RFR: 8264104: Eliminate unnecessary vector mask conversion during VectorUnbox for floating point VectorMask [v4]

[Jatin Bhateja]

On Tue, 13 Apr 2021 03:21:21 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> The Vector API defines different element types for floating point VectorMask. For example, the bitwise related APIs use "`long/int`", while data related APIs use "`double/float`". This makes some optimizations that based on the type checking cannot work well.
>> 
>> For example, the VectorBox/Unbox elimination like `"VectorUnbox (VectorBox v) ==> v"` requires the types of output and
>> input are equal. Normally this is necessary. However, due to the different element type for floating point VectorMask with the same species, the VectorBox/Unbox pattern is optimized to:
>> 
>>   VectorLoadMask (VectorStoreMask vmask)
>> 
>> Actually the types can be treated as the same one for such cases. And considering the vector mask representation is the same for
>> vectors with the same element size and vector length, it's safe to do the optimization:
>> 
>>   VectorLoadMask (VectorStoreMask vmask) ==> vmask
>> 
>> The same issue exists for GVN which is based on the type of a node. Making the mask node's` hash()/cmp()` methods depends on the element size instead of the element type can fix it.
>
> Xiaohong Gong has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use "Matcher::match_rule_supported_vector" for vector nodes checking

src/hotspot/cpu/aarch64/aarch64_sve_ad.m4 line 861:

> 859: instruct vmaskcast(vReg dst) %{
> 860:   predicate(UseSVE > 0);
> 861:   match(Set dst (VectorMaskCast dst));

A strict check based on in-type and out-type in predicate could strengthen the pattern.

src/hotspot/share/opto/vectornode.hpp line 1240:

> 1238: };
> 1239: 
> 1240: class VectorMaskCastNode : public VectorNode {

VectorMaskReinterpret seems better choice, since its a re-interpretation and not a casting (up/down).

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PR: https://git.openjdk.java.net/jdk/pull/3238