Re: Definition of BUG on x86

Petr Tesarik
2009-02-19T12:38:19Z

Ingo Molnar píše v Čt 19. 02. 2009 v 13:22 +0100:
> * Petr Tesarik <ptesarik@suse.cz> wrote:
>
> > Ingo Molnar píše v Čt 19. 02. 2009 v 13:10 +0100:
> > > * Petr Tesarik <ptesarik@suse.cz> wrote:
> > >
> > > > So, the only method I could invent was using gas macros. It
> > > > works but is quite ugly, because it relies on the actual
> > > > assembler instruction which is generated by the compiler. Now,
> > > > AFAIK gcc has always translated "for(;;)" into a jump to self,
> > > > and that with any conceivable compiler options, but I don't
> > > > know anything about Intel cc.
> > >
> > > > +static inline __noreturn void discarded_jmp(void)
> > > > +{
> > > > + asm volatile(".macro jmp target\n"
> > > > + "\t.purgem jmp\n"
> > > > + ".endm\n");
> > > > + for (;;) ;
> > > > +}
> > >
> > > hm, that's very fragile.
> > >
> > > Why not just:
> > >
> > > static inline __noreturn void x86_u2d(void)
> > > {
> > > asm volatile("u2d\n");
> > > }
> > >
> > > If GCC emits a bogus warning about _that_, then it's a bug in
> > > the compiler that should be fixed.
> >
> > I wouldn't call it a bug. The compiler has no idea about what
> > the inline assembly actualy does. So it cannot recognize that
> > the ud2 instruction does not return (which BTW might not even
> > be the case, depending on the implementation of the Invalid
> > Opcode exception).
>
> No, i'm not talking about the inline assembly.
>
> I'm talking about the x86_u2d() _inline function_, which has
> the __noreturn attribute.
>
> Shouldnt that be enough to tell the compiler that it ... wont
> return?

Nope, that's not how it works.

You _may_ specify a noreturn attribute to any function (and GCC will
honour it AFAICS), but if GCC _thinks_ that the function does return, it
will issue the above-mentioned warning:

/usr/src/linux-2.6/arch/x86/include/asm/bug.h:10: warning: 'noreturn' function does return

And that's what your function will do. :-(

Yes, I also thinks that this behaviour is counter-intuitive. Besides, I
haven't found a gcc switch to turn this warning off, which would be my
next recommendation, since the GCC heuristics is broken, of course.

Petr Tesarik