[Branch probability] Recompute branch weights of tail-merged basic blocks.

BranchFolderPass was not correctly setting the basic block branch weights when
tail-merging created or merged blocks. This patch recomutes the weights of
tail-merged blocks using the following formula:

branch_weight(merged block to successor j) =
sum(block_frequency(bb) * branch_probability(bb -> j))

bb is a block that is in the set of merged blocks.

<rdar://problem/16256423>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215135 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/IfConversion.cpp

@@ -17,6 +17,7 @@
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/LivePhysRegs.h"
+#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
@@ -161,6 +162,7 @@ namespace {
     const TargetLoweringBase *TLI;
     const TargetInstrInfo *TII;
     const TargetRegisterInfo *TRI;
+    const MachineBlockFrequencyInfo *MBFI;
     const MachineBranchProbabilityInfo *MBPI;
     MachineRegisterInfo *MRI;
 
@@ -177,6 +179,7 @@ namespace {
     }
 
     void getAnalysisUsage(AnalysisUsage &AU) const override {
+      AU.addRequired<MachineBlockFrequencyInfo>();
       AU.addRequired<MachineBranchProbabilityInfo>();
       MachineFunctionPass::getAnalysisUsage(AU);
     }
@@ -272,6 +275,7 @@ bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
   TLI = MF.getSubtarget().getTargetLowering();
   TII = MF.getSubtarget().getInstrInfo();
   TRI = MF.getSubtarget().getRegisterInfo();
+  MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
   MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
   MRI = &MF.getRegInfo();
 
@@ -286,7 +290,7 @@ bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
   bool BFChange = false;
   if (!PreRegAlloc) {
     // Tail merge tend to expose more if-conversion opportunities.
-    BranchFolder BF(true, false);
+    BranchFolder BF(true, false, *MBFI, *MBPI);
     BFChange = BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(),
                                    getAnalysisIfAvailable<MachineModuleInfo>());
   }
@@ -419,7 +423,7 @@ bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
   BBAnalysis.clear();
 
   if (MadeChange && IfCvtBranchFold) {
-    BranchFolder BF(false, false);
+    BranchFolder BF(false, false, *MBFI, *MBPI);
     BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(),
                         getAnalysisIfAvailable<MachineModuleInfo>());
   }