RFR: 8302167: Avoid allocating register in fast_lock()

[Kelvin Nilsen]

On Thu, 9 Feb 2023 18:09:08 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> In x86's C2MacroAssembler::fast_lock(), we are never really using the cx1Reg register, except in the DiagnoseSyncOnValueBasedClasses path, where we can just as well use the scrReg register.
> 
> What's worse, in x86_32.ad, we don't allocate the cx1 register and pass noreg, so the DiagnoseSyncOnValueBasedClasses would be broken on x86_32.
> 
> We can simply avoid allocating that extra register (which may help performance under register pressure), and use scr in DiagnoseSyncOnValueBasedClasses.
> 
> Testing:
>  - [x] tier1
>  - [ ] tier2

Marked as reviewed by kdnilsen (no project role).

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PR: https://git.openjdk.org/jdk/pull/12499