pretty: lazy-load commit data when expanding user-format

When we expand a user-format, we try to avoid work that isn't necessary
for the output. For instance, we don't bother parsing the commit header
until we know we need the author, subject, etc.

But we do always load the commit object's contents from disk, even if
the format doesn't require it (e.g., just "%H"). Traditionally this
didn't matter much, because we'd have loaded it as part of the traversal
anyway, and we'd typically have those bytes attached to the commit
struct (or these days, cached in a commit-slab).

But when we have a commit-graph, we might easily get to the point of
pretty-printing a commit without ever having looked at the actual object
contents. We should push off that load (and reencoding) until we're
certain that it's needed.

I think the results of p4205 show the advantage pretty clearly (we serve
parent and tree oids out of the commit struct itself, so they benefit as
well):

  # using git.git as the test repo
  Test                          HEAD^             HEAD
  ----------------------------------------------------------------------
  4205.1: log with %H           0.40(0.39+0.01)   0.03(0.02+0.01) -92.5%
  4205.2: log with %h           0.45(0.44+0.01)   0.09(0.09+0.00) -80.0%
  4205.3: log with %T           0.40(0.39+0.00)   0.04(0.04+0.00) -90.0%
  4205.4: log with %t           0.46(0.46+0.00)   0.09(0.08+0.01) -80.4%
  4205.5: log with %P           0.39(0.39+0.00)   0.03(0.03+0.00) -92.3%
  4205.6: log with %p           0.46(0.46+0.00)   0.10(0.09+0.00) -78.3%
  4205.7: log with %h-%h-%h     0.52(0.51+0.01)   0.15(0.14+0.00) -71.2%
  4205.8: log with %an-%ae-%s   0.42(0.41+0.00)   0.42(0.41+0.01) +0.0%

  # using linux.git as the test repo
  Test                          HEAD^             HEAD
  ----------------------------------------------------------------------
  4205.1: log with %H           7.12(6.97+0.14)   0.76(0.65+0.11) -89.3%
  4205.2: log with %h           7.35(7.19+0.16)   1.30(1.19+0.11) -82.3%
  4205.3: log with %T           7.58(7.42+0.15)   1.02(0.94+0.08) -86.5%
  4205.4: log with %t           8.05(7.89+0.15)   1.55(1.41+0.13) -80.7%
  4205.5: log with %P           7.12(7.01+0.10)   0.76(0.69+0.07) -89.3%
  4205.6: log with %p           7.38(7.27+0.10)   1.32(1.20+0.12) -82.1%
  4205.7: log with %h-%h-%h     7.81(7.67+0.13)   1.79(1.67+0.12) -77.1%
  4205.8: log with %an-%ae-%s   7.90(7.74+0.15)   7.81(7.66+0.15) -1.1%

I added the final test to show where we don't improve (the 1% there is
just lucky noise), but also as a regression test to make sure we're not
doing anything stupid like loading the commit multiple times when there
are several placeholders that need it.

Reported-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 13 insertions, 12 deletions

diff --git a/pretty.c b/pretty.c
index 3922f6f9f2..b4ff3f602f 100644
--- a/pretty.c
+++ b/pretty.c
@@ -783,6 +783,7 @@ enum trunc_type {
 };
 
 struct format_commit_context {
+	struct repository *repository;
 	const struct commit *commit;
 	const struct pretty_print_context *pretty_ctx;
 	unsigned commit_header_parsed:1;
@@ -1373,10 +1374,13 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		return 2;
 	}
 
-
 	/* For the rest we have to parse the commit header. */
-	if (!c->commit_header_parsed)
+	if (!c->commit_header_parsed) {
+		msg = c->message =
+			repo_logmsg_reencode(c->repository, commit,
+					     &c->commit_encoding, "UTF-8");
 		parse_commit_header(c);
+	}
 
 	switch (placeholder[0]) {
 	case 'a':	/* author ... */
@@ -1667,6 +1671,7 @@ void repo_format_commit_message(struct repository *r,
 				const struct pretty_print_context *pretty_ctx)
 {
 	struct format_commit_context context = {
+		.repository = r,
 		.commit = commit,
 		.pretty_ctx = pretty_ctx,
 		.wrap_start = sb->len
@@ -1674,18 +1679,14 @@ void repo_format_commit_message(struct repository *r,
 	const char *output_enc = pretty_ctx->output_encoding;
 	const char *utf8 = "UTF-8";
 
-	/*
-	 * convert a commit message to UTF-8 first
-	 * as far as 'format_commit_item' assumes it in UTF-8
-	 */
-	context.message = repo_logmsg_reencode(r, commit,
-					       &context.commit_encoding,
-					       utf8);
-
 	strbuf_expand(sb, format, format_commit_item, &context);
 	rewrap_message_tail(sb, &context, 0, 0, 0);
 
-	/* then convert a commit message to an actual output encoding */
+	/*
+	 * Convert output to an actual output encoding; note that
+	 * format_commit_item() will always use UTF-8, so we don't
+	 * have to bother if that's what the output wants.
+	 */
 	if (output_enc) {
 		if (same_encoding(utf8, output_enc))
 			output_enc = NULL;
diff --git a/t/perf/p4205-log-pretty-formats.sh b/t/perf/p4205-log-pretty-formats.sh
index 7c26f4f337..609fecd65d 100755
--- a/t/perf/p4205-log-pretty-formats.sh
+++ b/t/perf/p4205-log-pretty-formats.sh
@@ -6,7 +6,7 @@ test_description='Tests the performance of various pretty format placeholders'
 
 test_perf_default_repo
 
-for format in %H %h %T %t %P %p %h-%h-%h
+for format in %H %h %T %t %P %p %h-%h-%h %an-%ae-%s
 do
 	test_perf "log with $format" "
 		git log --format=\"$format\" >/dev/null