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
