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
