RFR: 8348868: AArch64: Add backend support for SelectFromTwoVector [v3]
Andrew Haley
aph at openjdk.org
Tue Jun 17 08:57:31 UTC 2025
On Mon, 16 Jun 2025 10:12:14 GMT, Bhavana Kilambi <bkilambi at openjdk.org> wrote:
>> This patch adds aarch64 backend support for SelectFromTwoVector operation which was recently introduced in VectorAPI.
>>
>> It implements this operation using a two table vector lookup instruction - "tbl" which is available only in Neon and SVE2.
>>
>> For 128-bit vector length : Neon tbl instruction is generated if UseSVE < 2 and SVE2 "tbl" instruction is generated if UseSVE == 2.
>>
>> For > 128-bit vector length : Currently there are no machines which have vector length > 128-bit and support SVE2. For all those machines with vector length > 128-bit and UseSVE < 2, this operation is not supported. The inline expander for this operation would fail and lowered IR will be generated which is a mix of two rearrange and one blend operation.
>>
>> This patch also adds a boolean "need_load_shuffle" in the inline expander for this operation to test if the platform requires VectorLoadShuffle operation to be generated. Without this, the lowering IR was not being generated on aarch64 and the performance was quite poor.
>>
>> Performance numbers with this patch on a 128-bit, SVE2 supporting machine is shown below -
>>
>>
>> Benchmark (size) Mode Cnt Gain
>> SelectFromBenchmark.selectFromByteVector 1024 thrpt 9 1.43
>> SelectFromBenchmark.selectFromByteVector 2048 thrpt 9 1.48
>> SelectFromBenchmark.selectFromDoubleVector 1024 thrpt 9 68.55
>> SelectFromBenchmark.selectFromDoubleVector 2048 thrpt 9 72.07
>> SelectFromBenchmark.selectFromFloatVector 1024 thrpt 9 1.69
>> SelectFromBenchmark.selectFromFloatVector 2048 thrpt 9 1.52
>> SelectFromBenchmark.selectFromIntVector 1024 thrpt 9 1.50
>> SelectFromBenchmark.selectFromIntVector 2048 thrpt 9 1.52
>> SelectFromBenchmark.selectFromLongVector 1024 thrpt 9 85.38
>> SelectFromBenchmark.selectFromLongVector 2048 thrpt 9 80.93
>> SelectFromBenchmark.selectFromShortVector 1024 thrpt 9 1.48
>> SelectFromBenchmark.selectFromShortVector 2048 thrpt 9 1.49
>>
>>
>> Gain column refers to the ratio of thrpt between this patch and the master branch after applying changes in the inline expander.
>
> Bhavana Kilambi has updated the pull request incrementally with one additional commit since the last revision:
>
> Addressed review comments and added a JTREG test
src/hotspot/cpu/aarch64/aarch64_vector_ad.m4 line 5155:
> 5153: // values across function calls and usually used for long-lived values), we can use any two volatile
> 5154: // registers between V16-V31.
> 5155: instruct vselect_from_two_vectors_HS_Neon(vReg dst, vReg_V17 src1, vReg_V18 src2,
I think it's worth replicating this pattern a couple of times with more vector registers. The allocator might prefer some in the set v8-v15.
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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2151713846
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