> From: Morten Brørup > Sent: Tuesday, 18 April 2023 13.30 > > > From: Bruce Richardson [mailto:bruce.richard...@intel.com] > > Sent: Tuesday, 18 April 2023 13.07 > > > > On Tue, Apr 11, 2023 at 08:48:45AM +0200, Morten Brørup wrote:
[...] > > > + /* > > > + * 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. > > Regardless, your point here is still valid! I expected that any > unrolling capable compiler can unroll *dst++ = *--src; but I can > experiment with different compilers on godbolt.org to see if dst[index] > = src[-index] is better. Just for the record... I have now tried experimenting with the alternative, and it makes no difference.
