RFR: 8348868: AArch64: Add backend support for SelectFromTwoVector [v4]

[Xiaohong Gong]

On Wed, 25 Jun 2025 06:06:53 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> Bhavana Kilambi has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Addressed review comments
>
> src/hotspot/cpu/aarch64/aarch64_vector.ad line 262:
> 
>> 260:             (UseSVE == 2 && length_in_bytes > 8 && length_in_bytes < MaxVectorSize )) {
>> 261:           return false;
>> 262:         }
> 
> How about:
> 
> case Op_SelectFromTwoVector:
>   // The "tbl" instruction for two vector table is supported only in Neon and SVE2. Return
>   // false if vector length > 16B but supported SVE version < 2.
>   //
>   // Additionally, this operation is disabled for doubles and longs on machines with SVE < 2,
>   // Instead, the default VectorRearrange + VectorBlend is generated as the performance of
>   // the default pattern is slightly better.
>   if (UseSVE < 2 && (type2aelembytes(bt) == 8 || length_in_bytes > 16)) {
>     return false;
>   }
> 
>   // As the SVE2 "tbl" instruction is unpredicated and partial operations cannot be generated
>   // using masks, we currently disable this operation on machines where length_in_bytes <
>   // MaxVectorSize with the only exception of 8B vector length.
>   if (UseSVE == 2 && length_in_bytes > 8 && length_in_bytes < MaxVectorSize)) {
>     return false;
>   }
> 
>   break;

Maybe the NEON `tbl` can also be generated for SVE2 when `length_in_bytes == 16 && length_in_bytes < MaxVectorSize`. This is a special partial version for SVE2.  As a summary, The match rule's predicate will be:
1) NEON: UseSVE < 2 || (length_in_bytes < 16 || length_in_bytes < MaxVectorSize)
2) SVE: UseSVE ==2 && (length_in_bytes >= 16 && length_in_bytes == MaxVectorSize)

Seems this will make predicate or code here more complex. Advantage is this op with 128 vector shape on a SVE2 256 or larger size machine will also be intrinsified. It's not a block and change or not is up to you. We can also revisit this part once the 256-bit SVE2 machine exist in future.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2165895697