cache-tree: speed up consecutive path comparisons

The previous change reduced time spent in strlen() while comparing
consecutive paths in verify_cache(), but we can do better. The
conditional checks the existence of a directory separator at the correct
location, but only after doing a string comparison. Swap the order to be
logically equivalent but perform fewer string comparisons.

To test the effect on performance, I used a repository with over three
million paths in the index. I then ran the following command on repeat:

  git -c index.threads=1 commit --amend --allow-empty --no-edit

Here are the measurements over 10 runs after a 5-run warmup:

  Benchmark #1: v2.30.0
    Time (mean ± σ):     854.5 ms ±  18.2 ms
    Range (min … max):   825.0 ms … 892.8 ms

  Benchmark #2: Previous change
    Time (mean ± σ):     833.2 ms ±  10.3 ms
    Range (min … max):   815.8 ms … 849.7 ms

  Benchmark #3: This change
    Time (mean ± σ):     815.5 ms ±  18.1 ms
    Range (min … max):   795.4 ms … 849.5 ms

This change is 2% faster than the previous change and 5% faster than
v2.30.0.

Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

cache-tree.c

@@ -191,8 +191,8 @@ static int verify_cache(struct cache_entry **cache,
 		const char *next_name = next_ce->name;
 		int this_len = ce_namelen(this_ce);
 		if (this_len < ce_namelen(next_ce) &&
-		    strncmp(this_name, next_name, this_len) == 0 &&
-		    next_name[this_len] == '/') {
+		    next_name[this_len] == '/' &&
+		    strncmp(this_name, next_name, this_len) == 0) {
 			if (10 < ++funny) {
 				fprintf(stderr, "...\n");
 				break;