RFR: 8372285: G1: Micro-optimize x86 barrier code [v3]

[Aleksey Shipilev]

On Fri, 21 Nov 2025 10:07:35 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> We know from [JDK-8372284](https://bugs.openjdk.org/browse/JDK-8372284) that G1 C2 stubs can take ~10% of total instructions. So minor optimizations in hand-written assembly pay off for code density. This PR does a little x86-specific polishing: `testptr` where possible, short forward branches where possible. I rewired some code to make it abundantly clear the branches in question are short. It also makes clear that lots of the affected methods are essentially fall-through.
>> 
>> The patch is deliberately on simpler side, so we can backport it to 25u, if need arises.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `tier1`
>>  - [ ]  Linux x86_64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Make some backward branches explicitly short

GHA failure is due to https://github.com/openjdk/jdk/pull/28445.

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PR Comment: https://git.openjdk.org/jdk/pull/28446#issuecomment-3562313261