[PATCH] wc: Add AVX2 optimization when counting only lines

[Kristoffer Brånemyr]

Hi,
I wanted to practice some more using vector intrinsics, so I made a small AVX2 
optimization for wc -l. Depending on line length it is about 2-5x faster than 
previous version. (Well, only looking at user time it is much faster than that 
even.)

I put the patch at https://github.com/coreutils/coreutils/pull/50 . Maybe this 
is a pointless optimization, I guess not many people run wc -l on gigabytes of 
data, but maybe it could be useful for someone...

As an aside, I think that .gitignore should be updated to include 
src/libcksum_pclmul.a , and if patch is accepted also the libwc_avx2.a I added 
in this patch.

Some informal benchmark results with big files already in filecache:
-----

An HTML formatted e-book, concatenated many times. Most lines around 80 chars, 
with some shorter.
avx2 (1.98x faster)

38256750 /disk2/download/storfil4

real    0m0,292s
user    0m0,040s
sys     0m0,252s

normal wc
38256750 /disk2/download/storfil4

real    0m0,580s
user    0m0,346s
sys     0m0,234s

-------

A big file with only \n character in it.
avx2 (4.9x faster)
1328545792 /disk2/download/storfil6_bara_radbryt

real    0m0,160s
user    0m0,012s
sys     0m0,148s


normal wc
1328545792 /disk2/download/storfil6_bara_radbryt

real    0m0,768s
user    0m0,626s
sys     0m0,142s

----

A big file with no \n at all.
avx2 (I think they are basically equally fast, since running several times it 
varied who was faster)
0 /disk2/download/storfil7_inga_radbryt

real    0m0,277s
user    0m0,035s
sys     0m0,242s

normal wc
0 /disk2/download/storfil7_inga_radbryt

real    0m0,269s
user    0m0,039s
sys     0m0,230s


-- /Kristoffer Brånemyr