RFR: 8316645: RISC-V: Remove dependency on libatomic by adding cmpxchg 1b

[Robbin Ehn]

On Wed, 27 Sep 2023 07:37:52 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> Looks good from a build perspective. It's nice to get rid of such special cases.

Thank you @magicus !

> > This have been fixed in recent gcc, so latomic is no longer needed.
> 
> I just noticed this. Does this mean RISC-V requires a higher version of gcc than the current minimum of 6.0?

This patch uses gcc builtin after 13.2, before that it uses this new assembly implementation.
So once we move to 13 as minimum we can remove this asm.

The rv port is only in gcc 7 and above.
Debian trixie will be the first distro to official support rv64 and it looks like that will be shipped with gcc 13 as default and with 11 as installable.

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PR Comment: https://git.openjdk.org/jdk/pull/15919#issuecomment-1736931374