RFR: 8367292: VectorAPI: Optimize VectorMask.fromLong/toLong() for SVE [v5]

[Emanuel Peter]

On Wed, 29 Oct 2025 10:13:03 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> The current implementations of `VectorMask.fromLong()` and `toLong()` on AArch64 SVE are inefficient. SVE does not support naive predicate instructions for these operations. Instead, they are implemented with vector instructions, but the output/input of `fromLong/toLong` are defined as masks with predicate registers on SVE architectures.
>> 
>> For `toLong()`, the current implementation generates a vector mask stored in a vector register with bool type first, then converts the vector to predicate layout. For `fromLong()`, the opposite conversion is needed at the start of codegen.
>> 
>> These conversions are expensive and are implemented in the IR backend codegen, which is inefficient. The performance impact is significant on SVE architectures.
>> 
>> This patch optimizes the implementation by leveraging two existing C2 IRs (`VectorLoadMask/VectorStoreMask`) that can handle the conversion efficiently. By splitting this work at the mid-end IR level, we align with the current IR pattern used on architectures without predicate features (like AArch64 Neon) and enable sharing of existing common IR optimizations.
>> 
>> It also modifies the Vector API jtreg tests for well testing. Here is the details:
>> 
>> 1) Fix the smoke tests of `fromLong/toLong` to make sure these APIs are tested actually. These two APIs are not well tested before. Because in the original test, the C2 IRs for `fromLong` and `toLong` are optimized out completely by compiler due to following IR identity:
>> 
>>   VectorMaskToLong (VectorLongToMask l) => l
>> 
>> Besides, an additional warmup loop is necessary to guarantee the APIs are compiled by C2.
>> 
>> 2) Refine existing IR tests to verify the expected IR patterns after this patch. Also changed to use the exact required cpu feature on AArch64 for these ops. `fromLong` requires "svebitperm" instead of "sve2".
>> 
>> Performance shows significant improvement on NVIDIA's Grace CPU.
>> 
>> Here is the performance data with `-XX:UseSVE=2`:
>> 
>> Benchmark                                   bits inputs Mode   Unit     Before       After    Gain
>> MaskQueryOperationsBenchmark.testToLongByte  128    1  thrpt  ops/ms  322151.976  1318576.736 4.09
>> MaskQueryOperationsBenchmark.testToLongByte  128    2  thrpt  ops/ms  322187.144  1315736.931 4.08
>> MaskQueryOperationsBenchmark.testToLongByte  128    3  thrpt  ops/ms  322213.330  1353272.882 4.19
>> MaskQueryOperationsBenchmark.testToLongInt   128    1  thrpt  ops/ms 1009426.292  1339834.833 1.32
>> MaskQueryOperations...
>
> Xiaohong Gong has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update comments

Generally, this patch looks reasonable, but I'm not a aarch64 or x64 specialist for these ops.

I think we have sufficient aarch64 specialists look at this already.

But I'd like to ping @sviswa7 and @jatin-bhateja to sanity check the x64 changes, IR rules etc :)

After approval from x64 folks, I can offer to do some internal testing :)

test/jdk/jdk/incubator/vector/Long128VectorTests.java line 6850:

> 6848:             var vmask = VectorMask.fromLong(SPECIES, inputLong);
> 6849:             // Insert "not()" to avoid the "fromLong/toLong" being optimized out by compiler.
> 6850:             long outputLong = vmask.not().toLong();

That sounds a bit fragile. Is there something that would catch if it did ever get optimized away?

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PR Review: https://git.openjdk.org/jdk/pull/27481#pullrequestreview-3397777045
PR Review Comment: https://git.openjdk.org/jdk/pull/27481#discussion_r2476685457