RFR: 8365911: AArch64: Fix encoding error in sve_cpy for negative floats [v3]

[Andrew Haley]

On Wed, 3 Sep 2025 10:02:24 GMT, erifan <duke at openjdk.org> wrote:

>> 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.
>
> erifan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Code style fixes

Good.

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Marked as reviewed by aph (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26951#pullrequestreview-3179955144