RFR: 8356159: RISC-V: Add Zabha

[Fei Yang]

On Thu, 15 May 2025 14:08:48 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

> Hi, please consider.
> 
> This adds the byte and halfword atomic memory operations (Zabha) - https://github.com/riscv/riscv-zabha.
> All amo-instructions, except load-reserve and store-conditional, can also be performed on natural aligned half-words and bytes. (i.e. the extension do not add lr.h/b or sc.h/b) This includes amocas if zacas extension is present.
> 
> The majority of this patch is to support amocas.h/b. We are now starting to really feel the pain of all these extensions, as CAS:ing 16/8-bits can now be done in three different ways:
> - lr.w/sc.w 'narrow' CAS (no extension)
> - amocas.w 'narrow' CAS (Zacas)
> - amocas.h/b (Zacas + Zabha)
> 
> There is no hwprobe support yet.
> 
> Ran t1-3 with Zacas+Zabha and t1 without Zabha in qemu.
> 
> Thanks, Robbin

I witnessed performance regressions after this change on two of my OoO machines (around 7%-10% on P550 and SG2042) when doing a quick specjbb2005 test. There are no Zacas or Zabha extensions. This issue disappear if I revert changes in file `src/hotspot/cpu/riscv/assembler_riscv.hpp`. I haven't figured out the reason.

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PR Comment: https://git.openjdk.org/jdk/pull/25252#issuecomment-2889785836