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mmap on OS X

the patch 1f62d8d (34451) introduced mmap on OS X. This caused a
slowdown which I and Bart identified. Then the patch has been recently
reverted with 8934007 (36956), however before that a few optimizations
have been introduced. It turned out that they smoothed out effect of
34451. Starting from 9f8e3e8 (36834) mmap was no longer slowing things
down. I also checked the update of 36834, which is patch 506d592
(36943), and it's the same.

With the patch reverted the situation is as follows:
- for small data there is no change
- for large data reverted version is slower

So the whole thing is like this:
- OS X mmap is slower for smaller data
- recent optimizations smooth this out
- OS X mmap is faster for large data

So by restoring mmap use we have all cases covered and full performance.

Results for big data (max string size 1350000):
*Running [zsh-withmmap]:                  string_test      57625,00
Running [zsh-withoutmmap]:             string_test      85235,24
Running [zsh-first-mmap]:                 string_test     127042,96
Running [zsh-before-first-mmap]:      string_test      75629,60

Results for small data (max string size 225000):
Running [zsh-withmmap]:                  string_test       1496,85
Running [zsh-withoutmmap]:             string_test       1595,89
**Running [zsh-first-mmap]:                 string_test       3790,70
Running [zsh-before-first-mmap]:      string_test       1496,02

It can be seen that for small data the slowdown of ** is smoothed out
in recent Zsh, while for large data mmap in recent Zsh gives speed up

Best regards,
Sebastian Gniazdowski

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