RFR: 8354062: x86: Optimize stores of zero immediates with r12_heapbase

[Aleksey Shipilev]

On Tue, 8 Apr 2025 17:53:29 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> X86 does not have zero register. Except that it does for Hotspot, when compressed oops are enabled and heap base is zero. C2 routinely uses `r12` as zero register then. It makes the code considerably more compact. We can do the same in `MacroAssembler`. This would target the stores of known zeroes, which are surprisingly frequent in C1, mostly for zeroing out various `JavaThread` slots, e.g. for exception handling. 
> 
> (Kept x86_32 code intact, in case we want to backport it later. I don't mind removing x86_32 parts either.)
> 
> Additional testing:
>  - [ ] Linux x86_64 server fastdebug, `all`

Test failures. Back to draft.

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PR Comment: https://git.openjdk.org/jdk/pull/24519#issuecomment-2787444064