RFR: 8315841: RISC-V: Check for hardware TSO support [v4]

[Ludovic Henry]

On Thu, 7 Sep 2023 12:52:53 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> @theRealAph this way we will miss fence completely when UseZtso is false
>
> I prefer:
> 
> {
>   if (UseZtso) {
>     ....
>     return;
>   }
>   <the other code path without else>
> }

We still need to generate the fence in case of non-TSO, so with that code structure, it would look like that:

void fence(uint32_t predecessor, uint32_t successor) {
  if (UseZtso) {
    if (pred_succ_to_membar_mask(predecessor, successor) & StoreLoad) {
      // TSO allows for stores to be reordered after loads. When the compiler
      // generates a fence to disallow that, we are required to generate the
      // fence for correctness.
      Assembler::fence(predecessor, successor);
    } else {
      // TSO guarantees other fences already.
    }
  } else {
    Assembler::fence(predecessor, successor);
  }
}

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PR Review Comment: https://git.openjdk.org/jdk/pull/15613#discussion_r1318564436