RFR: 8348868: AArch64: Add backend support for SelectFromTwoVector [v3]

[Xiaohong Gong]

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.ad line 4990:

> 4988: %{
> 4989:   constraint(ALLOC_IN_RC(v17_veca_reg));
> 4990:   match(vReg);

Not sure whether it's better to use `match(VecA)` here.

src/hotspot/cpu/aarch64/aarch64_vector.ad line 7176:

> 7174:                                           vReg index, vReg tmp1) %{
> 7175:   predicate((Matcher::vector_element_basic_type(n) == T_SHORT ||
> 7176:             type2aelembytes(Matcher::vector_element_basic_type(n)) == 4) &&

To use the same basic type check condition, can we use `type2aelembytes(Matcher::vector_element_basic_type(n)) == 2` instead of `Matcher::vector_element_basic_type(n) == T_SHORT` here?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2151208483
PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2151217232