On Mon, 4 Sep 2023 14:30:06 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org>
wrote:
>> @TheRealMDoerr We've been discussing the shifts in order to wrap our heads
>> around it, and we've ended up with this diagram in order to try and
>> visualize what happens:
>>
>> Let's say we have a struct with 3 ints:
>>
>>
>> struct Foo {
>> int x;
>> int y;
>> int z;
>> };
>>
>>
>> If this struct is passed as an argument, then the load of the second 'half'
>> of the struct would look like this:
>>
>>
>> offset : 0 .... 32 ..... 64 ..... 96 .... 128
>> values : xxxxxxxx|yyyyyyyy|zzzzzzzz|???????? (can't touch bits
>> 96..128)
>> Load int : V +--------+
>> | |
>> +--------+ |
>> V V
>> In register : ????????|zzzzzzzz (MSBs are 0)
>> Shift left : zzzzzzzz|00000000 (LSBs are zero)
>> Write long : V V
>> Result : xxxxxxxx|yyyyyyyy|zzzzzzzz|00000000
>>
>>
>> So, the 'Result' is padded at the tail with zeros.
>>
>> Does that seem right? Does it seem useful to add this diagram as a comment
>> somewhere, for us when we come back to this code a year from now? Thanks
>
>> If this struct is passed as an argument, then the load of the second 'half'
>> of the struct would look like this:
>
> It would perhaps be cleaner if in the MSB/LSB comments we said:
>
> LSBs are zzz...z
> LSBs are 000...0
>
> (e.g. avoid to refer to MSBs in the first, since those bytes are not exactly
> zero, they are the padding bytes)
@mcimadamore: May I ask you or somebody else from the Panama team to provide a
2nd review? This PR requires Panama knowledge, not really PPC knowledge.
-------------
PR Comment: https://git.openjdk.org/jdk/pull/15417#issuecomment-1706217722
