GCC 12 miscompilation of volatile asm (was: Re: [PATCH] arm64/io: Remind compiler that there is a memory side effect)

[Mark Rutland]

On Tue, Apr 05, 2022 at 04:05:22PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 05, 2022 at 01:51:30PM +0100, Mark Rutland wrote:
> > Hi all,
> > 
> > [adding kernel folk who work on asm stuff]
> > 
> > As a heads-up, GCC 12 (not yet released) appears to erroneously optimize away
> > calls to functions with volatile asm. Szabolcs has raised an issue on the GCC
> > bugzilla:  
> > 
> >   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105160
> > 
> > ... which is a P1 release blocker, and is currently being investigated.
> > 
> > Jemery originally reported this as an issue with {readl,writel}_relaxed(), but
> > the underlying problem doesn't have anything to do with those specifically.
> > 
> > I'm dumping a bunch of info here largely for posterity / archival, and to find
> > out who (from the kernel side) is willing and able to test proposed compiler
> > fixes, once those are available.
> > 
> > I'm happy to do so for aarch64; Peter, I assume you'd be happy to look at the
> > x86 side?
> 
> Sure..

FWIW, compiler explorer now have a trunk build with the fix, and my x86-64
example now gets compiled to something which looks correct:

  https://godbolt.org/z/cveff9hq5

Thanks,
Mark.