RFR: 8282711: Accelerate Math.signum function for AVX and AVX512 target. [v2]

[Quan Anh Mai]

On Tue, 8 Mar 2022 13:11:24 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp line 4334:
>> 
>>> 4332:     vsubpd(xtmp3, zero, one, vec_enc);
>>> 4333:     vblendvpd(dst, xtmp3, one, xtmp2, vec_enc);
>>> 4334:     vblendvpd(dst, dst, src, xtmp1, vec_enc);
>> 
>> The same applies here, suggestion:
>> 
>>     vsubpd(dst, zero, one, vec_enc);
>>     vblendvpd(dst, src, one, dst, vec_enc);
>>     vcmppd(xtmp1, src, zero, Assembler::EQ_UQ, vec_enc);
>>     vblendvpd(dst, xtmp1, dst, zero, vec_enc);
>> 
>> Thanks.
>
> Same responses as above.

I don't fully understand what you mean by "same as above". The logic here is using `src` to blend `one` and `minus_one` so that every positive will get `one` and every negative will get `minus_one`, the others are `zero` and `Nan` will make the unordered comparison with zero return true, thus the final blend will result in the correct output.
Thanks.

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PR: https://git.openjdk.java.net/jdk/pull/7717