[PATCH v2 0/5] Optimization batch 12: miscellaneous unthemed stuff

This series depends on en/ort-perf-batch-11 textually, but is semantically independent of it.

Changes since v1 (all for the first patch):

 * Add more comments explaining the sorting function, its purpose, and how
   its expected to be called
 * Small style fixup
 * Switch back to using string_list_sort() instead of direct QSORT()

This short series has a few optimizations, but only one of which affects the testcases of interest (namely, reducing our time spent on sorting an array). It also fixes a few comments.

For the testcases mentioned in commit 557ac0350d ("merge-ort: begin performance work; instrument with trace2_region_* calls", 2020-10-28), the changes in just this series improves the performance as follows:

                     Before Series           After Series
no-renames:        5.622 s ±  0.059 s     5.235 s ±  0.042 s
mega-renames:     10.127 s ±  0.073 s     9.419 s ±  0.107 s
just-one-mega:   500.3  ms ±  3.8  ms   480.1  ms ±  3.9  ms

As a reminder, before any merge-ort/diffcore-rename performance work, the performance results we started with were:

no-renames-am: 6.940 s ± 0.485 s no-renames: 18.912 s ± 0.174 s mega-renames: 5964.031 s ± 10.459 s just-one-mega: 149.583 s ± 0.751 s

Elijah Newren (5):
  merge-ort: replace string_list_df_name_compare with faster alternative
  diffcore-rename: avoid unnecessary strdup'ing in break_idx
  diffcore-rename: enable limiting rename detection to relevant
    destinations
  Fix various issues found in comments
  merge-ort: miscellaneous touch-ups

 diffcore-rename.c                   | 52 ++++++++++++++---
 diffcore.h                          |  2 +
 merge-ort.c                         | 86 +++++++++++++++++++++--------
 t/t6423-merge-rename-directories.sh |  2 +-
 4 files changed, 110 insertions(+), 32 deletions(-)

base-commit: 76e253793c9a1d7fdd1836d5e4db26dabd3d713a
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-962%2Fnewren%2Fort-perf-batch-12-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-962/newren/ort-perf-batch-12-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/962

Range-diff vs v1:

 1:  5055dfce3281 ! 1:  c4a0f6a9510c merge-ort: replace string_list_df_name_compare with faster alternative
     @@ Commit message
          "-sort" to either rebase or merge, sorting an array takes more time than
          anything else.  Serves me right for naming my merge strategy this way.
      
     -    Rewrite the comparison function and remove as many levels of indirection
     -    as possible (e.g. the old code had
     -        cmp_items() ->
     -          string_list_df_name_compare() ->
     -            df_name_compare()
     -    now we just have sort_dirs_next_to_their_children()), and tweak it to be
     -    as optimized as possible for our specific case.  These changes reduced
     -    the time spent in "plist special sort" by ~25% in the mega-renames case.
     +    Rewrite the comparison function in a way that does not require finding
     +    out the lengths of the strings when comparing them.  While at it, tweak
     +    the code for our specific case -- no need to handle a variety of modes,
     +    for example.  The combination of these changes reduced the time spent in
     +    "plist special sort" by ~25% in the mega-renames case.
      
          For the testcases mentioned in commit 557ac0350d ("merge-ort: begin
          performance work; instrument with trace2_region_* calls", 2020-10-28),
     @@ merge-ort.c: static int detect_and_process_renames(struct merge_options *opt,
       /*** Function Grouping: functions related to process_entries() ***/
       
      -static int string_list_df_name_compare(const char *one, const char *two)
     -+static int sort_dirs_next_to_their_children(const void *a, const void *b)
     ++static int sort_dirs_next_to_their_children(const char *one, const char *two)
       {
      -	int onelen = strlen(one);
      -	int twolen = strlen(two);
     ++	unsigned char c1, c2;
     ++
       	/*
      -	 * Here we only care that entries for D/F conflicts are
      -	 * adjacent, in particular with the file of the D/F conflict
      -	 * appearing before files below the corresponding directory.
      -	 * The order of the rest of the list is irrelevant for us.
      +	 * Here we only care that entries for directories appear adjacent
     -+	 * to and before files underneath the directory.  In other words,
     -+	 * we do not want the natural sorting of
     ++	 * to and before files underneath the directory.  We can achieve
     ++	 * that by pretending to add a trailing slash to every file and
     ++	 * then sorting.  In other words, we do not want the natural
     ++	 * sorting of
      +	 *     foo
      +	 *     foo.txt
      +	 *     foo/bar
     @@ merge-ort.c: static int detect_and_process_renames(struct merge_options *opt,
      +	 * To achieve this, we basically implement our own strcmp, except that
      +	 * if we get to the end of either string instead of comparing NUL to
      +	 * another character, we compare '/' to it.
     ++	 *
     ++	 * If this unusual "sort as though '/' were appended" perplexes
     ++	 * you, perhaps it will help to note that this is not the final
     ++	 * sort.  write_tree() will sort again without the trailing slash
     ++	 * magic, but just on paths immediately under a given tree.
       	 *
      -	 * To achieve this, we sort with df_name_compare and provide
      -	 * the mode S_IFDIR so that D/F conflicts will sort correctly.
     @@ merge-ort.c: static int detect_and_process_renames(struct merge_options *opt,
      -	 * since in other cases any changes in their order due to
      -	 * sorting cause no problems for us.
      +	 * The reason to not use df_name_compare directly was that it was
     -+	 * just too expensive, so I had to reimplement it.
     ++	 * just too expensive (we don't have the string lengths handy), so
     ++	 * I had to reimplement it.
       	 */
      -	int cmp = df_name_compare(one, onelen, S_IFDIR,
      -				  two, twolen, S_IFDIR);
     --	/*
     ++
     + 	/*
      -	 * Now that 'foo' and 'foo/bar' compare equal, we have to make sure
      -	 * that 'foo' comes before 'foo/bar'.
     --	 */
     ++	 * NOTE: This function will never be called with two equal strings,
     ++	 * because it is used to sort the keys of a strmap, and strmaps have
     ++	 * unique keys by construction.  That simplifies our c1==c2 handling
     ++	 * below.
     + 	 */
      -	if (cmp)
      -		return cmp;
      -	return onelen - twolen;
     -+	const char *one = ((struct string_list_item *)a)->string;
     -+	const char *two = ((struct string_list_item *)b)->string;
     -+	unsigned char c1, c2;
      +
      +	while (*one && (*one == *two)) {
      +		one++;
     @@ merge-ort.c: static int detect_and_process_renames(struct merge_options *opt,
      +	if (c1 == c2) {
      +		/* Getting here means one is a leading directory of the other */
      +		return (*one) ? 1 : -1;
     -+	}
     -+	else
     ++	} else
      +		return c1-c2;
       }
       
     @@ merge-ort.c: static void process_entries(struct merge_options *opt,
       
       	trace2_region_enter("merge", "plist special sort", opt->repo);
      -	plist.cmp = string_list_df_name_compare;
     --	string_list_sort(&plist);
     -+	QSORT(plist.items, plist.nr, sort_dirs_next_to_their_children);
     ++	plist.cmp = sort_dirs_next_to_their_children;
     + 	string_list_sort(&plist);
       	trace2_region_leave("merge", "plist special sort", opt->repo);
       
     - 	trace2_region_leave("merge", "process_entries setup", opt->repo);
 2:  7212816c8d47 = 2:  38713ed48273 diffcore-rename: avoid unnecessary strdup'ing in break_idx
 3:  19150b575058 = 3:  45e1de5fe780 diffcore-rename: enable limiting rename detection to relevant destinations
 4:  98c9a419b313 = 4:  2f26d7e935c0 Fix various issues found in comments
 5:  e85dad887f95 = 5:  7156f26ab299 merge-ort: miscellaneous touch-ups