Possible bug in ValueType.GetHashCode() for fields smaller than 4 bytes

[tomspilman]

I found the results of GetHashCode() odd for a struct i was testing:

https://dotnetfiddle.net/BTgRga

The reason i get these results seem to be coming from this line:

https://github.com/dotnet/coreclr/blob/master/src/vm/comutilnative.cpp#L2129

It does (INT32)(fieldSize / sizeof(INT32)) and when fieldSize is less than 4 we get 0 which results in no hash contribution for the field.

While this is a valid result for GetHashCode() it doesn't seem to be the intent of the code written and may be a bug.

[danmoseley]

Same in FastGetValueTypeHashCodeHelper

[tomspilman]

@danmosemsft - Yes in the case of a struct that is tightly packed and is less than 4 bytes in size....
FastGetValueTypeHashCodeHelper would return 0.

[ionuttamas]

@tomspilman the same issue is by replacing char with byte or bool.

[tomspilman]

the same issue is by replacing char with byte or bool.

Yes it occurs with any type less than 4 bytes.

[RussKeldorph]

@jkotas / @sergiy-k Could you triage?

[jkotas]

The default ValueType hashcode is poor and has a lot of issues: #4414, #9092, ... . This is yet another one.

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