Re: Definition of BUG on x86

[Jeremy Fitzhardinge]

Ingo Molnar wrote:
> * Petr Tesarik <ptesarik@xxxxxxx> wrote:
>
>   
> > Ingo Molnar pÃÅe v Ät 19. 02. 2009 v 13:47 +0100:
> >     
> > > * Petr Tesarik <ptesarik@xxxxxxx> wrote:
> > >
> > >       
> > > > Ingo Molnar pÃÅe v Ät 19. 02. 2009 v 13:22 +0100:
> > > >         
> > > > > * Petr Tesarik <ptesarik@xxxxxxx> wrote:
> > > > >
> > > > >           
> > > > > > Ingo Molnar pÃÅe v Ät 19. 02. 2009 v 13:10 +0100:
> > > > > >             
> > > > > > > * Petr Tesarik <ptesarik@xxxxxxx> 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.
>
>   

I think the official answer for this case is to use __builtin_trap.  But:

-- Built-in Function: void __builtin_trap (void)
    This function causes the program to exit abnormally.  GCC
    implements this function by using a target-dependent mechanism
    (such as intentionally executing an illegal instruction) or by
    calling `abort'.  ***The mechanism used may vary from release to
    release so you should not rely on any particular implementation.***

which in principle is hard for us to make use of.  In practice I think 
it has always been ud2a on x86.

http://gcc.gnu.org/ml/gcc-patches/2000-01/msg00190.html

   J
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