Fix handling of implicit_pure by checking if non-pure procedures are called.

Procedures are marked as implicit_pure if they fulfill the criteria of
pure procedures.  In this case, a procedure was not marked as not being
implicit_pure which called another procedure, which had not yet been
marked as not being implicit_impure.

Fixed by iterating over all procedures, setting callers of procedures
which are non-pure and non-implicit_pure as non-implicit_pure and
doing this until no more procedure has been changed.

gcc/fortran/ChangeLog:

2020-07-19  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR fortran/96018
	* frontend-passes.c (gfc_check_externals): Adjust formatting.
	(implicit_pure_call): New function.
	(implicit_pure_expr): New function.
	(gfc_fix_implicit_pure): New function.
	* gfortran.h (gfc_fix_implicit_pure): New prototype.
	* parse.c (translate_all_program_units): Call gfc_fix_implicit_pure.

gcc/fortran/frontend-passes.c

@@ -5551,7 +5551,8 @@ gfc_check_externals0 (gfc_namespace *ns)
 
 /* Called routine.  */
 
-void gfc_check_externals (gfc_namespace *ns)
+void
+gfc_check_externals (gfc_namespace *ns)
 {
   gfc_clear_error ();
 
@@ -5566,3 +5567,76 @@ void gfc_check_externals (gfc_namespace *ns)
   gfc_errors_to_warnings (false);
 }
 
+/* Callback function. If there is a call to a subroutine which is
+   neither pure nor implicit_pure, unset the implicit_pure flag for
+   the caller and return -1.  */
+
+static int
+implicit_pure_call (gfc_code **c, int *walk_subtrees ATTRIBUTE_UNUSED,
+		    void *sym_data)
+{
+  gfc_code *co = *c;
+  gfc_symbol *caller_sym;
+  symbol_attribute *a;
+
+  if (co->op != EXEC_CALL || co->resolved_sym == NULL)
+    return 0;
+
+  a = &co->resolved_sym->attr;
+  if (a->intrinsic || a->pure || a->implicit_pure)
+    return 0;
+
+  caller_sym = (gfc_symbol *) sym_data;
+  gfc_unset_implicit_pure (caller_sym);
+  return 1;
+}
+
+/* Callback function. If there is a call to a function which is
+   neither pure nor implicit_pure, unset the implicit_pure flag for
+   the caller and return 1.  */
+
+static int
+implicit_pure_expr (gfc_expr **e, int *walk ATTRIBUTE_UNUSED, void *sym_data)
+{
+  gfc_expr *expr = *e;
+  gfc_symbol *caller_sym;
+  gfc_symbol *sym;
+  symbol_attribute *a;
+
+  if (expr->expr_type != EXPR_FUNCTION || expr->value.function.isym)
+    return 0;
+
+  sym = expr->symtree->n.sym;
+  a = &sym->attr;
+  if (a->pure || a->implicit_pure)
+    return 0;
+
+  caller_sym = (gfc_symbol *) sym_data;
+  gfc_unset_implicit_pure (caller_sym);
+  return 1;
+}
+
+/* Go through all procedures in the namespace and unset the
+   implicit_pure attribute for any procedure that calls something not
+   pure or implicit pure.  */
+
+bool
+gfc_fix_implicit_pure (gfc_namespace *ns)
+{
+  bool changed = false;
+  gfc_symbol *proc = ns->proc_name;
+
+  if (proc && proc->attr.flavor == FL_PROCEDURE && proc->attr.implicit_pure
+      && ns->code
+      && gfc_code_walker (&ns->code, implicit_pure_call, implicit_pure_expr,
+			  (void *) ns->proc_name))
+    changed = true;
+
+  for (ns = ns->contained; ns; ns = ns->sibling)
+    {
+      if (gfc_fix_implicit_pure (ns))
+	changed = true;
+    }
+
+  return changed;
+}

gcc/fortran/gfortran.h

@@ -3623,6 +3623,7 @@ int gfc_expr_walker (gfc_expr **, walk_expr_fn_t, void *);
 int gfc_code_walker (gfc_code **, walk_code_fn_t, walk_expr_fn_t, void *);
 bool gfc_has_dimen_vector_ref (gfc_expr *e);
 void gfc_check_externals (gfc_namespace *);
+bool gfc_fix_implicit_pure (gfc_namespace *);
 
 /* simplify.c */
 

gcc/fortran/parse.c

@@ -6447,6 +6447,11 @@ loop:
 
   gfc_resolve (gfc_current_ns);
 
+  /* Fix the implicit_pure attribute for those procedures who should
+     not have it.  */
+  while (gfc_fix_implicit_pure (gfc_current_ns))
+    ;
+
   /* Dump the parse tree if requested.  */
   if (flag_dump_fortran_original)
     gfc_dump_parse_tree (gfc_current_ns, stdout);
@@ -6492,6 +6497,23 @@ done:
   /* Do the resolution.  */
   resolve_all_program_units (gfc_global_ns_list);
 
+  /* Go through all top-level namespaces and unset the implicit_pure
+     attribute for any procedures that call something not pure or
+     implicit_pure.  Because the a procedure marked as not implicit_pure
+     in one sweep may be called by another routine, we repeat this
+     process until there are no more changes.  */
+  bool changed;
+  do
+    {
+      changed = false;
+      for (gfc_current_ns = gfc_global_ns_list; gfc_current_ns;
+	   gfc_current_ns = gfc_current_ns->sibling)
+	{
+	  if (gfc_fix_implicit_pure (gfc_current_ns))
+	    changed = true;
+	}
+    }
+  while (changed);
 
   /* Fixup for external procedures.  */
   for (gfc_current_ns = gfc_global_ns_list; gfc_current_ns;

gcc/testsuite/gfortran.dg/implicit_pure_5.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+
+extern int num_calls;
+int side_effect_c()
+{
+  num_calls ++;
+}

gcc/testsuite/gfortran.dg/implicit_pure_5.f90

@@ -0,0 +1,63 @@
+! { dg-do run }
+! { dg-additional-sources implicit_pure_5.c }
+! PR fortran/96018 - a wrongly marked implicit_pure
+! function caused wrong code.
+module wrapper
+  use, intrinsic :: iso_c_binding, only : c_int
+  implicit none
+  integer(kind=c_int), bind(C) :: num_calls
+contains
+
+  integer function call_side_effect() result(ierr)
+    call side_effect(ierr)
+  end function call_side_effect
+
+  integer function inner_3d(array) result(ierr)
+    real, intent(in) :: array(:,:,:)
+    integer dimensions(3)
+    dimensions = shape(array)
+    ierr = call_side_effect()
+  end function inner_3d
+
+  integer function inner_4d(array) result(ierr)
+    real, intent(in) :: array(:,:,:,:)
+    integer dimensions(4)
+    dimensions = shape(array)
+    ierr = call_side_effect()
+  end function inner_4d
+
+  subroutine write_3d()
+    real :: array(1,1,1)
+    integer ierr
+    ierr = inner_3d(array)
+    ierr = call_side_effect()
+  end subroutine write_3d
+
+  subroutine write_4d()
+    real array(1,1,1,1)
+    integer ierr
+    ierr = inner_4d(array)
+    ierr = call_side_effect()
+  end subroutine write_4d
+
+  subroutine side_effect(ierr)
+    integer, intent(out) :: ierr        ! Error code
+    interface
+       integer(c_int) function side_effect_c() bind(C,name='side_effect_c')
+         use, intrinsic :: iso_c_binding, only: c_int
+       end function side_effect_c
+    end interface
+    ierr = side_effect_c()
+  end subroutine side_effect
+
+end module wrapper
+
+program self_contained
+  use wrapper
+  implicit none
+  call write_3d()
+  if (num_calls /= 2) stop 1
+  call write_4d()
+  if (num_calls /= 4) stop 2
+end program self_contained
+