RFR: 8336464: C2: Force CastX2P to be a two-address instruction

[Fei Gao]

On Tue, 30 Jul 2024 17:40:42 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

> I am not sure about this change. There reason we keep `src `and `dst` in different register for different types is most likely for cases when `src` could be used in other operations. Overwriting `src` register may give you more spills than before.
> 
> If there are no other `src` usages RA should handle this I think in shared code.

Thanks for your review @vnkozlov .

In my initial idea, if we keep `src` and `dst` in the same register, when `src` is used in other operations, yes, we need to generate extra spill code like:

Spill src to new_src
CastX2P    src src
...  // Other operations use new_src

In the final code, we can remove `CastX2P`, it becomes:

mov    src new_src
...  // Other operations use new_src

If we keep `src` and `dst` in the different registers, we may get:

CastX2P    src dst
...  // Other operations use src

In the final code, it will be:

mov    src  dst
...  // Other operations use src

I thought that keeping `src` and `dst` might not generate extra `mov`s because we can remove `CastX2P` itself finally.

But I tried some written cases showed in the PR description, which violated my thoughts in an unexpected way. Then I'm also not sure about it.

Anyway, it's a try and comments are welcome :)

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

PR Comment: https://git.openjdk.org/jdk/pull/20159#issuecomment-2268744405