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SubjectRe: [PATCH] MIPS: Make BUG() __noreturn.
Andrew Morton wrote:
>
>
> 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.

As I said in the other part of the thread, We are working on a GCC patch
that adds a new built-in function '__builtin_noreturn()', that you could
substitute for 'for(;;);' that emits no instructions in this case.

David Daney


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