[glibc] stdlib: Avoid element self-comparisons in qsort

[Florian Weimer <fw@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=f8cfb6836e8d91bb789b2e7fd65338d6f5bd459c

commit f8cfb6836e8d91bb789b2e7fd65338d6f5bd459c
Author: Florian Weimer <fweimer@redhat.com>
Date:   Wed Nov 8 15:18:02 2023 +0100

    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>

Diff:
---
 stdlib/qsort.c | 8 +++++---
 1 file 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)