Re: Definition of BUG on x86

Ingo Molnar
2009-02-19T12:47:26Z

* Petr Tesarik <ptesarik@suse.cz> wrote:

> 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.

so GCC should be fixed and improved here, on several levels.

Ingo