@@ -767,12 +767,23 @@ GlobOptBlockData::MergeValueMaps(
767767
768768 if (iter2.IsValid () && bucket.value ->m_id == iter2.Data ().value ->m_id )
769769 {
770+ // Syms that are assigned to within the loop must have unique
771+ // value numbers in the loop header after merging; a single
772+ // prepass is not adequate to determine that sym values are
773+ // equivalent through all possible loop paths.
774+ bool forceUniqueValue =
775+ isLoopBackEdge &&
776+ !this ->globOpt ->IsLoopPrePass () &&
777+ loop &&
778+ loop->symsAssignedToInLoop ->Test (bucket.value ->m_id );
779+
770780 newValue =
771781 this ->MergeValues (
772782 bucket.element ,
773783 iter2.Data ().element ,
774784 iter2.Data ().value ,
775785 isLoopBackEdge,
786+ forceUniqueValue,
776787 symsRequiringCompensation,
777788 symsCreatedForMerge);
778789 }
@@ -847,6 +858,7 @@ GlobOptBlockData::MergeValues(
847858 Value *fromDataValue,
848859 Sym *fromDataSym,
849860 bool isLoopBackEdge,
861+ bool forceUniqueValue,
850862 BVSparse<JitArenaAllocator> *const symsRequiringCompensation,
851863 BVSparse<JitArenaAllocator> *const symsCreatedForMerge)
852864{
@@ -879,22 +891,30 @@ GlobOptBlockData::MergeValues(
879891 return toDataValue;
880892 }
881893
882- // There may be other syms in toData that haven't been merged yet, referring to the current toData value for this sym. If
883- // the merge produced a new value info, don't corrupt the value info for the other sym by changing the same value. Instead,
884- // create one value per source value number pair per merge and reuse that for new value infos.
885- Value *newValue = this ->globOpt ->valuesCreatedForMerge ->Lookup (sourceValueNumberPair, nullptr );
886- if (newValue)
894+ Value *newValue = nullptr ;
895+ if (forceUniqueValue)
887896 {
888- Assert (sameValueNumber == (newValue->GetValueNumber () == toDataValue->GetValueNumber ()));
889-
890- // This is an exception where Value::SetValueInfo is called directly instead of GlobOpt::ChangeValueInfo, because we're
891- // actually generating new value info through merges.
892- newValue->SetValueInfo (newValueInfo);
897+ newValue = this ->globOpt ->NewValue (newValueInfo);
893898 }
894899 else
895900 {
896- newValue = this ->globOpt ->NewValue (sameValueNumber ? sourceValueNumberPair.First () : this ->globOpt ->NewValueNumber (), newValueInfo);
897- this ->globOpt ->valuesCreatedForMerge ->Add (sourceValueNumberPair, newValue);
901+ // There may be other syms in toData that haven't been merged yet, referring to the current toData value for this sym. If
902+ // the merge produced a new value info, don't corrupt the value info for the other sym by changing the same value. Instead,
903+ // create one value per source value number pair per merge and reuse that for new value infos.
904+ newValue = this ->globOpt ->valuesCreatedForMerge ->Lookup (sourceValueNumberPair, nullptr );
905+ if (newValue)
906+ {
907+ Assert (sameValueNumber == (newValue->GetValueNumber () == toDataValue->GetValueNumber ()));
908+
909+ // This is an exception where Value::SetValueInfo is called directly instead of GlobOpt::ChangeValueInfo, because we're
910+ // actually generating new value info through merges.
911+ newValue->SetValueInfo (newValueInfo);
912+ }
913+ else
914+ {
915+ newValue = this ->globOpt ->NewValue (sameValueNumber ? sourceValueNumberPair.First () : this ->globOpt ->NewValueNumber (), newValueInfo);
916+ this ->globOpt ->valuesCreatedForMerge ->Add (sourceValueNumberPair, newValue);
917+ }
898918 }
899919
900920 // Set symStore if same on both paths.
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