Hi, Kaartic Sivaraam wrote: > On Saturday 16 September 2017 09:36 AM, Michael Haggerty wrote: >> Jonathan Nieder wrote:
>>> Does the following alternate fix work? I think I prefer it because >>> it doesn't require introducing a new global. [...] >>> #define for_each_string_list_item(item,list) \ >>> - for (item = (list)->items; item < (list)->items + (list)->nr; ++item) >>> + for (item = (list)->items; \ >>> + (list)->items && item < (list)->items + (list)->nr; \ >>> + ++item) >> >> This is the possibility that I was referring to as "add[ing] overhead to >> each iteration of the loop". I'd rather not add an extra test-and-branch >> to every iteration of a loop in which `list->items` is *not* NULL, which >> your solution appears to do. Or are compilers routinely able to optimize >> the check out? > > I t seems at least 'gcc' is able to optimize this out even with a -O1 > and 'clang' optimizes this out with a -O2. Taking a sneak peek at > the 'Makefile' shows that our default is -O2. > > For a proof, see https://godbolt.org/g/CPt73L >From that link: for ( ;valid_int && *valid_int < 10; (*valid_int)++) { printf("Valid instance"); } Both gcc and clang are able to optimize out the 'valid_int &&' because it is dereferenced on the RHS of the &&. For comparison, 'item < (list)->items + (list)->nr' does not dereference (list)->items. So that optimization doesn't apply here. A smart compiler could be able to take advantage of there being no object pointed to by a null pointer, which means item < (list)->items + (list)->nr is always false when (list)->items is NULL, which in turn makes a '(list)->items &&' test redundant. But a quick test with gcc 4.8.4 -O2 finds that at least this compiler does not contain such an optimization. The overhead Michael Haggerty mentioned is real. Thanks and hope that helps, Jonathan
