[PATCH] powerpc: merge include/asm/cputable.h

[Arnd Bergmann]

On Middeweken 14 September 2005 21:11, Kumar Gala wrote:
> I not sure I understand what the introduction of the enum's gets us.
> 
It doesn't have to be an enum, it could just as well be a #define,
if we find that to be better in some way (maybe compile-time).

The general idea is to convert run-time checks into compile-time
checks in order to improve the running kernel. If you have

// start code
enum {
        FEATURE_1 = 1,
        FEATURE_2 = 2,
        PLATFORM_1 = FEATURE_1, 
        PLATFORM_2 = FEATURE_2, 
        PLATFORM_3 = FEATURE_1 | FEATURE_2,
        FEATURE_POSSIBLE = 
#ifdef CONFIG_PLATFORM_1
                PLATFORM_1 |
#endif
#ifdef CONFIG_FEATURE_2
                PLATFORM_2 |
#endif
#ifdef CONFIG_FEATURE_3
                PLATFORM_3 |
#endif
                0,
        FEATURE_ALWAYS =
#ifdef CONFIG_PLATFORM_1
                PLATFORM_1 &
#endif
#ifdef CONFIG_PLATFORM_2
                PLATFORM_2 &
#endif
#ifdef CONFIG_PLATFORM_3
                PLATFORM_3 &
#endif
                FEATURE_POSSIBLE,
};

static inline int have_feature(unsigned long feature)
{
    return (FEATURE_ALWAYS & feature) ||
           (FEATURE_POSSIBLE & runtime_feature & feature);
}

int foo();
int bar();
int main(void)
{
        if (have_feature(FEATURE_1))
                return foo();
        if (have_feature(FEATURE_2))
                return bar();
        return 0;
}
// end code

Then gcc will produce optimal object code for any combination
of CONFIG_PLATFORM_{1,2,3}. Of course I have to admit that the
header file is not exactly elegant ;-).

        Arnd <><