[Fortran] Disable front-end optimization for OpenACC atomic (PR93462)

        PR fortran/93462
        * frontend-passes.c (gfc_code_walker): For EXEC_OACC_ATOMIC, set
        in_omp_atomic to true prevent front-end optimization.

        PR fortran/93462
        * gfortran.dg/goacc/atomic-1.f90: New.

gcc/fortran/ChangeLog

+2020-01-31  Tobias Burnus  <tobias@codesourcery.com>
+
+	PR fortran/93462
+	* frontend-passes.c (gfc_code_walker): For EXEC_OACC_ATOMIC, set
+	in_omp_atomic to true prevent front-end optimization.
+
 2020-01-30  Bernhard Reutner-Fischer  <aldot@gcc.gnu.org>
 
 	PR fortran/87103

gcc/fortran/frontend-passes.c

@@ -5258,6 +5258,7 @@ gfc_code_walker (gfc_code **c, walk_code_fn_t codefn, walk_expr_fn_t exprfn,
 	      WALK_SUBEXPR (co->ext.dt->extra_comma);
 	      break;
 
+	    case EXEC_OACC_ATOMIC:
 	    case EXEC_OMP_ATOMIC:
 	      in_omp_atomic = true;
 	      break;

gcc/testsuite/ChangeLog

+2020-01-31  Tobias Burnus  <tobias@codesourcery.com>
+
+	PR fortran/93462
+	* gfortran.dg/goacc/atomic-1.f90: New.
+
 2020-01-31  Tamar Christina  <tamar.christina@arm.com>
 
 	PR rtl-optimization/91838

gcc/testsuite/gfortran.dg/goacc/atomic-1.f90

+! { dg-do compile }
+!
+! PR fortran/93462
+!
+! Contributed by G. Steinmetz
+!
+program p
+   integer :: n = 1
+   integer :: a
+!$acc atomic write
+   a = f(n) - f(n)
+contains
+   integer function f(x)
+      integer, intent(in) :: x
+      f = x
+   end
+end