RFR: 8288300: AArch64: Remove the assertion in fmovs/fmovd(FloatRegister, FloatRegister) [v2]

[Andrew Haley]

On Wed, 15 Jun 2022 07:56:38 GMT, Hao Sun <haosun at openjdk.org> wrote:

>> The assertion, i.e. src and dst must be different registers, was
>> introduced years ago. But I don't think it's needed.
>> 
>> This limitation was added in [1]. Frankly speaking, I don't know the
>> reason. But I guess the assertion is probably used for debugging,
>> raising one warning of fmovs/fmovd usage in the scenario of moving
>> element at index zero from one **vector** register, to one float-point
>> scalar register. If the "src" vector register and the "dst" float-point
>> scalar register are the same one, it introduces a side-effect, i.e. the
>> higher bits are cleared to zeros[2].
>> 
>> If so, I argue that
>> 1) the assembler should align with the ISA.
>> 2) compiler developers should be aware of the side-effect when they want
>> to use fmovs/fmovd, and they should guarantee "dst != src" if they like
>> to higher bits untouched, e.g., [3].
>> 
>> Hence, I think we can remove this unnecessary assertion.
>> 
>> [1] http://hg.openjdk.java.net/aarch64-port/jdk8/hotspot/rev/9baee4e65ac5
>> [2] https://developer.arm.com/documentation/ddi0596/2021-12/SIMD-FP-Instructions/FMOV--register---Floating-point-Move-register-without-conversion-?lang=en
>> [3] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64_sve.ad#L4899
>
> Hao Sun has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove unnecessary whitespace
>   
>   As suggested by aph, remove the unnecessary whitespace and keep the
>   substantive changes only.

Thanks.

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Marked as reviewed by aph (Reviewer).

PR: https://git.openjdk.org/jdk/pull/9163