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give branch folding a simple heuristic to decide which block to split so that

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it inserts an uncond branch where it is less likely to cause a problem.  This
fixes some perf issues on ppc.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31354 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2006-11-01 19:36:29 +00:00
parent f4208026aa
commit d4bf3c2fd6
1 changed files with 49 additions and 6 deletions
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55
lib/CodeGen/BranchFolding.cpp
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@ -283,6 +283,44 @@ MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
return NewMBB;
}
/// EstimateRuntime - Make a rough estimate for how long it will take to run
/// the specified code.
static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
MachineBasicBlock::iterator E,
const TargetInstrInfo *TII) {
unsigned Time = 0;
for (; I != E; ++I) {
const TargetInstrDescriptor &TID = TII->get(I->getOpcode());
if (TID.Flags & M_CALL_FLAG)
Time += 10;
else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG))
Time += 2;
else
++Time;
}
return Time;
}
/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at
/// MBB2I and then insert an unconditional branch in the other block. Determine
/// which is the best to split
static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1,
MachineBasicBlock::iterator MBB1I,
MachineBasicBlock *MBB2,
MachineBasicBlock::iterator MBB2I,
const TargetInstrInfo *TII) {
// 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.
unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII);
unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII);
// If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the
// MBB1 block so it falls through. This will penalize the MBB2 path, but will
// have a lower overall impact on the program execution.
return MBB1Time < MBB2Time;
}
bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
MadeChange = false;
@ -355,12 +393,17 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
MergePotentials.pop_back();
continue;
}
// TODO: if we had some notion of which block was hotter, we could split
// the hot block, so it is the fall-through. For now, just split the
// second block.
MBB2 = SplitMBBAt(*MBB2, BBI2);
BBI2 = MBB2->begin();
(MergePotentials.end()-2)->second = MBB2;
// Decide whether we want to split CurMBB or MBB2.
if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII)) {
CurMBB = SplitMBBAt(*CurMBB, BBI1);
BBI1 = CurMBB->begin();
MergePotentials.back().second = CurMBB;
} else {
MBB2 = SplitMBBAt(*MBB2, BBI2);
BBI2 = MBB2->begin();
(MergePotentials.end()-2)->second = MBB2;
}
}
if (MBB2->begin() == BBI2) {