[Bug c/95378] __atomic_load will write to objects of cv-qualified types

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95378

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Every __atomic_xxx built-in has the same problem. They'll all accept
cv-qualified types as output parameters.

This seems to fix it, but I'll finish testing it and submit it tomorrow:

--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -6903,6 +6903,8 @@ get_atomic_generic_size (location_t loc, tree function,
 {
   unsigned int n_param;
   unsigned int n_model;
+  unsigned int n_first_output = 0;
+  unsigned int n_last_output = 0;
   unsigned int x;
   int size_0;
   tree type_0;
@@ -6915,11 +6917,14 @@ get_atomic_generic_size (location_t loc, tree function,
       n_model = 1;
       break;
     case BUILT_IN_ATOMIC_LOAD:
+      n_first_output = n_last_output = 1;
+      /* fallthrough  */
     case BUILT_IN_ATOMIC_STORE:
       n_param = 3;
       n_model = 1;
       break;
     case BUILT_IN_ATOMIC_COMPARE_EXCHANGE:
+      n_last_output = 1;
       n_param = 6;
       n_model = 2;
       break;
@@ -7010,6 +7015,23 @@ get_atomic_generic_size (location_t loc, tree function,
                    function);
          return 0;
        }
+
+      if (x >= n_first_output && x <= n_last_output)
+       {
+         tree type = TREE_TYPE (TREE_TYPE ((*params)[x]));
+         if (TYPE_QUALS (type) & (TYPE_QUAL_CONST|TYPE_QUAL_VOLATILE))
+           {
+             if (c_dialect_cxx ())
+               {
+                 error_at (loc, "argument %d of %qE must not be a pointer to "
+                           "a cv-qualified type", x + 1, function);
+                 return 0;
+               }
+             else
+               pedwarn (loc, OPT_Wincompatible_pointer_types, "argument %d "
+                        "of %qE discards qualifiers", x + 1, function);
+           }
+       }
     }

   /* Check memory model parameters for validity.  */