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stdlib: Avoid element self-comparisons in qsort

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This improves compatibility with applications which assume that qsort
does not invoke the comparison function with equal pointer arguments.

The newly introduced branches should be predictable, as leading to a
call to the comparison function.  If the prediction fails, we avoid
calling the function.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
This commit is contained in:
Florian Weimer 2023-11-08 15:18:02 +01:00
parent bf033c0072
commit f8cfb6836e
1 changed files with 5 additions and 3 deletions
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8
stdlib/qsort.c
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@ -136,7 +136,7 @@ siftdown (void *base, size_t size, size_t k, size_t n,
if (j < n && cmp (base + (j * size), base + ((j + 1) * size), arg) < 0)
j++;
if (cmp (base + (k * size), base + (j * size), arg) >= 0)
if (j == k || cmp (base + (k * size), base + (j * size), arg) >= 0)
break;
do_swap (base + (size * j), base + (k * size), size, swap_type);
@ -332,10 +332,12 @@ __qsort_r (void *const pbase, size_t total_elems, size_t size,
that this algorithm runs much faster than others. */
do
{
while ((*cmp) ((void *) left_ptr, (void *) mid, arg) < 0)
while (left_ptr != mid
&& (*cmp) ((void *) left_ptr, (void *) mid, arg) < 0)
left_ptr += size;
while ((*cmp) ((void *) mid, (void *) right_ptr, arg) < 0)
while (right_ptr != mid
&& (*cmp) ((void *) mid, (void *) right_ptr, arg) < 0)
right_ptr -= size;
if (left_ptr < right_ptr)