Re: [PATCH] RISCV: Add -m(no)-omit-leaf-frame-pointer support.

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/26/23 08:50, Kito Cheng wrote:
> LLVM will try to find scratch register even after RA to resolve the long 
> jump issue. so maybe we could consider similar approach? And I guess the 
> most complicate part would be the scratch register is not found, and 
> require spill/reload after RA.
Right.  And the spill/reload after RA is ta problem unless you 
pre-allocate the space.  Of course in a function near 1M in size, odds 
are there were some calls in there and thus $ra would be saved.  In the 
exceedingly rare case where it wasn't, allocating a single stack slot 
isn't going to be a major performance driver.

There's other things you can do as well.  Register scavenging, jump 
trampolines, etc.  Examples of both exist.

The point I'm trying to make is that I suspect we're better off burning 
$ra right now to address the correctness issue, then coming back to one 
of the schemes noted above when the cost/benefit analysis shows it's a 
reasonably high priority relative to other optimizations we could be doing.

Jeff