Re: [committed] [PR target/93062] RISC-V: Handle long conditional branches for RISC-V

[Jeff Law <jeffreyalaw@gmail.com>]

On 10/10/23 18:24, Andrew Waterman wrote:
> I remembered another concern since we discussed this patch privately.
> Using ra for long calls results in a sequence that will corrupt the
> return-address stack.
Yup.  We've actually got data on that internally, it's not showing up in 
a significant way in practice.


   I know nothing
> about the complexity of register scavenging, but it would be nice to
> opportunistically use a scratch register (other than t0), falling back
> to ra only when necessary.
The nice thing about making $ra fixed is some can add a register 
scavenging approach, then fall back to $ra if they're unable to find a 
register to reuse.

> 
> Tangentially, I noticed the patch uses `jump label, ra' for far
> branches but uses `call label' for far jumps.  These corrupt the RAS
> in opposite ways (the former pops the RAS and the latter pushes it.
> Any reason for using a different sequence in one than the other?
I'd noticed it as well -- that's the way it was in the patch that was 
already in Ventana's tree ;-)  My plan was to address that separately 
after dropping in enough infrastructure to allow me to force everything 
to be far branches for testing purposes.

jeff