stdlib: Avoid element self-comparisons in qsort

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>

1 files changed, 5 insertions, 3 deletions

diff --git a/stdlib/qsort.c b/stdlib/qsort.c
index fd32a165e7..ad110e8a89 100644
--- a/stdlib/qsort.c
+++ b/stdlib/qsort.c
@@ -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)