Re: Help with bit-field semantics in C and C++

[Mark Mitchell <mark at codesourcery dot com>]

Roger Sayle wrote:

> Could one of GCC's resident C/C++ language lawyers confirm that the
> behaviour of the following code is undefined?
>
>
> extern "C" void abort(void);
>
> enum E { e = 0x0f };
>
> void test(int n)
> {
>  enum E x, y;
>
>  x = (E)0x10;
>  
That conversion is unspecified in C++, as others have said.

Now, the next question is what behavior we should prefer.  I think that 
we should avoid generating any code for such a conversion, so that means 
that we should avoid, for example, generating mask instructions.  As 
Joseph points out, it's not crystal clear whether the result of the 
conversion must result in a valid value of the underlying enumeration 
type -- but all compilers of which I am aware will just store "0x10" in 
the memory for "x" without doing any truncation, so I think that's what 
we should do too. 

In other words, treat this like a conversion to the underlying integer 
type for the enumeration.

--
Mark Mitchell
CodeSourcery, LLC
(916) 791-8304
mark@codesourcery.com