[IfConversion] Only renormalize probabilities if branches are analyzable

[kparzysz]

Summary

If a block has non-analyzable branches, the listed successors don't need to add up to one. For example, if a block has a conditional tail call, that tail call will not have a corresponding successor in the successor list, but will still be a possible branch.

lib/CodeGen/IfConversion.cpp

void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI, bool AddEdges) {
   if (FromTI != FromMBB.end() && !TII->isPredicated(*FromTI))
     ToTI = ToBBI.BB->end();
   ToBBI.BB->splice(ToTI, &FromMBB, FromTI, FromMBB.end());
 
   // Force normalizing the successors' probabilities of ToBBI.BB to convert all
   // unknown probabilities into known ones.
   // FIXME: This usage is too tricky and in the future we would like to
   // eliminate all unknown probabilities in MBB.
+  if (ToBBI.IsBrAnalyzable)
     ToBBI.BB->normalizeSuccProbs();
 
   SmallVector<MachineBasicBlock *, 4> FromSuccs(FromMBB.succ_begin(),
                                                 FromMBB.succ_end());
   MachineBasicBlock *NBB = getNextBlock(FromMBB);
   MachineBasicBlock *FallThrough = FromBBI.HasFallThrough ? NBB : nullptr;
   // The edge probability from ToBBI.BB to FromMBB, which is only needed when
   // AddEdges is true and FromMBB is a successor of ToBBI.BB.
   auto To2FromProb = BranchProbability::getZero();
void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI, bool AddEdges) {
   }
 
   // Now FromBBI always falls through to the next block!
   if (NBB && !FromMBB.isSuccessor(NBB))
     FromMBB.addSuccessor(NBB);
 
   // Normalize the probabilities of ToBBI.BB's successors with all adjustment
   // we've done above.
+  if (ToBBI.IsBrAnalyzable && FromBBI.IsBrAnalyzable)
     ToBBI.BB->normalizeSuccProbs();
 
   ToBBI.Predicate.append(FromBBI.Predicate.begin(), FromBBI.Predicate.end());
   FromBBI.Predicate.clear();
 
   ToBBI.NonPredSize += FromBBI.NonPredSize;
   ToBBI.ExtraCost += FromBBI.ExtraCost;
   ToBBI.ExtraCost2 += FromBBI.ExtraCost2;
   FromBBI.NonPredSize = 0;

test/CodeGen/Hexagon/ifcvt-simple-bprob.ll

+; RUN: llc -march=hexagon < %s
+
+; Check that branch probabilities are set correctly after performing the
+; simple variant of if-conversion. The converted block has a branch that
+; is not analyzable.
+
+target triple = "hexagon"
+
+declare void @foo()
+
+; CHECK-LABEL: danny
+; CHECK: if (p0.new) jump:nt foo
+define void @danny(i32 %x) {
+  %t0 = icmp sgt i32 %x, 0
+  br i1 %t0, label %tail, label %exit, !prof !0
+tail:
+  tail call void @foo();
+  ret void
+exit:
+  ret void
+}
+
+; CHECK-LABEL: sammy
+; CHECK: if (!p0.new) jump:t foo
+define void @sammy(i32 %x) {
+  %t0 = icmp sgt i32 %x, 0
+  br i1 %t0, label %exit, label %tail, !prof !0
+tail:
+  tail call void @foo();
+  ret void
+exit:
+  ret void
+}
+
+!0 = !{!"branch_weights", i32 1, i32 2000}
+