RFR: 8315841: RISC-V: Check for hardware TSO support [v5]

[Vladimir Kempik]

On Thu, 7 Sep 2023 13:04:05 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>> With the Ztso extension [1], some hardware will support TSO on RISC-V. That allows us to reduce the generation of memory fences, given the stronger memory model compared to RVWMO.
>> 
>> [1] https://github.com/riscv/riscv-isa-manual/blob/6dcbc6da9ada01f0f57da83cda6059bdec57619f/src/ztso-st-ext.adoc#L1
>
> Ludovic Henry has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - fixup! 8315841: RISC-V: Check for hardware TSO support
>  - fixup! 8315841: RISC-V: Check for hardware TSO support

Maybe this way:
still needs a good comment for cases when we don't generate fence

   if (UseZtso) {
      if ((pred_succ_to_membar_mask(predecessor, successor) & StoreLoad) == 0) {
             return;
   }
      // always generate fence for RVWMO
      Assembler::fence(predecessor, successor);

-------------

PR Comment: https://git.openjdk.org/jdk/pull/15613#issuecomment-1710123100