RFR: 8338023: Support two vector selectFrom API [v10]

[Paul Sandoz]

On Mon, 16 Sep 2024 02:58:41 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> Hi All,
>> 
>> As per the discussion on panama-dev mailing list[1], patch adds the support for following new two vector permutation APIs.
>> 
>> 
>> Declaration:-
>>     Vector<E>.selectFrom(Vector<E> v1, Vector<E> v2)
>> 
>> 
>> Semantics:-
>>     Using index values stored in the lanes of "this" vector, assemble the values stored in first (v1) and second (v2) vector arguments. Thus, first and second vector serves as a table, whose elements are selected based on index value vector. API is applicable to all integral and floating-point types.  The result of this operation is semantically equivalent to expression v1.rearrange(this.toShuffle(), v2). Values held in index vector lanes must lie within valid two vector index range [0, 2*VLEN) else an IndexOutOfBoundException is thrown.  
>> 
>> Summary of changes:
>> -  Java side implementation of new selectFrom API.
>> -  C2 compiler IR and inline expander changes.
>> -  In absence of direct two vector permutation instruction in target ISA, a lowering transformation dismantles new IR into constituent IR supported by target platforms. 
>> -  Optimized x86 backend implementation for AVX512 and legacy target.
>> -  Function tests covering new API.
>> 
>> JMH micro included with this patch shows around 10-15x gain over existing rearrange API :-
>> Test System: Intel(R) Xeon(R) Platinum 8480+ [ Sapphire Rapids Server]
>> 
>> 
>>   Benchmark                                     (size)   Mode  Cnt      Score   Error   Units
>> SelectFromBenchmark.rearrangeFromByteVector     1024  thrpt    2   2041.762          ops/ms
>> SelectFromBenchmark.rearrangeFromByteVector     2048  thrpt    2   1028.550          ops/ms
>> SelectFromBenchmark.rearrangeFromIntVector      1024  thrpt    2    962.605          ops/ms
>> SelectFromBenchmark.rearrangeFromIntVector      2048  thrpt    2    479.004          ops/ms
>> SelectFromBenchmark.rearrangeFromLongVector     1024  thrpt    2    359.758          ops/ms
>> SelectFromBenchmark.rearrangeFromLongVector     2048  thrpt    2    178.192          ops/ms
>> SelectFromBenchmark.rearrangeFromShortVector    1024  thrpt    2   1463.459          ops/ms
>> SelectFromBenchmark.rearrangeFromShortVector    2048  thrpt    2    727.556          ops/ms
>> SelectFromBenchmark.selectFromByteVector        1024  thrpt    2  33254.830          ops/ms
>> SelectFromBenchmark.selectFromByteVector        2048  thrpt    2  17313.174          ops/ms
>> SelectFromBenchmark.selectFromIntVector         1024  thrpt    2  10756.804          ops/ms
>> S...
>
> Jatin Bhateja has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Disabling VectorLoadShuffle bypassing optimization to comply with rearrange semantics at IR level.

src/jdk.incubator.vector/share/classes/jdk/incubator/vector/X-Vector.java.template line 561:

> 559:         for (int i = 0; i < vlen; i++) {
> 560:             int index = ((int)vecPayload1[i]);
> 561:             res[i] = index >= vlen ? vecPayload3[index & (vlen - 1)] : vecPayload2[index];

This is incorrect as the index could be negative. You need to wrap in the range `[0, 2 * vlen - 1]` before the comparison and selection.

int index = ((int)vecPayload1[i]) & ((vlen << 1) - 1));
res[i] = index < vlen ? vecPayload2[index] : vecPayload3[index - vlen];

src/jdk.incubator.vector/share/classes/jdk/incubator/vector/X-Vector.java.template line 2974:

> 2972:     final $abstractvectortype$ selectFromTemplate(Class<? extends Vector<$Boxbitstype$>> indexVecClass,
> 2973:                                                   $abstractvectortype$ v1, $abstractvectortype$ v2) {
> 2974:         int twoVectorLen = length() * 2;

We should assert that the length is a power of two.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20508#discussion_r1761663646
PR Review Comment: https://git.openjdk.org/jdk/pull/20508#discussion_r1761667602