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    SubjectRe: Definition of BUG on x86

    * 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
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