Am 10.10.2016 um 02:00 schrieb Jeff King:
> On Sat, Oct 08, 2016 at 05:38:47PM +0200, René Scharfe wrote:
>
>> Call strbuf_add_unique_abbrev() to add abbreviated hashes to strbufs
>> instead of taking detours through find_unique_abbrev() and its static
>> buffer. This is shorter in most cases and a bit more efficient.
>>
>> The changes here are not easily handled by a semantic patch because
>> they involve removing temporary variables and deconstructing format
>> strings for strbuf_addf().
>
> Yeah, the thing to look for here is whether the sha1 variable holds the
> same value at both times.
>
> These all look OK to me. Mild rambling below.
>
>> diff --git a/merge-recursive.c b/merge-recursive.c
>> index 5200d5c..9041c2f 100644
>> --- a/merge-recursive.c
>> +++ b/merge-recursive.c
>> @@ -202,9 +202,9 @@ static void output_commit_title(struct merge_options *o, struct commit *commit)
>> strbuf_addf(&o->obuf, "virtual %s\n",
>> merge_remote_util(commit)->name);
>> else {
>> - strbuf_addf(&o->obuf, "%s ",
>> - find_unique_abbrev(commit->object.oid.hash,
>> - DEFAULT_ABBREV));
>> + strbuf_add_unique_abbrev(&o->obuf, commit->object.oid.hash,
>> + DEFAULT_ABBREV);
>> + strbuf_addch(&o->obuf, ' ');
>
> I've often wondered whether a big strbuf_addf() is more efficient than
> several strbuf_addstrs. It amortizes the size-checks, certainly, though
> those are probably not very big. It shouldn't matter much for amortizing
> the cost of malloc, as it's very unlikely to have a case where:
>
> strbuf_addf("%s%s", foo, bar);
>
> would require one malloc, but:
>
> strbuf_addstr(foo);
> strbuf_addstr(bar);
>
> would require two (one of the strings would have to be around the same
> size as the ALLOC_GROW() doubling).
>
> So it probably doesn't matter much in practice (not that most of these
> cases are very performance sensitive anyway). Mostly just something I've
> pondered while tweaking strbuf invocations.
Good question. ALLOC_GROW() doesn't double exactly, but indeed the
number of reallocations depends on the size of the added pieces. I
always thought of strbuf_addf() as an expensive function for
convenience, but never timed it.
Numbers vary a bit, but here's what I get for the crude test program
at the end:
$ time t/helper/test-strbuf strbuf_addf 123 123456789012345678901234567890
123123456789012345678901234567890
real 0m0.168s
user 0m0.164s
sys 0m0.000s
$ time t/helper/test-strbuf strbuf_addstr 123 123456789012345678901234567890
123123456789012345678901234567890
real 0m0.141s
user 0m0.140s
sys 0m0.000s
Just a data-point, but it confirms my bias, so I stop here. :)
> Just thinking aloud, I've also wondered if strbuf_addoid() would be
> handy. We already have oid_to_hex_r(). In fact, you could write it
> already as:
>
> strbuf_add_unique_abbrev(sb, oidp->hash, 0);
>
> but that is probably not helping maintainability. ;)
Yes, a function for adding full hashes without using a static
variable is useful for library functions that may end up being
called often or in parallel. I'd call it strbuf_add_hash,
though.
diff --git a/Makefile b/Makefile
index 1aad150..ad5e98c 100644
--- a/Makefile
+++ b/Makefile
@@ -628,6 +628,7 @@ TEST_PROGRAMS_NEED_X += test-scrap-cache-tree
TEST_PROGRAMS_NEED_X += test-sha1
TEST_PROGRAMS_NEED_X += test-sha1-array
TEST_PROGRAMS_NEED_X += test-sigchain
+TEST_PROGRAMS_NEED_X += test-strbuf
TEST_PROGRAMS_NEED_X += test-string-list
TEST_PROGRAMS_NEED_X += test-submodule-config
TEST_PROGRAMS_NEED_X += test-subprocess
diff --git a/t/helper/test-strbuf.c b/t/helper/test-strbuf.c
new file mode 100644
index 0000000..177f3e2
--- /dev/null
+++ b/t/helper/test-strbuf.c
@@ -0,0 +1,25 @@
+#include "cache.h"
+
+int cmd_main(int argc, const char **argv)
+{
+ struct strbuf sb = STRBUF_INIT;
+ int i = 1000000;
+
+ if (argc == 4) {
+ if (!strcmp(argv[1], "strbuf_addf")) {
+ while (i--) {
+ strbuf_release(&sb);
+ strbuf_addf(&sb, "%s%s", argv[2], argv[3]);
+ }
+ }
+ if (!strcmp(argv[1], "strbuf_addstr")) {
+ while (i--) {
+ strbuf_release(&sb);
+ strbuf_addstr(&sb, argv[2]);
+ strbuf_addstr(&sb, argv[3]);
+ }
+ }
+ puts(sb.buf);
+ }
+ return 0;
+}