Kyle Moffett wrote:
> Here's code that I've found works as well as can be expected under both
> GCC 3 and GCC 4. If xp is a known-NULL constant the whole function will
> be optimized out completely. If xp is known-not-NULL, then it will
> optimize to a kfree function without the null check. Otherwise it
> optimizes to call the out-of-line version.
>
> Cheers,
> Kyle Moffett
>
> static inline void kfree(void *ptr)
> {
> if (__builtin_constant_p((ptr == NULL))) {
> if (ptr)
> kfree_nonnull(ptr);
> } else {
> kfree_unknown(ptr);
> }
> }
>
> void kfree_nonnull(void *ptr)
> {
> /* kfree code here, no null check */
> }
>
> void kfree_unknown(void *ptr)
> {
> if (ptr)
> kfree_nonnull(ptr);
> }
I still think there is an inconsistency in gcc. If I call your kfree
with the following:
void test( char *ptr )
{
char *null = NULL;
kfree(ptr); /* unknown */
*ptr = 'a';
kfree(ptr); /* nonnull */
kfree(null); /* should be optimised away */
}
,the compiler (4.1) generates two calls to kfree_unknown instead of one
to kfree_nonnull and one to kfree_unknown. It seems that the
__builtin_constant_p((ptr==NULL)) check does not always trigger, even if
the compiler 'knows' ptr to be equal to NULL. I posted a nasty hack
around this problem yesterday.
Groeten,
Bart
--
Bart Hartgers - TUE Eindhoven - http://plasimo.phys.tue.nl/bart/contact/
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