Re: Definition of BUG on x86

Ingo Molnar
2009-02-19T12:22:34Z

* 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?

Ingo