Re: thumb compilation

[Mikko Rapeli <mikko.rapeli%teleca.com@localhost>]

On Wed, Apr 30, 2008 at 10:09:04AM +0100, Richard Earnshaw wrote:
> The compiler never directly generates SWP, so this must be an in-line
> assembler statement.  Since there's no SWP instruction in Thumb, the
> code that generates this will need reworking to push the required
> instructions out-of-line.

Ok, I tried finding the culprit asm code but failed to notice
sys/arch/arm/include/lock.h:

...
#if defined(_KERNEL)
static __inline int
__swp(int __val, volatile unsigned char *__ptr)
{

        __asm volatile("swpb %0, %1, [%2]"
            : "=&r" (__val) : "r" (__val), "r" (__ptr) : "memory");
        return __val;
}
#else
static __inline int
__swp(int __val, volatile int *__ptr)
{

        __asm volatile("swp %0, %1, [%2]"
            : "=&r" (__val) : "r" (__val), "r" (__ptr) : "memory");
        return __val;
}
#endif /* _KERNEL */
...
static __inline void __attribute__((__unused__))
__cpu_simple_lock(__cpu_simple_lock_t *alp)
{

        while (__swp(__SIMPLELOCK_LOCKED, alp) != __SIMPLELOCK_UNLOCKED)
                continue;
}

static __inline int __attribute__((__unused__))
__cpu_simple_lock_try(__cpu_simple_lock_t *alp)
{

        return (__swp(__SIMPLELOCK_LOCKED, alp) ==
__SIMPLELOCK_UNLOCKED);
}
...

I don't know ARM assembly that well, but it seems like an atomic version of swp
is needed here in thumb asm. Is this correct?

-Mikko