On Tue, Jun 17, 2014 at 01:08:05PM +0200, Torsten Bögershausen wrote:
> On 2014-06-17 09.34, Jeremiah Mahler wrote:
> > Add a strnncmp() function which behaves like strncmp() except it takes
> > the length of both strings instead of just one.
> >
> > Then simplify tree-walk.c and unpack-trees.c using this new function.
> > Replace all occurrences of name_compare() with strnncmp(). Remove
> > name_compare(), which they both had identical copies of.
> >
> > Version 2 includes suggestions from Jonathan Neider [1]:
> >
> > - Fix the logic which caused the new strnncmp() to behave differently
> > from the old version. Now it is identical to strncmp().
> >
> > - Improve description of strnncmp().
> >
> > Also, strnncmp() was switched from using memcmp() to strncmp()
> > internally to make it clear that this is meant for strings, not
> > general buffers.
> I don't think this is a good change, for 2 reasons:
> - It changes the semantics of existing code, which should be carefully
> reviewed, documented and may be put into a seperate commit.
> - Looking into the code for memcmp() and strncmp() in libc,
> I can see that memcmp() is written in 13 lines of assembler,
> (on a 386 system) with a fast
> repz cmpsb %es:(%edi),%ds:(%esi)
> working as the core engine.
>
> strncmp() uses 83 lines of assembler, because after each comparison
> the code needs to check of the '\0' in both strings.
> - I can't see a reason to replace efficient code with less efficient code,
> so moving the old function "as is" into a include file, and declare
> it "static inline" could be the first step.
>
That is not true, a rep cmpsb was fast for 486 but is relatively slow
for newer processors. For performance a correct answer is to measure it than do
blind guess. Are these strings null terminated or is giving a size just
a hint? If it is a hint then a plain strcmp could be faster (this
depends on implementation). A reason is that for implementations that
check more bytes at once it is easier to combine a terminating null mask with
difference than trying to first find which of first 16 bytes are different and
then compare if it is within size.
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