Subject: Re: [PATCH v2 7/7] unpack-trees: free cache_entry array members for merges

From: René Scharfe <[email-masked]>
Date: Mon, 03 Jun 2013 01:06:52 +0200

Am 03.06.2013 00:38, schrieb Felipe Contreras:
> On Sun, Jun 2, 2013 at 3:26 PM, René Scharfe
> <[email-masked]> wrote:
>> Am 02.06.2013 19:59, schrieb Felipe Contreras:
>>
>>> On Sun, Jun 2, 2013 at 12:54 PM, René Scharfe
>>> <[email-masked]> wrote:
>>>>
>>>> Am 02.06.2013 19:25, schrieb Felipe Contreras:
>>>>>
>>>>>
>>>>> On Sun, Jun 2, 2013 at 10:46 AM, René Scharfe
>>>>> <[email-masked]> wrote:
>>>>>>
>>>>>>
>>>>>> +               for (i = 0; i < n; i++) {
>>>>>> +                       struct cache_entry *ce = src[i + o->merge];
>>>>>> +                       if (ce != o->df_conflict_entry)
>>>>>
>>>>>
>>>>>
>>>>> It's possible that ce is NULL, but you didn't add that check because
>>>>> free(NULL) still works? Or because ce cannot be NULL?
>>>>>
>>>>> If it's the former, I think it's clearer if we check that ce is not
>>>>> NULL either way.
>>>>
>>>>
>>>>
>>>> It is NULL if one tree misses an entry (e.g. a new or removed file). free
>>>> handles NULL and we generally avoid duplicating its NULL-check.
>>>
>>>
>>> Yeah, but I can see somebody adding code inside that 'if' clause to
>>> print the cache entry, and see a crash only to wonder what's going on.
>>> And to save what? 5 characters?
>>
>>
>> The person adding code that depends on ce not being NULL needs to add that
>> check as well.  Let's not worry too much about future changes that may or
>> (more likely IMHO) may not be done.  The test suite covers this case
>> multiple times, so such a mistake doesn't have a realistic chance to hit
>> master.
>
> What do we gain by not doing this? 5 less characters?

By following the convention of not checking for NULL when freeing, we 
reduce the size of the code slightly and have one less branch 
instruction in the object code.  That's not particularly exciting in a 
single instance but makes a difference if followed throughout the code base.

What do we gain by adding a duplicate check?  A few minutes of debugging 
time by the person who will add some code and forget the NULL check? 
And how likely is that to happen?

René