[jdk17u-dev] RFR: 8265263: AArch64: Combine vneg with right shift count

[Andrew Haley]

On Thu, 20 Oct 2022 08:50:41 GMT, Hao Sun <haosun at openjdk.org> wrote:

>> This is a performance improvement for AArch64. There are several differences from the original change.
>> 
>> https://bugs.openjdk.org/browse/JDK-8267356 (Vector API SVE codegen support) is not in 17u, so `UseSVE == 0` parts in predicates are missing/excluded.
>> 
>> https://bugs.openjdk.org/browse/JDK-8288445 (C2 compilation fails) is a subsequent bugfix already backported in 17u, so some `immI` arguments in rules became `immI_positive`.
>> 
>> https://bugs.openjdk.org/browse/JDK-8277239 (SIGSEGV in vrshift_reg_maskedNode::emit) is also related to Vector API and is not in 17u, so `!n->as_ShiftV()->is_var_shift()` is replaced by `VectorNode::is_vshift_cnt(n->in(2))`. This substitution may raise doubts.
>> 
>> Testing: jtreg test/hotspot/jtreg/compiler, tier1, tier2 on aarch64.
>> 
>> Performance improvements in the added benchmark VectorShiftRight on Graviton 2 for default size=1024 correspond to the original review:
>> 
>> 
>> rShiftByte      16%
>> rShiftInt       27%
>> rShiftLong      16%
>> rShiftShort     20%
>> urShiftByte     0%
>> urShiftChar     20%
>> urShiftInt      27%
>> urShiftLong     16%
>
> src/hotspot/cpu/aarch64/aarch64_neon.ad line 4254:
> 
>> 4252: instruct vslcnt8B(vecD dst, iRegIorL2I cnt) %{
>> 4253:   predicate(n->as_Vector()->length_in_bytes() == 4 ||
>> 4254:                             n->as_Vector()->length_in_bytes() == 8);
> 
> We may improve the style here, i.e. removing the leading whitespace.

Not in a backport.

> src/hotspot/cpu/aarch64/aarch64_neon.ad line 4276:
> 
>> 4274: 
>> 4275: instruct vsrcnt8B(vecD dst, iRegIorL2I cnt) %{
>> 4276:   predicate(UseSVE == 0 && (n->as_Vector()->length_in_bytes() == 4 ||
> 
> As mentioned in the commit msg, I think `UseSVE == 0` check is not needed here.

Again, the place to fix that is JDK head.

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PR: https://git.openjdk.org/jdk17u-dev/pull/811