Re: [PATCH] Fix maybe-uninitialized warnings found by gcc 9 -flto

[Jeff King <peff@peff.net>]

On Fri, Sep 06, 2019 at 01:10:46AM +0200, Stephan Beyer wrote:

> I took a quick glance at yours. I also noticed the issue you address in
> [PATCH 2/6], but I was unsure if this is the way to go (I'm only
> occasionally reading on this list). I would prefer your patch series,
> with maybe one exception...

Yeah, we've been slowly removing these old "unsigned char *" references
(see a7db4c193d98f for the latest round touching this same code -- I'm
actually surprised I missed this one back then, as that's when
packlist_find() got converted).

> The thing is: I had *exactly* the same commit like your [PATCH 6/6]
> (except for the commit message and for the number), but I dropped it.
> Why? Because I had the feeling (no particular instance though) that the
> second locate_object_entry_hash() for each insertion *can* indeed take
> "too much" time. Also, I was wondering, if the "found = 1" case should
> be catched as a BUG("should not happen") or something.
> 
> I don't care much, though. The performance impact should probably be
> checked carefully.

I did measure it, like this:

  # Do the traversal separately. This would make any difference in the
  # pack-objects hash code stand out _more_, plus it makes it cheaper to
  # run multiple trials.
  git rev-list --objects --all >input

  # Make sure stderr is redirected to avoid progress, which again would
  # amplify any differences.
  git pack-objects --stdout --delta-base-offset <input >/dev/null 2>&1

Running on linux.git, I got:

  [before]
  Benchmark #1: git pack-objects --stdout --delta-base-offset <input >/dev/null 2>&1
    Time (mean ± σ):     26.225 s ±  0.233 s    [User: 24.089 s, System: 4.867 s]
    Range (min … max):   25.915 s … 26.555 s    10 runs

  [after]
  Benchmark #1: git pack-objects --stdout --delta-base-offset <input >/dev/null 2>&1
    Time (mean ± σ):     26.129 s ±  0.170 s    [User: 24.003 s, System: 4.958 s]
    Range (min … max):   25.974 s … 26.570 s    10 runs

So actually faster after, though not statistically significant. ;)

The BUG() on "found==1" might be worth doing.

-Peff