[CodeGenPrepare] use branch weight metadata to decide if a select should be turned into a branch

This is part of solving PR27344:
https://llvm.org/bugs/show_bug.cgi?id=27344

CGP should undo the SimplifyCFG transform for the same reason that earlier patches have used this
same mechanism: it's possible that passes between SimplifyCFG and CGP may be able to optimize the
IR further with a select in place.

For the TLI hook default, >99% taken or not taken is chosen as the default threshold for a highly
predictable branch. Even the most limited HW branch predictors will be correct on this branch almost
all the time, so even a massive mispredict penalty perf loss would be overcome by the win from all
the times the branch was predicted correctly.

As a follow-up, we could make the default target hook less conservative by using the SchedMachineModel's
MispredictPenalty. Or we could just let targets override the default by implementing the hook with that
and other target-specific options. Note that trying to statically determine mispredict rates for 
close-to-balanced profile weight data is generally impossible if the HW is sufficiently advanced. Ie, 
50/50 taken/not-taken might still be 100% predictable.

Finally, note that this patch as-is will not solve PR27344 because the current __builtin_unpredictable()
branch weight default values are 4 and 64. A proposal to change that is in D19435.

Differential Revision: http://reviews.llvm.org/D19488

llvm-svn: 267572

1 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringBase.cpp b/llvm/lib/CodeGen/TargetLoweringBase.cpp
index aa4e03548b9..0355b8dd135 100644
--- a/llvm/lib/CodeGen/TargetLoweringBase.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringBase.cpp
@@ -28,6 +28,7 @@
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
+#include "llvm/Support/BranchProbability.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
@@ -43,6 +44,17 @@ static cl::opt<bool> JumpIsExpensiveOverride(
     cl::desc("Do not create extra branches to split comparison logic."),
     cl::Hidden);
 
+// Although this default value is arbitrary, it is not random. It is assumed
+// that a condition that evaluates the same way by a higher percentage than this
+// is best represented as control flow. Therefore, the default value N should be
+// set such that the win from N% correct executions is greater than the loss
+// from (100 - N)% mispredicted executions for the majority of intended targets.
+static cl::opt<int> MinPercentageForPredictableBranch(
+    "min-predictable-branch", cl::init(99),
+    cl::desc("Minimum percentage (0-100) that a condition must be either true "
+             "or false to assume that the condition is predictable"),
+    cl::Hidden);
+
 /// InitLibcallNames - Set default libcall names.
 ///
 static void InitLibcallNames(const char **Names, const Triple &TT) {
@@ -1625,6 +1637,9 @@ bool TargetLoweringBase::allowsMemoryAccess(LLVMContext &Context,
   return allowsMisalignedMemoryAccesses(VT, AddrSpace, Alignment, Fast);
 }
 
+BranchProbability TargetLoweringBase::getPredictableBranchThreshold() const {
+  return BranchProbability(MinPercentageForPredictableBranch, 100);
+}
 
 //===----------------------------------------------------------------------===//
 //  TargetTransformInfo Helpers