linux-kernel - Re: Definition of BUG on x86

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Date:	Thu, 19 Feb 2009 13:22:11 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Petr Tesarik <ptesarik@...e.cz>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Definition of BUG on x86


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

> Ingo Molnar píše v Čt 19. 02. 2009 v 13:10 +0100:
> > * Petr Tesarik <ptesarik@...e.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
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