Re: [PATCH 1/3] add QSORT
- From
René Scharfe <l.s.r@web.de>
- Date
- Oct 1, 2016, 16:19 UTC
- Message-ID
- <83398160-555f-adab-6b1e-3283c533b5ff@web.de>
- In-Reply-To
- <xmqqponmcp07.fsf@gitster.mtv.corp.google.com>
Am 30.09.2016 um 00:36 schrieb Junio C Hamano:
Show 15 quoted lines
> 3. builtin/show-branch.c does this: > > qsort(ref_name + bottom, top - bottom, sizeof(ref_name[0]), > compare_ref_name); > > where ref_name[] is a file-scope global: > > static char *ref_name[MAX_REVS + 1]; > > and top and bottom are plain integers. The sizeof() does not take > the size of *base, so it is understandable that this does not get > automatically converted. > > It seems that some calls to this function _could_ send the same top > and bottom, asking for 0 element array to be sorted, by the way.
It's hard to imagine an implementation of qsort(3) that can't handle zero elements. QSORT's safety feature is that it prevents the compiler from removing NULL checks for the array pointer. E.g. the last two lines in the following example could be optimized away:
qsort(ptr, n, sizeof(*ptr), fn); if (!ptr) do_stuff();
You can see that on https://godbolt.org/g/JwS99b -- an awesome website for exploring compilation results for small snippets, by the way.
This optimization is dangerous when combined with the convention of using a NULL pointer for empty arrays. Diagnosing an affected NULL check is probably quite hard -- it's right there in the code after all and not all compilers remove it.
builtin/show-branch.c never passes NULL, so it's not affected by that hazard. We can (and should, IMHO) still use QSORT there for consistency and convenience, though:
-- >8 -- Subject: [PATCH] show-branch: use QSORT
Shorten the code by using QSORT instead of calling qsort(3) directly, as the former determines the element size automatically and checks if there are at least two elements to sort already.
Signed-off-by: Rene Scharfe <l.s.r@web.de> --- builtin/show-branch.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)
diff --git a/builtin/show-branch.c b/builtin/show-branch.c index 623ca56..974f340 100644 --- a/builtin/show-branch.c +++ b/builtin/show-branch.c @@ -353,8 +353,7 @@ static int compare_ref_name(const void *a_, const void *b_) static void sort_ref_range(int bottom, int top) { - qsort(ref_name + bottom, top - bottom, sizeof(ref_name[0]), - compare_ref_name); + QSORT(ref_name + bottom, top - bottom, compare_ref_name); } static int append_ref(const char *refname, const struct object_id *oid, @@ -540,8 +539,7 @@ static void append_one_rev(const char *av) if (saved_matches == ref_name_cnt && ref_name_cnt < MAX_REVS) error(_("no matching refs with %s"), av); - if (saved_matches + 1 < ref_name_cnt) - sort_ref_range(saved_matches, ref_name_cnt); + sort_ref_range(saved_matches, ref_name_cnt); return; } die("bad sha1 reference %s", av);
-- 2.10.0