Re: Definition of BUG on x86

Ingo Molnar
2009-02-19T14:49:27Z

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

> Ingo Molnar píše v Čt 19. 02. 2009 v 13:47 +0100:
> > * 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.
>
> Agree.
>
> But it takes some time, even if we start pushing right now.
> What's your suggestion for the meantime? Keep the dummy jmp?
> And in case anybody is concerned about saving every byte in
> the text section, they can apply my dirty patch?
>
> Actually, this doesn't sound too bad.

yeah. Please forward the problem to the appropriate GCC list in
any case.

Ingo