linux-kernel - Re: [PATCH] MIPS: Make BUG() _

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Date:	Fri, 21 Nov 2008 22:16:19 +0000
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	David Daney <ddaney@...iumnetworks.com>, gcc@....gnu.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-mips <linux-mips@...ux-mips.org>,
	linux-kernel@...r.kernel.org,
	Adam Nemet <anemet@...iumnetworks.com>
Subject: Re: [PATCH] MIPS: Make BUG() __noreturn.

On Fri, Nov 21, 2008 at 07:46:43PM +0100, Geert Uytterhoeven wrote:

> > 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);
> > }

I had suggested this one before ...

> > 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:

I like this interface.

> 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.

MIPS ISA newer than MIPS I also have conditional break codes allowing
something like this:

#define BUG_ON(condition)                                               \
do {                                                                    \
        __asm__ __volatile__("tne $0, %0, %1"                           \
                             : : "r" (condition), "i" (BRK_BUG));       \
} while (0)

that is test of condition and the trap as a single instruction.  Note there
are break and trap instructions on MIPS and they are basically doing the
same job ...

  Ralf
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