RFR: 8316186: RISC-V: Remove PlatformCmpxchg<4>

[Robbin Ehn]

On Wed, 13 Sep 2023 12:25:41 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

> Hi, please consider.
> 
> I don't know the history behind the Cmpxhg<4>, but it is not needed anymore.
> Using compiler as in 8 byte case is just fine.
> 
> Passes hotspot-tier (qemu, vf2 still running), some manual extra testing via gtest, looked at compiler: output, e.g. https://godbolt.org/z/a31Gdqn8q
> 
> If you know the history please share, and if you have a reason why we should keep it please speak up!

So we basically have some rubbish in top 32 bits, we clear them, right.
And lr.w automatically sign extends, so old_value is fine.

Checking for when/where the debug was/is in tool-chain.

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PR Comment: https://git.openjdk.org/jdk/pull/15715#issuecomment-1718993013