On Fri, 3 May 2024 10:46:05 +0100
Daniel Gregory <daniel.greg...@bytedance.com> wrote:
> On Thu, May 02, 2024 at 02:48:26PM -0700, Stephen Hemminger wrote:
> > There are already constant checks like this elsewhere in the file.
>
> Yes, but they're in macros, rather than inlined functions, so my
> understanding was that at compile time, macro expansion has put the
> memorder constant in the _Static_assert call as opposed to still being
> a function parameter in the inline definition.
Gcc and clang are smart enough that it is possible to use the internal
__builtin_constant_p() in the function. Some examples in DPDK:
static __rte_always_inline int
rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
unsigned int n, struct rte_mempool_cache *cache)
{
int ret;
unsigned int remaining;
uint32_t index, len;
void **cache_objs;
/* No cache provided */
if (unlikely(cache == NULL)) {
remaining = n;
goto driver_dequeue;
}
/* The cache is a stack, so copy will be in reverse order. */
cache_objs = &cache->objs[cache->len];
if (__extension__(__builtin_constant_p(n)) && n <= cache->len) {
It should be possible to use RTE_BUILD_BUG_ON() or static_assert here.
Changing a compile time check into a runtime check means that buggy programs
blow up much later and in a confusing manner. And it impacts all code that
is doing a spin lock, certainly one of the hottest paths in DPDK.
