Re: [PATCH v2] cat-file: skip expanding default format

[Taylor Blau <me@ttaylorr.com>]

On Tue, Mar 08, 2022 at 02:54:23AM +0000, John Cai via GitGitGadget wrote:
>  /*
>   * If "pack" is non-NULL, then "offset" is the byte offset within the pack from
>   * which the object may be accessed (though note that we may also rely on
> @@ -363,6 +371,11 @@ static void batch_object_write(const char *obj_name,
>  			       struct packed_git *pack,
>  			       off_t offset)
>  {
> +	struct strbuf type_name = STRBUF_INIT;
> +
> +	if (!opt->format)
> +		data->info.type_name = &type_name;

Hmmm, I'm not quite sure I understand why the extra string buffer is
necessary here. When we do the first expansion with the DEFAULT_FORMAT,
we set data->info.typep to point at data->type.

So by the time we do our actual query (either with
`packed_object_info()` or just `oid_object_info_extended()`), we will
get the type filled into data->type.

And we should be able to use that in print_default_format(), which saves
us in a couple of ways:

  - We don't have to (stack) allocate a string buffer, which then needs
    to be cleaned up after printing each object.

  - (More importantly) we can avoid the extra string copy through the
    intermediate buffer by using type_name() directly.

Here's a small patch on top that you could apply, if you're interested:

--- >8 ---


diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index ab9a49e13a..9f55afa75b 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -355,5 +355,5 @@ static int print_default_format(char *buf, int len, struct expand_data *data)
 {
 	return xsnprintf(buf, len, "%s %s %"PRIuMAX"\n", oid_to_hex(&data->oid),
-		 data->info.type_name->buf,
+			 type_name(data->type),
 		 (uintmax_t)*data->info.sizep);

@@ -372,9 +372,4 @@ static void batch_object_write(const char *obj_name,
 			       off_t offset)
 {
-	struct strbuf type_name = STRBUF_INIT;
-
-	if (!opt->format)
-		data->info.type_name = &type_name;
-
 	if (!data->skip_object_info) {
 		int ret;
@@ -391,5 +386,5 @@ static void batch_object_write(const char *obj_name,
 			       obj_name ? obj_name : oid_to_hex(&data->oid));
 			fflush(stdout);
-			goto cleanup;
+			return;
 		}
 	}
@@ -410,7 +405,4 @@ static void batch_object_write(const char *obj_name,
 		batch_write(opt, "\n", 1);
 	}
-
-cleanup:
-	strbuf_release(&type_name);
 }

--- 8< ---

On my copy of git.git., it shaves off around ~7ms that we're spending
just copying type names back and forth.

    (without the above)
    Benchmark 1: git.compile cat-file --batch-check --batch-all-objects
      Time (mean ± σ):     578.7 ms ±  12.7 ms    [User: 553.4 ms, System: 25.2 ms]
      Range (min … max):   568.1 ms … 600.1 ms    10 runs

    (with the above)
    Benchmark 1: git.compile cat-file --batch-check --batch-all-objects
      Time (mean ± σ):     571.5 ms ±   7.9 ms    [User: 544.0 ms, System: 27.3 ms]
      Range (min … max):   558.8 ms … 583.2 ms    10 runs

Thanks,
Taylor