RFR: 8356760: VectorAPI: Optimize VectorMask.fromLong for all-true/all-false cases [v3]

[Jatin Bhateja]

On Thu, 3 Jul 2025 07:10:22 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 `maskAll` is relative smaller than that of `fromLong`. This patch does the conversion for these cases if `l` is a compile time constant.
>> 
>> And this conversion also enables further optimizations that recognize maskAll patterns, see [1].
>> 
>> Some JTReg test cases are added to ensure the optimization is effective.
>> 
>> I tried many different ways to write a JMH benchmark, but failed. Since the input of `VectorMask.fromLong(SPECIES, l)` needs to be a specific compile-time constant, the statement will be hoisted out of the loop. If we don't use a loop, the hotspot will become other instructions, and no obvious performance change was observed. However, combined with the optimization of [1], we can observe a performance improvement of about 7% on both aarch64 and x64.
>> 
>> The patch was tested on both aarch64 and x64, all of tier1 tier2 and tier3 tests passed.
>> 
>> [1] https://github.com/openjdk/jdk/pull/24674
>
> erifan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Simplify the test code

> > public static final VectorSpecies FSP = FloatVector.SPECIES_512;
> > public static long micro1(long a) {
> > long mask = Math.min(-1, Math.max(-1, a));
> > return VectorMask.fromLong(FSP, mask).toLong();
> > }
> > public static long micro2() {
> > return FSP.maskAll(true).toLong();
> > }
> 
> With this JMH method we can not see obvious performance improvement, because the hot spots are other instructions. Adding a loop is better.

There is no hard and fast rule for the inclusion of a loop in a JMH micro in that case?

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PR Comment: https://git.openjdk.org/jdk/pull/25793#issuecomment-3035920476