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

[Hao Sun]

On Wed, 15 Jun 2022 06:20:12 GMT, Andrew Haley <aph 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
>
> src/hotspot/cpu/aarch64/assembler_aarch64.hpp line 1935:
> 
>> 1933:   INSN(fmovs,  0b000, 0b00, 0b000000);
>> 1934:   INSN(fabss,  0b000, 0b00, 0b000001);
>> 1935:   INSN(fnegs,  0b000, 0b00, 0b000010);
> 
> There's unnecessary whitespace here.
> Suggestion:
> 
>   INSN(fmovs, 0b000, 0b00, 0b000000);
>   INSN(fabss, 0b000, 0b00, 0b000001);
>   INSN(fnegs, 0b000, 0b00, 0b000010);

Thanks for your review.
It's a style issue. I added one whitespace for `fmov/fabs/fneg` and the later `fcvt` so as to align the arguments to those of `fsqrt`. I saw such a style in several other sites in this header.

If you don't like it, I can remove the added whitespace. But I guess you may also want me to remove the whitespace I added for the `fcvt` below.

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