[lworld] RFR: 8373787: [lworld] InstanceKlass::size mixes bytes and words

[Paul Hübner]

On Tue, 16 Dec 2025 12:49:20 GMT, Paul Hübner <phubner at openjdk.org> wrote:

>> The following function in `InstanceKlass` is supposed to return size in words:
>> 
>>   static int size(int vtable_length, int itable_length,
>>                   int nonstatic_oop_map_size,
>>                   bool is_interface,
>>                   bool is_inline_type) {
>>     return align_metadata_size(header_size() +
>>            vtable_length +
>>            itable_length +
>>            nonstatic_oop_map_size +
>>            (is_interface ? (int)sizeof(Klass*)/wordSize : 0) +
>>            (is_inline_type ? (int)sizeof(InlineKlassFixedBlock) : 0));
>>   }
>> 
>> but `sizeof(InlineKlassFixedBlock)` returns a size in bytes. This adds 640 bytes instead of 80 bytes to all InlineKlasses.
>> 
>> This can be seen by running `jcmd <pid> VM.classes` to check the size of the InlineKlass of Integer before and after the fix for this:
>> 
>> Before:
>> 
>> 0x00000ffc0024d260 167 fully_initialized WS java.lang.Integer
>> 
>> After:
>> 
>> 0x000001800024d260 97 fully_initialized WS java.lang.Integer
>> 
>> 
>> That is in words. So this bug added an extra `(167 - 97) * 8 == 560` bytes. 
>> 
>> I've so far only done some local smoke testing with TEST=hotspot_valhalla
>
> src/hotspot/share/oops/flatArrayKlass.hpp line 95:
> 
>> 93: 
>> 94:   // sizing
>> 95:   static int header_size()  { return sizeof(FlatArrayKlass) / wordSize; }
> 
> Curiously enough we actually got this right for `RefArrayKlass::header_size()` and I'm guessing the following assertion didn't actually do anything meaningful:
> 
> assert(RefArrayKlass::header_size() <= InstanceKlass::header_size(),
>          "array klasses must be same size as InstanceKlass");

Maybe it could even make sense to tighten the `<=` into a `=`, going by the error message. Thoughts?

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1804#discussion_r2623165585