[Bug c/59852] Support sparse-style __attribute__((bitwise)) (type attribute)

["josh at joshtriplett dot org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59852

--- Comment #7 from Josh Triplett <josh at joshtriplett dot org> ---
(In reply to Josh Triplett from comment #4)
> Also note that arithmetic operations between a bitwise and a known-zero
> value do not warn.
> 
> The warning on ~ of a value smaller than int only occurs if the value is not
> subsequently stuffed back into the same bitwise type.  For instance, this
> does not warn:
> 
> typedef unsigned short __attribute__((bitwise)) le16;
> 
> le16 i, j;
> 
> le16 k = ~i | j;

To elaborate on this with some implementation details of Sparse: applying ~ to
a bitwise type smaller than an int produces a value of a corresponding bitwise
type with the added attribute "fouled".  Bitwise operations propagate the
fouled bit if either operand has it, without warning.  == and != will warn
about fouled types.  Assignments or conversions to the original unfouled
bitwise type will work without warning, discarding the fouled bit.  And any
arithmetic operation that would warn about a bitwise type will warn about a
fouled type, complaining that the type degraded to "int".