6502/llvm-6502
1
0
Fork 0
mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2026-04-21 08:17:40 +00:00
Code Issues Projects Releases Wiki Activity

Add trace accessor methods, implement primitive if-conversion heuristic.

Browse Source 
Compare the critical paths of the two traces through an if-conversion
candidate. If the difference is larger than the branch brediction
penalty, reject the if-conversion. If would never pay.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161433 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jakob Stoklund Olesen
2012-08-07 18:02:19 +00:00
parent 3f5d1a2396
commit 84ef6ba443
3 changed files with 84 additions and 21 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/CodeGen/EarlyIfConversion.cpp
+18 -4
View File
@@ -601,10 +601,24 @@ void EarlyIfConverter::invalidateTraces() {
bool EarlyIfConverter::shouldConvertIf() {
if (!MinInstr)
MinInstr = Traces->getEnsemble(MachineTraceMetrics::TS_MinInstrCount);
DEBUG({
dbgs() << MinInstr->getTrace(IfConv.Head);
MinInstr->print(dbgs());
});
// MCSchedModel doesn't yet provide a misprediction penalty.
unsigned MispredictPenalty = 10;
// Compare the critical path through TBB and FBB. If the difference is
// greater than the branch misprediction penalty, it would never pay to
// if-convert. The triangle/diamond topology guarantees that these traces
// have the same head and tail, so they can be compared.
MachineTraceMetrics::Trace TBBTrace = MinInstr->getTrace(IfConv.TBB);
MachineTraceMetrics::Trace FBBTrace = MinInstr->getTrace(IfConv.FBB);
DEBUG(dbgs() << "TBB: " << TBBTrace << "FBB: " << FBBTrace);
unsigned TBBCrit = TBBTrace.getCriticalPath();
unsigned FBBCrit = FBBTrace.getCriticalPath();
unsigned ExtraCrit = TBBCrit > FBBCrit ? TBBCrit-FBBCrit : FBBCrit-TBBCrit;
if (ExtraCrit >= MispredictPenalty) {
DEBUG(dbgs() << "Critical path difference too large.\n");
return false;
}
return true;
}