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

[Thomas Schatzl]

On Fri, 21 Nov 2025 10:04:56 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

Looks good, thanks.

I assume that the jmh writebarrier micros were run just in case. Fwiw, also the GHA failures earlier looked like infra issues.

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Marked as reviewed by tschatzl (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28446#pullrequestreview-3492026049