RFR: 8318650: Optimized subword gather for x86 targets. [v13]

[Jatin Bhateja]

On Tue, 20 Feb 2024 08:36:29 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Jatin Bhateja has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Review comments resolutions.
>
> src/hotspot/cpu/x86/x86.ad line 4120:
> 
>> 4118:     BasicType elem_bt = Matcher::vector_element_basic_type(this);
>> 4119:     __ lea($tmp$$Register, $mem$$Address);
>> 4120:     __ vgather8b(elem_bt, $dst$$XMMRegister, $tmp$$Register, $idx$$Register, $rtmp$$Register, vlen_enc);
> 
> The `LE8B` and `Matcher::vector_length_in_bytes(n) <= 8` suggest we can perform this with 4 bytes as well.
> Is that correct?
> Would that not lead to issues, when we are then reading `base_index` at bytes 4...7, which possibly have garbage, and then use that to gather?
> Do we have tests for that?

64 bit sub-word SPECIES will either hold 8 bytes values or 4 short values, algorithm appropriately handle it.

> src/jdk.incubator.vector/share/classes/jdk/incubator/vector/ByteVector.java line 3071:
> 
>> 3069:                 .fromArray(lsp, indexMap, mapOffset + i)
>> 3070:                 .add(offset);
>> 3071:             vix = VectorIntrinsics.checkIndex(vix, a.length);
> 
> are you using the `vix` after this assignment?

Its purpose is to check out of bounds indices.

> src/jdk.incubator.vector/share/classes/jdk/incubator/vector/ShortVector.java line 3809:
> 
>> 3807: 
>> 3808:         // Check indices are within array bounds.
>> 3809:         // FIXME: Check index under mask controlling.
> 
> Did you mean to leave a FIXME? If so, please reference a JIRA bug number where you intend to fix it.

This is in line with already existing FIXME https://github.com/openjdk/jdk/blob/master/src/jdk.incubator.vector/share/classes/jdk/incubator/vector/X-Vector.java.template#L5035

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PR Review Comment: https://git.openjdk.org/jdk/pull/16354#discussion_r1501753441
PR Review Comment: https://git.openjdk.org/jdk/pull/16354#discussion_r1501753416
PR Review Comment: https://git.openjdk.org/jdk/pull/16354#discussion_r1501753413