[Ada] Fix corner case issue with discriminated record type

[Eric Botcazou]

The compiler generates code that writes too much data into a component of a 
record subject to a representation clause, when the source of the assignment 
is a call to a function that returns a discriminated record type with default 
discriminants, variable size and a statically known upper bound for this size, 
and the size of the component given by the representation clause is lower than 
the value of this bound rounded up to the alignment.

Tested on x86-64/Linux, applied on the mainline.


2020-12-07  Eric Botcazou  <ebotca...@adacore.com>

        * gcc-interface/trans.c (Call_to_gnu): Also create a temporary for
        the return value if the LHS is a bit-field and the return type is
        a type padding a self-referential type.
        (gnat_to_gnu): Do not remove the padding on the result if it is too
        small with regard to the natural padding size.

-- 
Eric Botcazou
diff --git a/gcc/ada/gcc-interface/trans.c b/gcc/ada/gcc-interface/trans.c
index 0eec1788e05..07e5a285b2b 100644
--- a/gcc/ada/gcc-interface/trans.c
+++ b/gcc/ada/gcc-interface/trans.c
@@ -4513,7 +4513,11 @@ Call_to_gnu (Node_Id gnat_node, tree *gnu_result_type_p, tree gnu_target,
 	  and the return type has variable size, because the gimplifier
 	  doesn't handle these cases.
 
-       4. There is no target and we have misaligned In Out or Out parameters
+       4. There is a target which is a bit-field and the function returns an
+	  unconstrained record type with default discriminant, because the
+	  return may copy more data than the bit-field can contain.
+
+       5. There is no target and we have misaligned In Out or Out parameters
 	  passed by reference, because we need to preserve the return value
 	  before copying back the parameters.  However, in this case, we'll
 	  defer creating the temporary, see below.
@@ -4536,7 +4540,11 @@ Call_to_gnu (Node_Id gnat_node, tree *gnu_result_type_p, tree gnu_target,
 		  || (TREE_CODE (TREE_TYPE (gnu_target)) == ARRAY_TYPE
 		      && TREE_CODE (TYPE_SIZE (TREE_TYPE (gnu_target)))
 			 == INTEGER_CST))
-	      && TREE_CODE (TYPE_SIZE (gnu_result_type)) != INTEGER_CST)))
+	      && TREE_CODE (TYPE_SIZE (gnu_result_type)) != INTEGER_CST)
+	  || (gnu_target
+	      && TREE_CODE (gnu_target) == COMPONENT_REF
+	      && DECL_BIT_FIELD (TREE_OPERAND (gnu_target, 1))
+	      && type_is_padding_self_referential (gnu_result_type))))
     {
       gnu_retval = create_temporary ("R", gnu_result_type);
       DECL_RETURN_VALUE_P (gnu_retval) = 1;
@@ -8249,8 +8257,10 @@ gnat_to_gnu (Node_Id gnat_node)
       /* Remove padding only if the inner object is of self-referential
 	 size: in that case it must be an object of unconstrained type
 	 with a default discriminant and we want to avoid copying too
-	 much data.  */
-      if (type_is_padding_self_referential (TREE_TYPE (gnu_result)))
+	 much data.  But do not remove it if it is already too small.  */
+      if (type_is_padding_self_referential (TREE_TYPE (gnu_result))
+	  && !(TREE_CODE (gnu_result) == COMPONENT_REF
+	       && DECL_BIT_FIELD (TREE_OPERAND (gnu_result, 1))))
 	gnu_result = convert (TREE_TYPE (TYPE_FIELDS (TREE_TYPE (gnu_result))),
 			      gnu_result);
     }