git/stable-qsort.c
René Scharfe b80741e5b9 stable-qsort: avoid using potentially unaligned access
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>
2022-01-07 15:48:40 -08:00

57 lines
986 B
C

#include "git-compat-util.h"
/*
* A merge sort implementation, simplified from the qsort implementation
* by Mike Haertel, which is a part of the GNU C Library.
*/
static void msort_with_tmp(void *b, size_t n, size_t s,
int (*cmp)(const void *, const void *),
char *t)
{
char *tmp;
char *b1, *b2;
size_t n1, n2;
if (n <= 1)
return;
n1 = n / 2;
n2 = n - n1;
b1 = b;
b2 = (char *)b + (n1 * s);
msort_with_tmp(b1, n1, s, cmp, t);
msort_with_tmp(b2, n2, s, cmp, t);
tmp = t;
while (n1 > 0 && n2 > 0) {
if (cmp(b1, b2) <= 0) {
memcpy(tmp, b1, s);
tmp += s;
b1 += s;
--n1;
} else {
memcpy(tmp, b2, s);
tmp += s;
b2 += s;
--n2;
}
}
if (n1 > 0)
memcpy(tmp, b1, n1 * s);
memcpy(b, t, (n - n2) * s);
}
void git_stable_qsort(void *b, size_t n, size_t s,
int (*cmp)(const void *, const void *))
{
const size_t size = st_mult(n, s);
char *tmp;
tmp = xmalloc(size);
msort_with_tmp(b, n, s, cmp, tmp);
free(tmp);
}