From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 6E2D73858418; Mon,  7 Aug 2023 12:55:05 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 6E2D73858418
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1691412905;
	bh=eUebi57SoM1JU9rC2lUcXDITkG0RCmTyLoXqY6eBJus=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=x+SMo5Bb9rwBKm0s5S24u4F8dJX6IIHa7WQHM+/79qdgr7JEzCGz7J22frDkx3I9d
	 /A6ZxSUYg3c2oUQrd5J1QFSnvSW3XUf1kz6HzN/4iG9eIVdMRnv9SZwKw/HGGsqg0f
	 0MjhjbRWYagMb1n+IceAq7xluUfkGgI1yvOttL2o=
From: "matz 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 12:55:04 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: matz at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-110899-4-1msugaiDAZ@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-110899-4@http.gcc.gnu.org/bugzilla/>
References: <bug-110899-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110899

--- Comment #3 from Michael Matz <matz at gcc dot gnu.org> ---
Huh, since when does clang implement this?  See also=20
  https://gcc.gnu.org/pipermail/gcc-patches/2023-July/624004.html
where I asked for comments about a similar, but not same, mechanism.  I came
from
the angle of preserving a couple of SSE registers on x86-64.

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, b=
ut
meanwhile irrelevant, reasons, so my approach above tried to rectify this.

The clang attributes seem to go against that general idea, they move all re=
gs
(or all general regs) into being callee-saved (except, strangely for aarch6=
4?).
It also makes argument registers be callee-saved, which is very unconventio=
nal.

Does the clang implementation take into account the various problematic cas=
es
that arise when calling a normal function from a (say) preserve_all function
(hint: such call can't usually be done)?  Does exception handling,
setjmp/longjmp
and make/swapcontext interact with the clang attributes?

That said: the implementation itself (after a more detailed spec) can be
implemented in the same framework like the above patch.  (It's possible that
something more needs doing for the unusual situation that argument regs are
then
callee-saved).=