On Thu, 27 Feb 2025 16:00:47 GMT, Chen Liang <li...@openjdk.org> wrote:
>> Simplify the layout access var handles to be direct in some common cases.
>> Also made `VarHandle::isAccessModeSupported` report if an access mode is
>> supported for a VH.
>>
>> Reduces the instructions to execute this code in a simple main by 47%:
>>
>> long[] arr = new long[8];
>> var ms = MemorySegment.ofArray(arr);
>> ms.setAtIndex(ValueLayout.JAVA_BYTE, 12, (byte) 3);
>>
>>
>> Main overheads in FFM are identified to be:
>> 1. Eager initialization of direct MethodHandle; can be CDS archived
>> 2. MH combinator forms via LambdaFormEditor, not cached right now and always
>> have large overhead
>>
>> Still need other measures to deal with common user patterns of
>> `MethodHandles.insertCoordinates(vh, 1, 0L)` which currently is still very
>> slow.
>>
>> Tests: 2 unrelated failures on tier 1-3
>
> Chen Liang has updated the pull request incrementally with one additional
> commit since the last revision:
>
> noStride -> constantOffset, optimize VH classes to have only 2 instead of 3
> classes for each type
src/java.base/share/classes/java/lang/invoke/X-VarHandleSegmentView.java.template
line 53:
> 51:
> 52: static final VarForm CONSTANT_OFFSET_FORM = new
> VarForm(VarHandleSegmentAs$Type$s.class, MemorySegment.class, $type$.class,
> long.class);
> 53: static final VarForm TAKE_OFFSET_FORM = new
> VarForm(VarHandleSegmentAs$Type$s.class, MemorySegment.class, $type$.class,
> long.class, long.class);
what does TAKE mean here? (the old names weren't great either, I realize). I
think it would be better to decide, once and for all, how to refer to these
things, and then use consistent naming throughout. I see we have discussed
using "noStride" vs. "fixedOffset". Since the implementation now uses the term
"constantOffset", let's just call the other "variableOffset" ?
src/java.base/share/classes/java/lang/invoke/X-VarHandleSegmentView.java.template
line 120:
> 118:
> 119: // This class must be accessed through non-aligned
> VarHandleSegmentAs$Type$s
> 120: final class VarHandleSegmentAs$Type$sAligned extends
> VarHandleSegmentAs$Type$s {
I kind of liked the nested classes -- why the change?
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1975192481
PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1975193951
