RFR: 8356760: VectorAPI: Optimize VectorMask.fromLong for all-true/all-false cases [v5]

[erifan]

On Fri, 25 Jul 2025 03:35:11 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> erifan has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:
>> 
>>  - Add JMH benchmarks for cast chain transformation
>>  - Merge branch 'master' into JDK-8356760
>>  - Refactor the implementation
>>    
>>    Do the convertion in C2's IGVN phase to cover more cases.
>>  - Merge branch 'master' into JDK-8356760
>>  - Simplify the test code
>>  - Address some review comments
>>    
>>    Add support for the following patterns:
>>      toLong(maskAll(true))  => (-1ULL >> (64 -vlen))
>>      toLong(maskAll(false)) => 0
>>    
>>    And add more test cases.
>>  - Merge branch 'master' into JDK-8356760
>>  - 8356760: VectorAPI: Optimize VectorMask.fromLong for all-true/all-false cases
>>    
>>    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
>
> src/hotspot/share/opto/vectornode.cpp line 1986:
> 
>> 1984: Node* VectorMaskToLongNode::Ideal_MaskAll(PhaseGVN* phase) {
>> 1985:   Node* in1 = in(1);
>> 1986:   // VectorMaskToLong follows a VectorStoreMask if predicate is not supported.
> 
> It's always good to add an assertion check for coding assumptions.

Done, thanks!

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PR Review Comment: https://git.openjdk.org/jdk/pull/25793#discussion_r2230351518