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Add a proper if-conversion cost model.

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Detect when there is not enough available ILP, so if-conversion can't
speculate instructions for free.

Compute the lengthening of the critical path when inserting a select
instruction that depends on the condition as well as both sides of the
if.

Reject conversions that would stretch the critical path by more than
half a mispredict penalty.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161713 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jakob Stoklund Olesen 2012-08-10 22:27:31 +00:00
parent a35bf506f4
commit eb74c08192
1 changed files with 77 additions and 13 deletions
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90
lib/CodeGen/EarlyIfConversion.cpp
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@ -599,10 +599,16 @@ void EarlyIfConverter::invalidateTraces() {
Traces->invalidate(IfConv.Tail);
Traces->invalidate(IfConv.TBB);
Traces->invalidate(IfConv.FBB);
DEBUG(if (MinInstr) MinInstr->print(dbgs()));
Traces->verifyAnalysis();
}
// Adjust cycles with downward saturation.
static unsigned adjCycles(unsigned Cyc, int Delta) {
if (Delta < 0 && Cyc + Delta > Cyc)
return 0;
return Cyc + Delta;
}
/// Apply cost model and heuristics to the if-conversion in IfConv.
/// Return true if the conversion is a good idea.
///
@ -614,21 +620,79 @@ bool EarlyIfConverter::shouldConvertIf() {
if (!MinInstr)
MinInstr = Traces->getEnsemble(MachineTraceMetrics::TS_MinInstrCount);
// 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);
MachineTraceMetrics::Trace TBBTrace = MinInstr->getTrace(IfConv.getTPred());
MachineTraceMetrics::Trace FBBTrace = MinInstr->getTrace(IfConv.getFPred());
DEBUG(dbgs() << "TBB: " << TBBTrace << "FBB: " << FBBTrace);
unsigned TBBCrit = TBBTrace.getCriticalPath();
unsigned FBBCrit = FBBTrace.getCriticalPath();
unsigned ExtraCrit = TBBCrit > FBBCrit ? TBBCrit-FBBCrit : FBBCrit-TBBCrit;
if (ExtraCrit >= SchedModel->MispredictPenalty) {
DEBUG(dbgs() << "Critical path difference larger than "
<< SchedModel->MispredictPenalty << ".\n");
unsigned MinCrit = std::min(TBBTrace.getCriticalPath(),
FBBTrace.getCriticalPath());
// Set a somewhat arbitrary limit on the critical path extension we accept.
unsigned CritLimit = SchedModel->MispredictPenalty/2;
// If-conversion only makes sense when there is unexploited ILP. Compute the
// maximum-ILP resource length of the trace after if-conversion. Compare it
// to the shortest critical path.
SmallVector<const MachineBasicBlock*, 1> ExtraBlocks;
if (IfConv.TBB != IfConv.Tail)
ExtraBlocks.push_back(IfConv.TBB);
unsigned ResLength = FBBTrace.getResourceLength(ExtraBlocks);
DEBUG(dbgs() << "Resource length " << ResLength
<< ", minimal critical path " << MinCrit << '\n');
if (ResLength > MinCrit + CritLimit) {
DEBUG(dbgs() << "Not enough available ILP.\n");
return false;
}
// Assume that the depth of the first head terminator will also be the depth
// of the select instruction inserted, as determined by the flag dependency.
// TBB / FBB data dependencies may delay the select even more.
MachineTraceMetrics::Trace HeadTrace = MinInstr->getTrace(IfConv.Head);
unsigned BranchDepth =
HeadTrace.getInstrCycles(IfConv.Head->getFirstTerminator()).Depth;
DEBUG(dbgs() << "Branch depth: " << BranchDepth << '\n');
// Look at all the tail phis, and compute the critical path extension caused
// by inserting select instructions.
MachineTraceMetrics::Trace TailTrace = MinInstr->getTrace(IfConv.Tail);
for (unsigned i = 0, e = IfConv.PHIs.size(); i != e; ++i) {
SSAIfConv::PHIInfo &PI = IfConv.PHIs[i];
unsigned Slack = TailTrace.getInstrSlack(PI.PHI);
unsigned MaxDepth = Slack + TailTrace.getInstrCycles(PI.PHI).Depth;
DEBUG(dbgs() << "Slack " << Slack << ":\t" << *PI.PHI);
// The condition is pulled into the critical path.
unsigned CondDepth = adjCycles(BranchDepth, PI.CondCycles);
if (CondDepth > MaxDepth) {
unsigned Extra = CondDepth - MaxDepth;
DEBUG(dbgs() << "Condition adds " << Extra << " cycles.\n");
if (Extra > CritLimit) {
DEBUG(dbgs() << "Exceeds limit of " << CritLimit << '\n');
return false;
}
}
// The TBB value is pulled into the critical path.
unsigned TDepth = adjCycles(TBBTrace.getPHIDepth(PI.PHI), PI.TCycles);
if (TDepth > MaxDepth) {
unsigned Extra = TDepth - MaxDepth;
DEBUG(dbgs() << "TBB data adds " << Extra << " cycles.\n");
if (Extra > CritLimit) {
DEBUG(dbgs() << "Exceeds limit of " << CritLimit << '\n');
return false;
}
}
// The FBB value is pulled into the critical path.
unsigned FDepth = adjCycles(FBBTrace.getPHIDepth(PI.PHI), PI.FCycles);
if (FDepth > MaxDepth) {
unsigned Extra = FDepth - MaxDepth;
DEBUG(dbgs() << "FBB data adds " << Extra << " cycles.\n");
if (Extra > CritLimit) {
DEBUG(dbgs() << "Exceeds limit of " << CritLimit << '\n');
return false;
}
}
}
return true;
}