Re: [PATCH] docs/memory-barriers.txt: volatile is not a barrier() substitute

[Nick Desaulniers]

On Tue, Feb 1, 2022 at 2:15 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Tue, 1 Feb 2022 at 20:40, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> >
> > On Tue, Feb 1, 2022 at 1:32 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > >
> > > On Mon, 31 Jan 2022 at 23:53, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> > > >
> > > > +     around an asm statement so long as clobbers are not violated. For example,
> > > > +
> > > > +       asm volatile ("");
> > > > +       flag = true;
> > > > +
> > > > +     May be modified by the compiler to:
> > > > +
> > > > +       flag = true;
> > > > +       asm volatile ("");
> > > > +
> > > > +     Marking an asm statement as volatile is not a substitute for barrier(),
> > > > +     and is implicit for asm goto statements and asm statements that do not
> > > > +     have outputs (like the above example). Prefer either:
> > > > +
> > > > +       asm ("":::"memory");
> > > > +       flag = true;
> > > > +
> > > > +     Or:
> > > > +
> > > > +       asm ("");
> > > > +       barrier();
> > > > +       flag = true;
> > > > +
> > >
> > > I would expect the memory clobber to only hazard against the
> > > assignment of flag if it results in a store, but looking at your
> > > Godbolt example, this appears to apply even if flag is kept in a
> > > register.
> > >
> > > Is that behavior documented/codified anywhere? Or are we relying on
> > > compiler implementation details here?
> >
> > https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile
> > "Note that the compiler can move even volatile asm instructions
> > relative to other code, including across jump instructions."
> >
>
> That doesn't really answer my question. We are documenting here that
> asm volatile does not prevent reordering but non-volatile asm with a
> "memory" clobber does, and even prevents reordering of instructions
> that do not modify memory to begin with.
>
> Why is it justified to rely on this undocumented behavior?

I see your point.  You're right, I couldn't find anywhere where such
behavior was specified.  So the suggestion to use barrier() would rely
on unspecified behavior and should not be suggested.

Probably worth still mentioning that `volatile` qualifying an asm
statement doesn't prevent such reordering in this document somehow,
and perhaps that it's (currently) unspecified whether a barrier() can
prevent re-ordering with regards to non-memory-modifying instructions.
-- 
Thanks,
~Nick Desaulniers