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From: "s_gccbugzilla at nedprod dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/108659] Suboptimal 128 bit atomics codegen on AArch64
 and x64
Date: Fri, 03 Feb 2023 21:22:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108659

--- Comment #7 from Niall Douglas <s_gccbugzilla at nedprod dot com> ---
(In reply to Andrew Pinski from comment #4)
> (In reply to Niall Douglas from comment #3)=20
> > You may be interested in reading https://reviews.llvm.org/D110069. It w=
anted
> > to have LLVM generate a 128 bit AArch64 CAS for atomics. LLVM merged th=
at
> > change, it'll be in the next release.
>=20
> Using CAS for atomic load is not valid thing to do ...
> Because atomic load from constant rodata needs to work.
> LLVM breaks this case as they don't care about it. GCC does though.

I've heard that argument before, and I've always wondered why _Atomic128 ty=
pes
couldn't have an attribute which applies attribute section to their static
const variable incarnations to force them into r/w memory. That would also
solve the LLVM issue. Said attribute is not unuseful in general actually, it
would help avoid having to mess with mprotect to apply copy on write perms =
on
regions in .rodata when you need to modify static const variable values.

I don't think that the standard *guarantees* that static const variables go
into read only memory, and besides, before C23 128 bit integers weren't
supported anyway so one could argue as a proprietary extension (__int128) y=
ou
get proprietary special casing.

Niall=