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

To: Geert Uytterhoeven <geert@linux-m68k.org>, gcc@gcc.gnu.org
Subject: Re: [PATCH] MIPS: Make BUG() __noreturn.
From: David Daney <ddaney@caviumnetworks.com>
Date: Fri, 21 Nov 2008 08:40:57 -0800
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-mips <linux-mips@linux-mips.org>, linux-kernel@vger.kernel.org, Adam Nemet <anemet@caviumnetworks.com>
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Geert Uytterhoeven wrote:
On Fri, 21 Nov 2008, Alan Cox wrote:
On Thu, 20 Nov 2008 17:26:36 -0800
David Daney <ddaney@caviumnetworks.com> 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

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