[TTI][NFC] Introduce option to set predictable branch threshold

[aleksandr.popov]

Summary

Currently TargetTransformInfo::getPredictableBranchThreshold() method
returns hardcoded value 99. This value affects the decision whether to
convert select instruction to branch or not in several passes:
SelectOptimize, CodeGenPrepare, SimplifyCFG.

It would be useful to make possible to play with that threshold in order
to test select-optimize heuristics.

Option was originally introduced in the TargetLoweringBase, but was
removed in the revision 664d0c052c315 and not restored in the TTI

[spatel]

LGTM

llvm/lib/Analysis/TargetTransformInfo.cpp

static cl::opt<bool> EnableReduxCost("costmodel-reduxcost", cl::init(false),
                                      cl::Hidden,
                                      cl::desc("Recognize reduction patterns."));
 
 static cl::opt<unsigned> CacheLineSize(
     "cache-line-size", cl::init(0), cl::Hidden,
     cl::desc("Use this to override the target cache line size when "
              "specified by the user."));
 
+static cl::opt<unsigned> PredictableBranchThreshold(
+    "predictable-branch-threshold", cl::init(99), cl::Hidden,
+    cl::desc(
+        "Use this to override the target's predictable branch threshold (%)."));
+
 namespace {
 /// No-op implementation of the TTI interface using the utility base
 /// classes.
 ///
 /// This is used when no target specific information is available.
 struct NoTTIImpl : TargetTransformInfoImplCRTPBase<NoTTIImpl> {
   explicit NoTTIImpl(const DataLayout &DL)
       : TargetTransformInfoImplCRTPBase<NoTTIImpl>(DL) {}
TargetTransformInfo::getInstructionCost(const User *U,
                                         enum TargetCostKind CostKind) const {
   InstructionCost Cost = TTIImpl->getInstructionCost(U, Operands, CostKind);
   assert((CostKind == TTI::TCK_RecipThroughput || Cost >= 0) &&
          "TTI should not produce negative costs!");
   return Cost;
 }
 
 BranchProbability TargetTransformInfo::getPredictableBranchThreshold() const {
-  return TTIImpl->getPredictableBranchThreshold();
+  return PredictableBranchThreshold.getNumOccurrences() > 0
+             ? BranchProbability(PredictableBranchThreshold, 100)
+             : TTIImpl->getPredictableBranchThreshold();
 }
 
 bool TargetTransformInfo::hasBranchDivergence() const {
   return TTIImpl->hasBranchDivergence();
 }
 
 bool TargetTransformInfo::useGPUDivergenceAnalysis() const {
   return TTIImpl->useGPUDivergenceAnalysis();