[TargetTransformInfo] move branch probability query from TargetLoweringInfo

This is no-functional-change intended (NFC), but needed to allow
optimizer passes to use the API. See D98898 for a proposed usage
by SimplifyCFG.

I'm simplifying the code by removing the cl::opt. That was added
back with the original commit in D19488, but I don't see any
evidence in regression tests that it was used. Target-specific
overrides can use the usual patterns to adjust as necessary.
We could also restore that cl::opt, but it was not clear to me
exactly how to do it in the convoluted TTI class structure.

Author: rotateright

llvm/include/llvm/Analysis/TargetTransformInfo.h

@@ -27,6 +27,7 @@
 #include "llvm/IR/PassManager.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/AtomicOrdering.h"
+#include "llvm/Support/BranchProbability.h"
 #include "llvm/Support/DataTypes.h"
 #include "llvm/Support/InstructionCost.h"
 #include <functional>
@@ -328,6 +329,10 @@ class TargetTransformInfo {
     return getUserCost(U, Operands, CostKind);
   }
 
+  /// If a branch or a select condition is skewed in one direction by more than
+  /// this factor, it is very likely to be predicted correctly.
+  BranchProbability getPredictableBranchThreshold() const;
+
   /// Return true if branch divergence exists.
   ///
   /// Branch divergence has a significantly negative impact on GPU performance
@@ -1400,6 +1405,7 @@ class TargetTransformInfo::Concept {
                                    BlockFrequencyInfo *BFI) = 0;
   virtual int getUserCost(const User *U, ArrayRef<const Value *> Operands,
                           TargetCostKind CostKind) = 0;
+  virtual BranchProbability getPredictableBranchThreshold() = 0;
   virtual bool hasBranchDivergence() = 0;
   virtual bool useGPUDivergenceAnalysis() = 0;
   virtual bool isSourceOfDivergence(const Value *V) = 0;
@@ -1691,6 +1697,9 @@ class TargetTransformInfo::Model final : public TargetTransformInfo::Concept {
                   TargetCostKind CostKind) override {
     return Impl.getUserCost(U, Operands, CostKind);
   }
+  BranchProbability getPredictableBranchThreshold() override {
+    return Impl.getPredictableBranchThreshold();
+  }
   bool hasBranchDivergence() override { return Impl.hasBranchDivergence(); }
   bool useGPUDivergenceAnalysis() override {
     return Impl.useGPUDivergenceAnalysis();

llvm/include/llvm/Analysis/TargetTransformInfoImpl.h

@@ -78,6 +78,16 @@ class TargetTransformInfoImplBase {
     return TTI::TCC_Expensive;
   }
 
+  // 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.
+  BranchProbability getPredictableBranchThreshold() const {
+    return BranchProbability(99, 100);
+  }
+
   bool hasBranchDivergence() const { return false; }
 
   bool useGPUDivergenceAnalysis() const { return false; }

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -528,10 +528,6 @@ class TargetLoweringBase {
     return false;
   }
 
-  /// If a branch or a select condition is skewed in one direction by more than
-  /// this factor, it is very likely to be predicted correctly.
-  virtual BranchProbability getPredictableBranchThreshold() const;
-
   /// Return true if the following transform is beneficial:
   /// fold (conv (load x)) -> (load (conv*)x)
   /// On architectures that don't natively support some vector loads

llvm/lib/Analysis/TargetTransformInfo.cpp

@@ -228,6 +228,10 @@ int TargetTransformInfo::getUserCost(const User *U,
   return Cost;
 }
 
+BranchProbability TargetTransformInfo::getPredictableBranchThreshold() const {
+  return TTIImpl->getPredictableBranchThreshold();
+}
+
 bool TargetTransformInfo::hasBranchDivergence() const {
   return TTIImpl->hasBranchDivergence();
 }

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -6587,7 +6587,7 @@ static bool isFormingBranchFromSelectProfitable(const TargetTransformInfo *TTI,
     uint64_t Sum = TrueWeight + FalseWeight;
     if (Sum != 0) {
       auto Probability = BranchProbability::getBranchProbability(Max, Sum);
-      if (Probability > TLI->getPredictableBranchThreshold())
+      if (Probability > TTI->getPredictableBranchThreshold())
         return true;
     }
   }

llvm/lib/CodeGen/TargetLoweringBase.cpp

@@ -45,7 +45,6 @@
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Type.h"
-#include "llvm/Support/BranchProbability.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
@@ -114,17 +113,6 @@ static bool darwinHasSinCos(const Triple &TT) {
   return true;
 }
 
-// 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);
-
 void TargetLoweringBase::InitLibcalls(const Triple &TT) {
 #define HANDLE_LIBCALL(code, name) \
   setLibcallName(RTLIB::code, name);
@@ -1736,10 +1724,6 @@ bool TargetLoweringBase::allowsMemoryAccess(LLVMContext &Context,
                             MMO.getAlign(), MMO.getFlags(), Fast);
 }
 
-BranchProbability TargetLoweringBase::getPredictableBranchThreshold() const {
-  return BranchProbability(MinPercentageForPredictableBranch, 100);
-}
-
 //===----------------------------------------------------------------------===//
 //  TargetTransformInfo Helpers
 //===----------------------------------------------------------------------===//

llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp

@@ -46,7 +46,7 @@ STATISTIC(ExpectIntrinsicsHandled,
 // WARNING: these values are internal implementation detail of the pass.
 // They should not be exposed to the outside of the pass, front-end codegen
 // should emit @llvm.expect intrinsics instead of using these weights directly.
-// Transforms should use TargetLowering getPredictableBranchThreshold() hook.
+// Transforms should use TargetTransformInfo's getPredictableBranchThreshold().
 static cl::opt<uint32_t> LikelyBranchWeight(
     "likely-branch-weight", cl::Hidden, cl::init(2000),
     cl::desc("Weight of the branch likely to be taken (default = 2000)"));