[PATCH] unpack-trees: correctly compute result count

The clear_ce_flags_dir() method processes the cache entries within a common directory. The returned int is the number of cache entries processed by that directory. When using the sparse-checkout feature in cone mode, we can skip the pattern matching for entries in the directories that are entirely included or entirely excluded.

eb42feca (unpack-trees: hash less in cone mode, 2019-11-21) introduced this performance feature. The old mechanism relied on the counts returned by calling clear_ce_flags_1(), but the new mechanism calculated the number of rows by subtracting "cache_end" from "cache" to find the size of the range. However, the equation is wrong because it divides by sizeof(struct cache_entry *). This is not how pointer arithmetic works!

A coverity build of Git for Windows in preparation for the 2.25.0 release found this issue with the warning, "Pointer differences, such as cache_end - cache, are automatically scaled down by the size (8 bytes) of the pointed-to type (struct cache_entry *). Most likely, the division by sizeof(struct cache_entry *) is extraneous and should be eliminated." This warning is correct.

This leaves us with the question "how did this even work?" The problem that occurs with this incorrect pointer arithmetic is a performance-only bug, and a very slight one at that. Since the entry count returned by clear_ce_flags_dir() is reduced by a factor of 8, the loop in clear_ce_flags_1() will re-process entries from those directories.

By inserting global counters into unpack-tree.c and tracing them with trace2_data_intmax() (in a private change, for testing), I was able to see count how many times the loop inside clear_ce_flags_1() processed an entry and how many times clear_ce_flags_dir() was called. Each of these are reduced by at least a factor of 8 with the current change. A factor larger than 8 happens when multiple levels of directories are repeated.

Specifically, in the Linux kernel repo, the command

	git sparse-checkout set LICENSES

restricts the working directory to only the files at root and in the LICENSES directory. Here are the measured counts:

clear_ce_flags_1 loop blocks:
	Before: 11,520
	After:   1,621

clear_ce_flags_dir calls:
	Before: 7,048
	After:    606

While these are dramatic counts, the time spent in clear_ce_flags_1() is under one millisecond in each case, so the improvement is not measurable as an end-to-end time.

Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
    unpack-trees: correctly compute result count
    
    Here is a very small fix to the cone-mode pattern-matching in
    unpack-trees.c. Johannes found this while running a Coverity scan for
    other issues. He alerted me to the problem and I could immediately see 
    my error here. In creating this patch, most of my time was spent asking
    "how did this work before?" and "why didn't this hurt performance?"
    Hopefully my commit message explains this thoroughly enough.
    
    As for making it into the release, I don't know. The change is small, it
    has a very limited scope, but this flaw is also not really hurting
    anything in a major way.
    
    Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-520%2Fderrickstolee%2Funpack-trees-division-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-520/derrickstolee/unpack-trees-division-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/520

 unpack-trees.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)