Re: [PATCH] Enable shrink wrapping for the RISC-V target.

[Jeff Law <jeffreyalaw@gmail.com>]

On 10/18/22 11:35, Palmer Dabbelt wrote:
>
>> I would have expected things to work fine with libcalls, perhaps with
>> the exception of the save/restore libcalls.  So that needs deeper
>> investigation.
>
> The save/restore libcalls only support saving/restoring a handful of 
> register configurations (just the saved X registers in the order 
> they're usually saved in by GCC).  It should be OK for correctness to 
> over-save registers, but it kind of just un-does the shrink wrapping 
> so not sure it's worth worrying about at that point.
>
> There's also some oddness around the save/restore libcall ABI, it's 
> not the standard function ABI but instead a GCC-internal one.  IIRC it 
> just uses the alternate link register (ie, t0 instead of ra) but I may 
> have forgotten something else.

I hadn't really dug into it -- I was pretty sure they weren't following 
the standard ABI based on its name and how I've used similar routines to 
save space on some targets in the past.  So if we're having problems 
with shrink-wrapping and libcalls, those two might be worth investigating.


But I think the most important takeaway is that shrink wrapping should 
work with libcalls, there's nothing radically different about libcalls 
that would make them inherently interact poorly with shrink-wrapping.  
So that aspect of the shrink-wrapping patch needs deeper investigation.

Jeff