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Change pre-regalloc tail duplication to only duplicate indirect branch blocks.

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The pre-regalloc pass caused some regressions in both compile time and
performance of the generated code, and it did not improve performance, except
for indirect branches.  I also moved the check for single-block loops to speed
up the common case when running the taildup pass before reg allocation.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93505 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bob Wilson 2010-01-15 06:29:17 +00:00
parent 283ba2fbb4
commit bfdcf3bd49
1 changed files with 11 additions and 4 deletions
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15
lib/CodeGen/TailDuplication.cpp
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@ -253,7 +253,7 @@ bool TailDuplicatePass::TailDuplicateBlocks(MachineFunction &MF) {
SSAUpdateVals.clear();
}
// Eliminate some of the copies inserted tail duplication to maintain
// Eliminate some of the copies inserted by tail duplication to maintain
// SSA form.
for (unsigned i = 0, e = Copies.size(); i != e; ++i) {
MachineInstr *Copy = Copies[i];
@ -437,8 +437,11 @@ bool
TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
SmallVector<MachineBasicBlock*, 8> &TDBBs,
SmallVector<MachineInstr*, 16> &Copies) {
// Don't try to tail-duplicate single-block loops.
if (TailBB->isSuccessor(TailBB))
// 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
@ -446,7 +449,7 @@ TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
// duplicate only one, because one branch instruction can be eliminated to
// compensate for the duplication.
unsigned MaxDuplicateCount;
if (!TailBB->empty() && TailBB->back().getDesc().isIndirectBranch())
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
@ -457,6 +460,10 @@ TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
else
MaxDuplicateCount = TailDuplicateSize;
// Don't try to tail-duplicate single-block loops.
if (TailBB->isSuccessor(TailBB))
return false;
// Check the instructions in the block to determine whether tail-duplication
// is invalid or unlikely to be profitable.
unsigned InstrCount = 0;