RFR: 8292898: [vectorapi] Unify vector mask cast operation [v4]

[Xiaohong Gong]

On Mon, 26 Sep 2022 18:27:33 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> Xiaohong Gong has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add assertion to the elem num for mast cast
>
> src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp line 4626:
> 
>> 4624:     switch (dst_bt_size / src_bt_size) {
>> 4625:       case 2: {
>> 4626:         if (vlen_enc == AVX_512bit && !VM_Version::supports_avx512bw()) {
> 
> Ideal type for a valid mask casting involving 512 bit species should be TypeVectMask, thus this code may never get executed since mask will be propagated using opmask register.

@merykitty , please take a look at this comment. Yes, I know that the mask use a different register for AVX-512. But I'm not quite familiar with the avx512 feature (i.e. `avx512bw`). Thanks a lot!

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PR: https://git.openjdk.org/jdk/pull/10192