RFR: 8370863: VectorAPI: Optimize the VectorMaskCast chain in specific patterns [v2]

[Eric Fang]

On Fri, 28 Nov 2025 09:21:18 GMT, Galder Zamarreño <galder at openjdk.org> wrote:

>> Eric Fang 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 three additional commits since the last revision:
>> 
>>  - Don't read and write the same memory in the JMH benchmarks
>>  - Merge branch 'master' into JDK-8370863-mask-cast-opt
>>  - 8370863: VectorAPI: Optimize the VectorMaskCast chain in specific patterns
>>    
>>    `VectorMaskCastNode` is used to cast a vector mask from one type to
>>    another type. The cast may be generated by calling the vector API `cast`
>>    or generated by the compiler. For example, some vector mask operations
>>    like `trueCount` require the input mask to be integer types, so for
>>    floating point type masks, the compiler will cast the mask to the
>>    corresponding integer type mask automatically before doing the mask
>>    operation. This kind of cast is very common.
>>    
>>    If the vector element size is not changed, the `VectorMaskCastNode`
>>    don't generate code, otherwise code will be generated to extend or narrow
>>    the mask. This IR node is not free no matter it generates code or not
>>    because it may block some optimizations. For example:
>>    1. `(VectorStoremask (VectorMaskCast (VectorLoadMask x)))`
>>    The middle `VectorMaskCast` prevented the following optimization:
>>    `(VectorStoremask (VectorLoadMask x)) => (x)`
>>    2. `(VectorMaskToLong (VectorMaskCast (VectorLongToMask x)))`, which
>>    blocks the optimization `(VectorMaskToLong (VectorLongToMask x)) => (x)`.
>>    
>>    In these IR patterns, the value of the input `x` is not changed, so we
>>    can safely do the optimization. But if the input value is changed, we
>>    can't eliminate the cast.
>>    
>>    The general idea of this PR is introducing an `uncast_mask` helper
>>    function, which can be used to uncast a chain of `VectorMaskCastNode`,
>>    like the existing `Node::uncast(bool)` function. The funtion returns
>>    the first non `VectorMaskCastNode`.
>>    
>>    The intended use case is when the IR pattern to be optimized may
>>    contain one or more consecutive `VectorMaskCastNode` and this does not
>>    affect the correctness of the optimization. Then this function can be
>>    called to eliminate the `VectorMaskCastNode` chain.
>>    
>>    Current optimizations related to `VectorMaskCastNode` include:
>>    1. `(VectorMaskCast (VectorMaskCast x)) => (x)`, see JDK-8356760.
>>    2. `(XorV (VectorMa...
>
> Nice improvement @erifan, just some small comments from me

Hi @galderz would you mind taking another look of this PR, thanks~

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PR Comment: https://git.openjdk.org/jdk/pull/28313#issuecomment-3641419459