tail-duplicate single-block loops.
if (TailBB->isSuccessor(TailBB))
return false;