https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71307
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
If the BIT_FIELD_REF covers the whole object then it should be simplified to
a VIEW_CONVERT. There is "related" simplification in match.pd:
(simplify
(BIT_FIELD_REF @0 @1 @2)
(switch
(if (TREE_CODE (TREE_TYPE (@0)) == COMPLEX_TYPE
&& tree_int_cst_equal (@1, TYPE_SIZE (TREE_TYPE (TREE_TYPE (@0)))))
(switch
(if (integer_zerop (@2))
(view_convert (realpart @0)))
(if (tree_int_cst_equal (@2, TYPE_SIZE (TREE_TYPE (TREE_TYPE (@0)))))
(view_convert (imagpart @0)))))
(if (INTEGRAL_TYPE_P (TREE_TYPE (@0))
&& INTEGRAL_TYPE_P (type)
/* On GIMPLE this should only apply to register arguments. */
&& (! GIMPLE || is_gimple_reg (@0))
/* A bit-field-ref that referenced the full argument can be stripped.
*/
&& ((compare_tree_int (@1, TYPE_PRECISION (TREE_TYPE (@0))) == 0
&& integer_zerop (@2))
...
(convert @0))))
but restricted to integral types as we use convert and not view_convert.
Other than that, BIT_FIELD_REF expansion might simply miss a trick.
