On Sat, Mar 9, 2013 at 2:50 PM, Junio C Hamano <gits...@pobox.com> wrote:
> Nguyễn Thái Ngọc Duy <pclo...@gmail.com> writes:
>> strncmp provides length information, compared to strcmp, which could
>> be taken advantage by the implementation. Even better, we could check
>> if the lengths are equal before calling strncmp, eliminating a bit of
>> strncmp calls.
> I think I am a bit slower than my usual self tonight, but I am
> utterly confused by the above.
> strncmp() compares _only_ up to the first n bytes, so when you are
> using it for equality, it is not "we could check length", but is "we
> MUST check they match to the length of the shorter string", if you
> want to obtain not just faster but correct result.
> Am I mistaken?
Yeap, the description is a bit misleading. Although you could get away
with length check by doing !strncmp(a, b, strlen(a)+1).
> Even if you are using strcmp() that yields ordering not just
> equality, it can return a correct result as soon as it hits the
> first bytes that are different; I doubt using strncmp() contributes
> to the performance very much. Comparing lengths before doing
> byte-for-byte comparison could help because you can reject two
> strings with different lengths without looking at them.
> At the same time, I wonder if we can take advantage of the fact that
> these call sites only care about equality and not ordering.
I tried to push it further and compare hash before do the actual
string comparison. It slowed things down (hopefully because the cost
of hashing, the same one from name-hash.c, not because I did it
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