Add trace accessor methods, implement primitive if-conversion heuristic.
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
diff --git a/lib/CodeGen/EarlyIfConversion.cpp b/lib/CodeGen/EarlyIfConversion.cpp
index e49b9d3..8adaa6d 100644
--- a/lib/CodeGen/EarlyIfConversion.cpp
+++ b/lib/CodeGen/EarlyIfConversion.cpp
@@ -601,10 +601,24 @@
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;
}