From 75d9d5159e70b9255a65aed45fd1adefde0be7c8 Mon Sep 17 00:00:00 2001 From: Jakob Stoklund Olesen Date: Tue, 7 Aug 2012 18:02:19 +0000 Subject: 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. llvm-svn: 161433 --- llvm/lib/CodeGen/EarlyIfConversion.cpp | 22 ++++++++++++++++++---- 1 file changed, 18 insertions(+), 4 deletions(-) (limited to 'llvm/lib/CodeGen/EarlyIfConversion.cpp') diff --git a/llvm/lib/CodeGen/EarlyIfConversion.cpp b/llvm/lib/CodeGen/EarlyIfConversion.cpp index e49b9d3e18e..8adaa6dd0bf 100644 --- a/llvm/lib/CodeGen/EarlyIfConversion.cpp +++ b/llvm/lib/CodeGen/EarlyIfConversion.cpp @@ -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; } -- cgit v1.2.3