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. This makes the new
> BUG() function 3 instructions long instead of just 1, but I think it
> is worth it as it is now unnecessary to do extra work to silence the
> 'used uninitialized' warnings.
>
> I also re-wrote BUG_ON so that if it is given a constant condition, it
> just does BUG() instead of loading a constant value in to a register
> and testing it.
>
Yup, this change will fix some compile warnings which will never be
fixed in any other way for mips.
> +static inline void __noreturn BUG(void)
> +{
> + __asm__ __volatile__("break %0" : : "i" (BRK_BUG));
> + /* Fool GCC into thinking the function doesn't return. */
> + while (1)
> + ;
> +}
This kind of sucks, doesn't it? It adds instructions into the kernel
text, very frequently on fast paths. Those instructions are never
executed, and we're blowing away i-cache just to quash compiler
warnings.
For example, this:
--- a/arch/x86/include/asm/bug.h~a
+++ a/arch/x86/include/asm/bug.h
@@ -22,14 +22,12 @@ do { \
".popsection" \
: : "i" (__FILE__), "i" (__LINE__), \
"i" (sizeof(struct bug_entry))); \
- for (;;) ; \
} while (0)
#else
#define BUG() \
do { \
asm volatile("ud2"); \
- for (;;) ; \
} while (0)
#endif
_
reduces the size of i386 mm/vmalloc.o text by 56 bytes.
I wonder if there is any clever way in which we can do this without
introducing additional runtime cost.
