> From: Thomas Monjalon [mailto:tho...@monjalon.net] > Sent: Wednesday, 7 June 2023 10.10 > > 07/06/2023 10:03, Morten Brørup: > > > From: Thomas Monjalon [mailto:tho...@monjalon.net] > > > Sent: Wednesday, 7 June 2023 09.52 > > > > > > 18/04/2023 14:55, Bruce Richardson: > > > > On Tue, Apr 18, 2023 at 01:29:49PM +0200, Morten Brørup wrote: > > > > > From: Bruce Richardson [mailto:bruce.richard...@intel.com] > > > > > > On Tue, Apr 11, 2023 at 08:48:45AM +0200, Morten Brørup wrote: > > > > > > > + if (__extension__(__builtin_constant_p(n)) && n <= cache->len) { > > > > > > > + /* > > > > > > > + * The request size is known at build time, and > > > > > > > + * the entire request can be satisfied from the cache, > > > > > > > + * so let the compiler unroll the fixed length copy > > > > > > > loop. > > > > > > > + */ > > > > > > > + cache->len -= n; > > > > > > > + for (index = 0; index < n; index++) > > > > > > > + *obj_table++ = *--cache_objs; > > > > > > > + > > > > > > > > > > > > This loop looks a little awkward to me. Would it be clearer (and > perhaps > > > > > > easier for compilers to unroll efficiently if it was rewritten as: > > > > > > > > > > > > cache->len -= n; > > > > > > cache_objs = &cache->objs[cache->len]; > > > > > > for (index = 0; index < n; index++) > > > > > > obj_table[index] = cache_objs[index]; > > > > > > > > > > The mempool cache is a stack, so the copy loop needs get the objects > in > > > decrementing order. I.e. the source index decrements and the destination > index > > > increments. > > > > > > > > > > > > > BTW: Please add this as a comment in the code too, above the loop to > avoid > > > > future developers (or even future me), asking this question again! > > > > > > Looks like this request was missed. > > > > I intentionally omitted it, because I disagree with the suggestion. > > > > Otherwise, reminders that the mempool cache is a stack should be plastered > all over the source code, not just here. For reference, this copy loop > (without such a reminder) also exists elsewhere in the same file. > > > > Apologies for not responding to Bruce's request earlier. > > What about doing a general comment at the top of the function, > with the assignment of the pointer at the end of the array: > > /* The cache is a stack, so copy will be in reverse order. */ > cache_objs = &cache->objs[cache->len]; > > I could do it on apply if there is an agreement. > ACK from me.
For consistency, please also add the reminder to the two existing reverse order copy loops in rte_mempool_do_generic_get().
