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From: "fw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/110899] RFE: Attributes preserve_most and preserve_all
Date: Mon, 07 Aug 2023 13:42:57 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110899

--- Comment #5 from Florian Weimer <fw at gcc dot gnu.org> ---
(In reply to Michael Matz from comment #3)
> For ABIs you generally want a good mix between caller- and callee-saved
> registers. The x86-64 psABI didn't do that on the SSE regs for conscious,=
 but
> meanwhile irrelevant, reasons, so my approach above tried to rectify this.
>=20
> The clang attributes seem to go against that general idea, they move all =
regs
> (or all general regs) into being callee-saved (except, strangely for
> aarch64?).

This is intended for functions that are called conditionally, but rarely, s=
uch
as debug logging. It's not a generally useful calling convention.

> It also makes argument registers be callee-saved, which is very
> unconventional.

Isn't this done for the this pointer in some C++ ABIs?

> Does the clang implementation take into account the various problematic
> cases that arise when calling a normal function from a (say) preserve_all
> function
> (hint: such call can't usually be done)?

How so? We need to version the __preserve_most__ attribute with the ISA lev=
el,
of course.=