[MERGE] Accelerate merge by skipping file existence check when merging execute bit

[Aaron Bentley]

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John Arbash Meinel wrote:
> Aaron Bentley wrote:
> | Hi all,
> |
> | This patch speeds up merge by avoiding unnecessary file-existence tests.

> Do you have any numbers to back this up?

I'm merging two branches of launchpad for my test, because it's a big
tree.  I'm running merge --force, but it is still finding a common base,
etc.  They are best of 4 (I use best of 4 because the first run is
almost always very slow).:

Current:
real    0m7.146s
user    0m6.476s
sys     0m0.520s

Patched:
real    0m4.768s
user    0m4.320s
sys     0m0.376s

So 1.50x faster.

> Is it only for files involved in the merge? (So if there were 10 changed
> files
> in a tree of 10,000 this would only skip the existence check on 9 files
> if the
> 10th had an exec bit changed.)

No, I believe it is for all files in the tree.

Aaron
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