Nguyễn Thái Ngọc Duy [off-list ref] 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?
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.
On Fri, Mar 08, 2013 at 11:50:04PM -0800, Junio C Hamano wrote:
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 did an RFC-patch for that (that I mistakenly didn't sent as a reply to
this e-mail). And I believe that you're correct. My solution is inspired
of curl's strequal.
Is the reason for git not to care about lower/upper-case for beeing able
to support windows? Or is there any other smart reason?
I was also thinking about discarding files by looking at their
modification date. If the modification timestamp is older than/or equal to
the latest commit, there's probably no reason for examine that file any
further. I'm not sure about the side effects this may imply though. I
think they can be quite nasty. Is this something worth digging more in
or am I already on the wrong path?
--
Med vänliga hälsningar
Fredrik Gustafsson
tel: 0733-608274
e-post: iveqy@iveqy.com
On Sat, Mar 9, 2013 at 2:50 PM, Junio C Hamano [off-list ref] wrote:
Nguyễn Thái Ngọc Duy [off-list ref] writes:
quoted
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
wrong).
--
Duy