[PGO][CHR] Guard against 0-to-0 branch weight and avoid division by zero crash.

[hjyamauchi]

Summary

This fixes a crash in internal builds under SamplePGO.

[davidxl]

lgtm

llvm/lib/Transforms/Instrumentation/ControlHeightReduction.cpp

if (!TrueWeight || !FalseWeight)
     return false;
   uint64_t TrueWt = TrueWeight->getValue().getZExtValue();
   uint64_t FalseWt = FalseWeight->getValue().getZExtValue();
   uint64_t SumWt = TrueWt + FalseWt;
 
   assert(SumWt >= TrueWt && SumWt >= FalseWt &&
          "Overflow calculating branch probabilities.");
 
+  // Guard against 0-to-0 branch weights to avoid a division-by-zero crash.
+  if (SumWt == 0)
+    return false;
+
   TrueProb = BranchProbability::getBranchProbability(TrueWt, SumWt);
   FalseProb = BranchProbability::getBranchProbability(FalseWt, SumWt);
   return true;
 }
 
 static BranchProbability getCHRBiasThreshold() {
   return BranchProbability::getBranchProbability(
       static_cast<uint64_t>(CHRBiasThreshold * 1000000), 1000000);

llvm/test/Transforms/PGOProfile/chr.ll

body.9:
   %v8_9 = add i64 %v7_9, %v6_9
   %v9_9 = add i64 %v8_9, %v7_9
   br label %end
 
 end:
   ret i64 99
 }
 
+; Test to not crash upon a 0:0 branch_weight metadata.
+define void @test_chr_24(i32* %i) !prof !14 {
+entry:
+  %0 = load i32, i32* %i
+  %1 = and i32 %0, 1
+  %2 = icmp eq i32 %1, 0
+  br i1 %2, label %bb1, label %bb0, !prof !17
+
+bb0:
+  call void @foo()
+  br label %bb1
+
+bb1:
+  %3 = and i32 %0, 2
+  %4 = icmp eq i32 %3, 0
+  br i1 %4, label %bb3, label %bb2, !prof !17
+
+bb2:
+  call void @foo()
+  br label %bb3
+
+bb3:
+  ret void
+}
+
 !llvm.module.flags = !{!0}
 !0 = !{i32 1, !"ProfileSummary", !1}
 !1 = !{!2, !3, !4, !5, !6, !7, !8, !9}
 !2 = !{!"ProfileFormat", !"InstrProf"}
 !3 = !{!"TotalCount", i64 10000}
 !4 = !{!"MaxCount", i64 10}
 !5 = !{!"MaxInternalCount", i64 1}
 !6 = !{!"MaxFunctionCount", i64 1000}
 !7 = !{!"NumCounts", i64 3}
 !8 = !{!"NumFunctions", i64 3}
 !9 = !{!"DetailedSummary", !10}
 !10 = !{!11, !12, !13}
 !11 = !{i32 10000, i64 100, i32 1}
 !12 = !{i32 999000, i64 100, i32 1}
 !13 = !{i32 999999, i64 1, i32 2}
 
 !14 = !{!"function_entry_count", i64 100}
 !15 = !{!"branch_weights", i32 0, i32 1}
 !16 = !{!"branch_weights", i32 1, i32 1}
+!17 = !{!"branch_weights", i32 0, i32 0}
 ; CHECK: !15 = !{!"branch_weights", i32 1000, i32 0}
 ; CHECK: !16 = !{!"branch_weights", i32 0, i32 1}
 ; CHECK: !17 = !{!"branch_weights", i32 1, i32 1}
 ; CHECK: !18 = !{!"branch_weights", i32 1, i32 0}
 ; CHECK: !19 = !{!"branch_weights", i32 0, i32 1000}