Re: [PATCH] for_each_string_list_item(): behave correctly for empty list

From: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
Date: Sep 19, 2017, 14:38 UTC
Message-ID: <b03c7b09-853f-a2ed-f73e-7d946c90cedb@gmail.com>
In-Reply-To: <b8951886-feab-a87a-9683-3c155cfa98a8@alum.mit.edu>

On Saturday 16 September 2017 09:36 AM, Michael Haggerty wrote:

Show 12 quoted lines

>> 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.

For a proof, see https://godbolt.org/g/CPt73L

--- Kaartic