Re: Make GIT_USE_LOOKUP default?

Jeff King [off-list ref] writes:

I do not see anything obviously wrong in it, though I did not walk
through all of the ofs calculation to look for any clever speedups.
However, I think it is clear from the other timings and Ingo's thread
that glibc 2.11's memcmp does not perform very well on many short reads.
And sha1_entry_pos will do memcmps even smaller than 20 bytes.

What happens if you do this?

The overall trend is the same.

[without GIT_USE_LOOKUP]
real    0m40.044s
real    0m40.054s
real    0m40.072s
real    0m40.097s
real    0m40.159s

[with GIT_USE_LOOKUP]
real    0m40.257s
real    0m40.281s
real    0m40.311s
real    0m40.366s
real    0m40.407s

I suspect that after the first few rounds the range shrinks small
enough that the difference between a simple "mi = (hi + lo)/2" and
convoluted ofs computation becomes dominant.  Perhaps we should only
do N-R for the initial midpoint selection and then fall back to a
stupid binary search?