RFR: 8365911: AArch64: Fix encoding error in sve_cpy for negative floats [v2]

[erifan]

On Wed, 3 Sep 2025 08:11:55 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> erifan has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
>> 
>>  - Don't rename sve_cpy as sve_fcpy
>>  - Merge branch 'master' into JDK-8365911
>>  - 8365911: AArch64: Fix encoding error in sve_cpy for negative floats
>>    
>>    The sve_cpy instruction is not correctly implemented for negative
>>    floating-point values. The issues include:
>>    
>>    1. When a negative floating-point number (e.g. `-1.0`) is passed, the
>>    `checked_cast<int8_t>(pack(d))` check fails. For example, assume `d = -1.0`:
>>    - `pack(-1.0)` returns an unsigned int with the 7th bit set, i.e., `0xf0`.
>>    - `checked_cast<int8_t>(0xf0)` casts `0xf0` to an int8_t value, which is `-16`.
>>    - Casting this int8_t `-16` back to unsigned int results in `0xfffffff0`.
>>    - The check compares `0xf0` to `0xfffffff0`, which obviously fails.
>>    
>>    2. Additionally, the encoding of the negative floating-point number is incorrect:
>>    - The imm8 field can fall outside the valid range of **[-128, 127]**.
>>    - Bit **13** should be encoded as **0** for floating-point numbers.
>>    
>>    This PR fixes these issues and renames floating-point `sve_cpy` as `sve_fcpy`.
>>    
>>    Some test cases are added to aarch64-asmtest.py, and all tests passed.
>
> src/hotspot/cpu/aarch64/assembler_aarch64.hpp line 3831:
> 
>> 3829:     int m = isMerge ? 1 : 0;
>> 3830:     f(0b00000101, 31, 24), f(T, 23, 22), f(0b01, 21, 20);
>> 3831:     prf(Pg, 16), f(isFloat ? 1 : 0, 15), f(m, 14), f(sh, 13), f(imm8&0xff, 12, 5), rf(Zd, 0);
> 
> Suggestion:
> 
>     prf(Pg, 16), f(isFloat ? 1 : 0, 15), f(m, 14), f(sh, 13), f(imm8 & 0xff, 12, 5), rf(Zd, 0);
> 
> General HotSpot style.

Done, thanks.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26951#discussion_r2318454592