On Fri, 21 Nov 2008, David Daney wrote:
> Geert Uytterhoeven wrote:
> > On Fri, 21 Nov 2008, Alan Cox wrote:
> > > On Thu, 20 Nov 2008 17:26:36 -0800
> > > David Daney <ddaney@xxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > > MIPS: Make BUG() __noreturn.
> > > >
> > > > Often we do things like put BUG() in the default clause of a case
> > > > statement. Since it was not declared __noreturn, this could sometimes
> > > > lead to bogus compiler warnings that variables were used
> > > > uninitialized.
> > > >
> > > > There is a small problem in that we have to put a magic while(1); loop
> > > > to
> > > > fool GCC into really thinking it is noreturn.
> > > That sounds like your __noreturn macro is wrong.
> > >
> > > Try using __attribute__ ((__noreturn__))
> > >
> > > if that works then fix up the __noreturn definitions for the MIPS and gcc
> > > you have.
> >
> > Nope, gcc is too smart:
> >
> > $ cat a.c
> >
> > int f(void) __attribute__((__noreturn__));
> >
> > int f(void)
> > {
> > }
> >
> > $ gcc -c -Wall a.c
> > a.c: In function f:
> > a.c:6: warning: `noreturn' function does return
> > $
>
> That's right.
>
> I was discussing this issue with my colleague Adam Nemet, and we came
> up with a couple of options:
>
> 1) Enhance the _builtin_trap() function so that we can specify the
> break code that is emitted. This would allow us to do something
> like:
>
> static inline void __attribute__((noreturn)) BUG()
> {
> __builtin_trap(0x200);
> }
>
> 2) Create a new builtin '__builtin_noreturn()' that expands to nothing
> but has no CFG edges leaving it, which would allow:
>
> static inline void __attribute__((noreturn)) BUG()
> {
> __asm__ __volatile__("break %0" : : "i" (0x200));
> __builtin_noreturn();
> }
Now I remember, yes, __builtin_trap() is how we fixed it on m68k:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=e8006b060f3982a969c5170aa869628d54dd30d8
Of course, if you need a different trap code than the default, you're in
trouble.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
