[PATCH 7/6] fortran: fix pair_cmp qsort comparator

[Alexander Monakov <amonakov@ispras.ru>]

Hello,

The final tie-breaker in pair_cmp comparator looks strange, it correctly
yields zero for equal expr->symtree-n.sym values, but for unequal values
it produces 0 or 1.  This would be correct for C++ STL-style comparators
that require "less-than" predicate to be computed, but not for C qsort.

The comment before the function seems to confirm that the intent was to
indeed sort in ascending gfc_symbol order, but the code is doing mostly
the opposite.

Make the comparator properly anti-commutative by returning -1 in the last
tie-breaker when appropriate.

Bootstrapped and regtested on x86-64, OK for trunk?

	* interface.c (pair_cmp): Fix gfc_symbol comparison.  Adjust comment.
---
 gcc/fortran/interface.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/gcc/fortran/interface.c b/gcc/fortran/interface.c
index 6fe0647..13e2bdd 100644
--- a/gcc/fortran/interface.c
+++ b/gcc/fortran/interface.c
@@ -3294,7 +3294,7 @@ argpair;
    order:
     - p->a->expr == NULL
     - p->a->expr->expr_type != EXPR_VARIABLE
-    - growing p->a->expr->symbol.  */
+    - by gfc_symbol pointer value (larger first).  */
 
 static int
 pair_cmp (const void *p1, const void *p2)
@@ -3320,6 +3320,8 @@ pair_cmp (const void *p1, const void *p2)
     }
   if (a2->expr->expr_type != EXPR_VARIABLE)
     return 1;
+  if (a1->expr->symtree->n.sym > a2->expr->symtree->n.sym)
+    return -1;
   return a1->expr->symtree->n.sym < a2->expr->symtree->n.sym;
 }
 
-- 
1.8.3.1