RFR: 8354242: VectorAPI: combine vector not operation with compare [v9]

[erifan]

On Tue, 8 Jul 2025 09:49:30 GMT, erifan <duke at openjdk.org> wrote:

>> Thanks for your answers @erifan !
>> 
>> Can you please answer these as well?
>> 
>>> predicate cannot be negated AND the vector is all ones. Can you explain this condition?
>> 
>> A code comment would be helpful for this case. I'm a little bit struggling to understand the bracket/negation here.
>> 
>>> Why do you guard against VectorNode::is_all_ones_vector(in2) at all?
>> 
>> Is this necessary? Why?
>
>> predicate cannot be negated AND the vector is all ones. Can you explain this condition?
> 
> Ok, I'll add a comment for it.
> 
>> Why do you guard against VectorNode::is_all_ones_vector(in2) at all?
> 
> Because one of the nodes in the supported patterns by this PR needs to be `MaskAll` or `Replicate`. And this function `VectorNode::is_all_ones_vector` just meets our check for `MaskAll` and `Replicate`. Actually I don't quite understand your question. I have two understandings:
> 1. Not all nodes that `VectorNode::is_all_ones_vector` returns true are `MaskAll` or `Replicate`, but other nodes that do not meet the conditions.
> 2. Here, it does not need to be a vector with every bit 1, it only needs to be an `all true` mask.
> 
> Which one do you mean? Or something else? Thanks~

The purpose of this PR is optimizing the following kinds of patterns:

XXXVector va, vb;
va.compare(EQ, vb).not()

And the generated IR of `va.compare(EQ, vb).not()` is `(XorVMask (VectorMaskCmp va vb EQ) (MaskAll -1))`. On platforms like aarch64 NEON, `MaskAll` is `Replicate`. And `MaskAll` and `Replicate` are both all ones vectors, so we do this check `VectorNode::is_all_ones_vector(in2)`

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PR Review Comment: https://git.openjdk.org/jdk/pull/24674#discussion_r2192087590