Re: [PATCH] MIPS: Make BUG() __noreturn.

[David Daney]

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();
}


David Daney
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/