RFR: 8356760: VectorAPI: Optimize VectorMask.fromLong for all-true/all-false cases [v6]

[Jatin Bhateja]

On Fri, 25 Jul 2025 07:24:28 GMT, erifan <duke at openjdk.org> wrote:

>> If the input long value `l` of `VectorMask.fromLong(SPECIES, l)` would set or unset all lanes, `VectorMask.fromLong(SPECIES, l)` is equivalent to `maskAll(true)` or `maskAll(false)`. But the cost of the `maskAll` is
>> relative smaller than that of `fromLong`. So this patch does the conversion for these cases.
>> 
>> The conversion is done in C2's IGVN phase. And on platforms (like Arm NEON) that don't support `VectorLongToMask`, the conversion is done during intrinsiication process if `MaskAll` or `Replicate` is supported.
>> 
>> Since this optimization requires the input long value of `VectorMask.fromLong` to be specific compile-time constants, and such expressions are usually hoisted out of the loop. So we can't see noticeable performance change.
>> 
>> This conversion also enables further optimizations that recognize maskAll patterns, see [1]. And we can observe a performance improvement of about 7% on both aarch64 and x64.
>> 
>> As `VectorLongToMask` is converted to `MaskAll` or `Replicate`, some existing optimizations recognizing the `VectorLongToMask` will be affected, like
>> 
>>   VectorMaskToLong (VectorLongToMask x) => x
>> 
>> 
>> Hence, this patch also added the following optimizations:
>> 
>>   VectorMaskToLong (MaskAll x) => (x & (-1ULL >> (64 - vlen)))    // x is -1 or 0
>>   VectorMaskToLong (VectorStoreMask (Replicate x)) => (x & (-1ULL >> (64 - vlen)))  // x is -1 or 0
>> 
>>   VectorMaskCast (VectorMaskCast x) => x
>> 
>> And we can see noticeable performance improvement with the above optimizations for floating-point types.
>> 
>> Benchmarks on Nvidia Grace machine with option `-XX:UseSVE=2`:
>> 
>> Benchmark				Unit	Before		Error		After		Error		Uplift
>> microMaskFromLongToLong_Double128	ops/s	1522384.986	1324881.46	2835774480	403575069.7	1862.71
>> microMaskFromLongToLong_Double256	ops/s	4275.415598	28.560622	4285.587451	27.633101	1
>> microMaskFromLongToLong_Double512	ops/s	3702.171936	9.528497	3692.747579	18.47744	0.99
>> microMaskFromLongToLong_Double64	ops/s	4624.452243	37.388427	4616.320519	23.455954	0.99
>> microMaskFromLongToLong_Float128	ops/s	1239661.887	1286803.852	2842927993	360468218.3	2293.3
>> microMaskFromLongToLong_Float256	ops/s	3681.64954	15.153633	3685.411771	21.737124	1
>> microMaskFromLongToLong_Float512	ops/s	3007.563025	10.189944	3022.002986	14.137287	1
>> microMaskFromLongToLong_Float64		ops/s	1646664.258	1375451.279	2948453900	397472562.4	1790.56
>> 
>> 
>> Benchmarks on AMD EPYC 9124 16-Core Processor with option `-XX:UseAVX=3`:
>> 
>> Benchm...
>
> erifan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add an assertion

Still looks good.

src/hotspot/share/opto/vectornode.cpp line 1989:

> 1987:   if (in1->Opcode() == Op_VectorStoreMask) {
> 1988:     in1 = in1->in(1);
> 1989:     assert(!in1->bottom_type()->isa_vectmask(), "sanity");

Assertion should precede before any other statement in the block :-)

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

Marked as reviewed by jbhateja (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25793#pullrequestreview-3054381237
PR Review Comment: https://git.openjdk.org/jdk/pull/25793#discussion_r2230359167