Re: [PATCH 1/5] Fix asymmetric comparison functions

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Dec 17, 2015 at 11:58:30AM +0300, Yury Gribov wrote:
> 2015-12-17  Yury Gribov  <tetra2005@gmail.com>
> 
> 	* c-family/c-common.c (resort_field_decl_cmp):
> 	Make symmteric.
> 	* cp/class.c (method_name_cmp): Ditto.
> 	(resort_method_name_cmp): Ditto.
> 	* fortran/interface.c (pair_cmp): Ditto.

Note, c-family, cp and fortran have their own ChangeLog files, so
the entries without those prefixes need to go into each one and can't
refer to other ChangeLog through Ditto/Likewise etc.
Typo in symmteric.

That said, is this actually really a problem?  I mean, is qsort
allowed to call the comparison function with the same arguments?
I think lots of the comparison functions just assume that
for int cmpfn (const void *x, const void *y) x != y.
And if qsort can't call the comparison function with the same argument,
then perhaps the caller has some knowledge your checker does not, say
that the entries that would compare equal by the comparison function
simply can't appear in the array (so the caller knows that the comparison
function should never return 0).

> --- a/gcc/tree-vrp.c
> +++ b/gcc/tree-vrp.c
> @@ -5882,7 +5882,9 @@ compare_case_labels (const void *p1, const void *p2)
>    else if (idx1 == idx2)
>      {
>        /* Make sure the default label is first in a group.  */
> -      if (!CASE_LOW (ci1->expr))
> +      if (!CASE_LOW (ci1->expr) && !CASE_LOW (ci2->expr))
> +	return 0;
> +      else if (!CASE_LOW (ci1->expr))
>  	return -1;
>        else if (!CASE_LOW (ci2->expr))
>  	return 1;
> -- 
> 1.9.1

Say here, we know there is at most one default label in a switch, never
more.  So, unless qsort is allowed to call compare_case_labels
with p1 == p2 (which really doesn't make sense), this case just won't
happen.

	Jakub