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stable-qsort: avoid using potentially unaligned access

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Like in the previous patch for compat/qsort_s.c, remove the optimization
of using an on-stack buffer to avoid small allocations.  This ensures
maximum alignment for the array elements and simplifies the code a bit.

The performance impact for the current callers is unlikely to be
noticeable:

 * compat/mingw.c::make_environment_block() uses ALLOC_ARRAY and
   ALLOC_GROW several times already, so another allocation of up to 1KB
   should not matter much.

 * diffcore-rename.c::diffcore_rename_extended() is called once per diff
   or twice per merge, and those require allocations for each object and
   more already.

 * merge-ort.c::detect_and_process_renames() is called once per merge.
   It's responsible for the two per-merge diffcore_rename_extended()
   calls mentioned above as well, though.  So this is possibly the most
   impacted caller.  Per-object allocations are likely to dwarf the
   additional small allocations in git_stable_qsort(), though.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
René Scharfe 2022-01-08 00:30:44 +01:00 committed by Junio C Hamano
parent 9bfa5fbae2
commit b80741e5b9
1 changed files with 4 additions and 10 deletions
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10
stable-qsort.c
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@ -48,15 +48,9 @@ void git_stable_qsort(void *b, size_t n, size_t s,
int (*cmp)(const void *, const void *)) int (*cmp)(const void *, const void *))
{ {
const size_t size = st_mult(n, s); const size_t size = st_mult(n, s);
char buf[1024]; char *tmp;
if (size < sizeof(buf)) { tmp = xmalloc(size);
/* The temporary array fits on the small on-stack buffer. */
msort_with_tmp(b, n, s, cmp, buf);
} else {
/* It's somewhat large, so malloc it. */
char *tmp = xmalloc(size);
msort_with_tmp(b, n, s, cmp, tmp); msort_with_tmp(b, n, s, cmp, tmp);
free(tmp); free(tmp);
} }
}