RFR: 8353176: C1: x86 patching stub always calls Thread::current()

[Vladimir Ivanov]

On Fri, 28 Mar 2025 11:05:33 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Noticed this while looking at compiled code density. In C1 PatchingStub code, we _always_ perform a runtime call to `Thread::current()`, even though we can rely on `r15_thread` to be available. This currently emits a huge sequence of instructions for register save/restore and the call itself.
> 
> Current code calls to `MacroAssembler::get_thread()`, which is always doing that slowpath. This kind of accident would be less likely / impossible once we cleanup uses of `MacroAssembler::get_thread()` with [JDK-8353174](https://bugs.openjdk.org/browse/JDK-8353174).
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `tier1`
>  - [x] Linux x86_64 server fastdebug, `tier1` + `-XX:TieredStopAtLevel=1`
>  - [ ] Linux x86_64 server fastdebug, `all`
>  - [x] Linux x86_64 server fastdebug, `all` + `-XX:TieredStopAtLevel=1`

Looks good.

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Marked as reviewed by vlivanov (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24291#pullrequestreview-2726274704