fortran: Assume there is no cyclic reference with submodule symbols [PR99798]

This prevents a premature release of memory with procedure symbols from
submodules, causing random compiler crashes.

The problem is a fragile detection of cyclic references, which can match
with procedures host-associated from a module in submodules, in cases where it
shouldn't.  The formal namespace is released, and with it the dummy arguments
symbols of the procedure.  But there is no cyclic reference, so the procedure
symbol itself is not released and remains, with pointers to its dummy arguments
now dangling.

The fix adds a condition to avoid the case, and refactors to a new predicate
by the way.  Part of the original condition is also removed, for lack of a
reason to keep it.

	PR fortran/99798

gcc/fortran/ChangeLog:

	* symbol.cc (gfc_release_symbol): Move the condition guarding
	the handling cyclic references...
	(cyclic_reference_break_needed): ... here as a new predicate.
	Remove superfluous parts.  Add a condition preventing any premature
	release with submodule symbols.

gcc/testsuite/ChangeLog:

	* gfortran.dg/submodule_33.f08: New test.

(cherry picked from commit 38d1761c0c94b77a081ccc180d6e039f7a670468)

gcc/fortran/symbol.cc

@@ -3179,6 +3179,57 @@ gfc_free_symbol (gfc_symbol *&sym)
 }
 
 
+/* Returns true if the symbol SYM has, through its FORMAL_NS field, a reference
+   to itself which should be eliminated for the symbol memory to be released
+   via normal reference counting.
+
+   The implementation is crucial as it controls the proper release of symbols,
+   especially (contained) procedure symbols, which can represent a lot of memory
+   through the namespace of their body.
+
+   We try to avoid freeing too much memory (causing dangling pointers), to not
+   leak too much (wasting memory), and to avoid expensive walks of the symbol
+   tree (which would be the correct way to check for a cycle).  */
+
+bool
+cyclic_reference_break_needed (gfc_symbol *sym)
+{
+  /* Normal symbols don't reference themselves.  */
+  if (sym->formal_ns == nullptr)
+    return false;
+
+  /* Procedures at the root of the file do have a self reference, but they don't
+     have a reference in a parent namespace preventing the release of the
+     procedure namespace, so they can use the normal reference counting.  */
+  if (sym->formal_ns == sym->ns)
+    return false;
+
+  /* If sym->refs == 1, we can use normal reference counting.  If sym->refs > 2,
+     the symbol won't be freed anyway, with or without cyclic reference.  */
+  if (sym->refs != 2)
+    return false;
+
+  /* Procedure symbols host-associated from a module in submodules are special,
+     because the namespace of the procedure block in the submodule is different
+     from the FORMAL_NS namespace generated by host-association.  So there are
+     two different namespaces representing the same procedure namespace.  As
+     FORMAL_NS comes from host-association, which only imports symbols visible
+     from the outside (dummy arguments basically), we can assume there is no
+     self reference through FORMAL_NS in that case.  */
+  if (sym->attr.host_assoc && sym->attr.used_in_submodule)
+    return false;
+
+  /* We can assume that contained procedures have cyclic references, because
+     the symbol of the procedure itself is accessible in the procedure body
+     namespace.  So we assume that symbols with a formal namespace different
+     from the declaration namespace and two references, one of which is about
+     to be removed, are procedures with just the self reference left.  At this
+     point, the symbol SYM matches that pattern, so we return true here to
+     permit the release of SYM.  */
+  return true;
+}
+
+
 /* Decrease the reference counter and free memory when we reach zero.
    Returns true if the symbol has been freed, false otherwise.  */
 
@@ -3188,8 +3239,7 @@ gfc_release_symbol (gfc_symbol *&sym)
   if (sym == NULL)
     return false;
 
-  if (sym->formal_ns != NULL && sym->refs == 2 && sym->formal_ns != sym->ns
-      && (!sym->attr.entry || !sym->module))
+  if (cyclic_reference_break_needed (sym))
     {
       /* As formal_ns contains a reference to sym, delete formal_ns just
 	 before the deletion of sym.  */

gcc/testsuite/gfortran.dg/submodule_33.f08

@@ -0,0 +1,20 @@
+! { dg-do compile }
+!
+! PR fortran/99798
+! This example used to trigger an ICE caused by a premature release of the G
+! symbol (with its argument X) following the rejection of the subroutine in
+! the submodule.
+
+module m
+   interface
+      module integer function g(x)
+         integer, intent(in) :: x
+      end
+   end interface
+end
+submodule(m) m2
+contains
+   subroutine g(x)      ! { dg-error "FUNCTION attribute conflicts with SUBROUTINE" }
+     integer, intent(in) :: x  ! { dg-error "Unexpected data declaration" }
+   end
+end