RFR: 8341471: [macOS_aarch64] Reversed field layout caused by unstable sorting

[Andrew Haley]

On Mon, 7 Oct 2024 08:55:17 GMT, Fei Gao <fgao at openjdk.org> wrote:

> For class Test:
> 
>     public class Test {
>         char a000;
>         char a001;
>         char a002;
>         char a003;
>         char a004;
>         char a005;
>         char a006;
>         char a007;
>         char a008;
>         char a009;
>         char a00a;
>         char a00b;
>     }
> 
> 
> We found its field layout on macOS was:
> 
> 
> Layout of class Test
>   Instance fields:
>    @0 12/- RESERVED
>    @12 "a00b" C 2/2 REGULAR
>    @14 "a001" C 2/2 REGULAR
>    @16 "a002" C 2/2 REGULAR
>    @18 "a003" C 2/2 REGULAR
>    @20 "a004" C 2/2 REGULAR
>    @22 "a005" C 2/2 REGULAR
>    @24 "a006" C 2/2 REGULAR
>    @26 "a007" C 2/2 REGULAR
>    @28 "a008" C 2/2 REGULAR
>    @30 "a009" C 2/2 REGULAR
>    @32 "a00a" C 2/2 REGULAR
>    @34 "a000" C 2/2 REGULAR
> 
> 
> `a000` was put in the end while `a00b` was put in the beginning.
> 
> Fields get sorted according to size in [1]. `qsort()` on macOS reverses the order of fields with the same size. We should extend the comparison function to preserve the order on macOS, as we did on Windows.
> 
> Tier 1-3 passed on macOS.
> 
> [1] https://github.com/openjdk/jdk/blob/3ee94e040a7395d11a294a6b660d707c97f188f8/src/hotspot/share/classfile/fieldLayoutBuilder.cpp#L102

src/hotspot/share/classfile/fieldLayoutBuilder.hpp line 106:

> 104: #if defined (_WINDOWS) || defined (__APPLE__)
> 105:     // qsort() on Windows/macOS reverse the order of fields with the same size
> 106:     // the extension of the comparison function below preserves this order

`qsort()` isn't defined to be stable anywhere. I can't see any reason to restrict thus bug fix to Windows and MacOS.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21382#discussion_r1789853123