RFR: 8307795: AArch64: Optimize VectorMask.truecount() on Neon [v3]

[Andrew Haley]

On Thu, 18 May 2023 09:50:13 GMT, Chang Peng <duke at openjdk.org> wrote:

>> In Vector API Java level, vector mask is represented as a boolean array with 0x00/0x01 (8 bits of each element) as values, aka in-memory format. When it is loaded into vector register, e.g. Neon, the in-memory format will be converted to in-register format with 0/-1 value for each lane (lane width aligned to its type) by VectorLoadMask [1] operation, and convert back to in-memory format by VectorStoreMask[2]. In Neon, a typical VectorStoreMask operation will first narrow given vector registers by xtn insn [3] into byte element type, and then do a vector negate to convert to 0x00/0x01 value for each element.
>> 
>> For most of the vector mask operations, the input mask is in-register format. And a vector mask also works in-register format all through the compilation. But for some operations like VectorMask.trueCount()[4] which counts the elements of true value, the expected input mask is in-memory format. So a VectorStoreMask is generated to convert the mask from in-register format to in-memory format before those operations.
>> 
>> However, for trueCount() these xtn instructions in VectorStoreMask can be saved, since the narrowing operations will not influence the number of active lane (value of 0x01) of its input.
>> 
>> This patch adds an optimized rule `VectorMaskTrueCount (VectorStoreMask mask)` to save the unnecessary narrowing operations.
>> 
>> For example,
>> 
>> 
>> var m = VectorMask.fromArray(IntVector.SPECIES_PREFERRED, ba, 0);
>> m.not().trueCount();
>> 
>> 
>> will produce following assembly on a Neon machine before this patch:
>> 
>> 
>> ...
>> mvn     v16.16b, v16.16b           // VectorMask.not()
>> xtn     v16.4h, v16.4s
>> xtn     v16.8b, v16.8h
>> neg     v16.8b, v16.8b             // VectorStoreMask
>> addv    b17, v16.8b
>> umov    w0, v17.b[0]               // VectorMask.trueCount()
>> ...
>> 
>> 
>> After this patch:
>> 
>> 
>> ...
>> mvn     v16.16b, v16.16b           // VectorMask.not()
>> addv    s17, v16.4s
>> smov    x0, v17.b[0]
>> neg     x0, x0                     // Optimized VectorMask.trueCount()
>> ...
>> 
>> 
>> In this case, we can save two xtn insns.
>> 
>> Performance:
>> 
>> Benchmark     Before               After                          Unit
>> testInt           723.822 ± 1.029  1182.375 ± 12.363    ops/ms
>> testLong       632.154 ± 0.197  1382.74 ± 2.188        ops/ms
>> testShort      788.665 ± 1.852   1152.38 ± 3.77         ops/ms
>> 
>> [1]: https://github.com/openjdk/jdk/blob/e1e758a7b43c29840296d337bd2f0213ab0ca3c9/src/hotspot/cpu/aarch64/aarch64_vect...
>
> Chang Peng has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update benchmark to avoid potential optimization

Marked as reviewed by aph (Reviewer).

src/hotspot/cpu/aarch64/aarch64_vector_ad.m4 line 3825:

> 3823: %}
> 3824: 
> 3825: // Combined rule for VectorStoreMask + VectorMaskTrueCount when the vector element type is not T_BYTE.

Suggestion:

// Combined rule for VectorMaskTrueCount (VectorStoreMask) when the vector element type is not T_BYTE.

Using `+` is unnecessarily confusing, as is swapping the operands.

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

PR Review: https://git.openjdk.org/jdk/pull/13974#pullrequestreview-1443414797
PR Review Comment: https://git.openjdk.org/jdk/pull/13974#discussion_r1205210387