Hi, Junio C Hamano wrote: > Kaartic Sivaraam <kaarticsivaraam91196@xxxxxxxxx> writes: >> 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? >> >> It 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. > > But doesn't the versions of gcc and clang currently available do the > right thing with the current code without this change anyway? I've > been operating under the assumption that this is to future-proof the > code even when the compilers change to use the "NULL+0 is undefined" > as an excuse to make demons fly out of your nose, so unfortunately I > do not think it is not so huge a plus to find that the current > compilers do the right thing to the code with proposed updates. I think you and Kaartic are talking about different things. Kaartic was checking that this wouldn't introduce a performance regression (thanks!). You are concerned about whether the C standard and common practice treat the resulting code as exhibiting undefined behavior. Fortunately the C standard is pretty clear about this. The undefined behavior here is at run time, not compile time. As you suggested in an earlier reply, the 'list->items &&' effectively guards the 'list->items + list->nr' to prevent that undefined behavior. I'll send a patch with a commit message saying so to try to close out this discussion. Thanks, Jonathan
