[systemd-devel] compile with clang broken

[David Herrmann]

Hi

On Fri, Aug 15, 2014 at 5:22 PM, Daniele Nicolodi <daniele at grinta.net> wrote:
> On 15/08/2014 16:30, David Herrmann wrote:
>> Ok, took me a while, but I now figured out how to cause compilation to
>> fail even in expressions that initialize types (_Static_assert is not
>> allowed there):
>>   #define assert_const(expr)
>> ((void)(__builtin_types_compatible_p(int[(expr) ? 1 : -1], int[1])))
>>
>> Btw., I like that more than our current assert_cc() fallback. But I
>> leave it up to you to decide.
>>
>> Anyhow, I found a way to make CONST_MAX work:
>> #define CONST_MAX(_A, _B)
>> (__builtin_choose_expr(__builtin_constant_p(_A) &&
>> __builtin_constant_p(_B), ((_A) > (_B)) ? (_A) : (_B), (void)0))
>>
>> This will return (void) in case _A or _B is not constant. Works fine
>> on LLVM, I now have to test it on gcc. If it works, I will commit it
>> and fix resolvd.
>
> Hello,
>
> this may be completely stupid, but if the only use case you have for
> CONST_MAX() is for computing the size of a data structure, I find
> something like
>
> #define MAXSIZE(A, B) sizeof(union { __typeof(A) a; __typeof(B) b;})
>
> a little more clear and less magic, and I believe it has the same
> guarantees that the solution you found.

Your MAXSIZE macro might add padding:
        union A {
                int a;
                char b[5];
        };
This union has size 8, not 5 (64bit). But CONST_MAX would return 5.
Not sure whether that really matters, though. And we could probably
add __packed__ to the definition.

However, I noticed that GCC complains about using
statement-expressions to initialize static-const structure members,
even with my 'const' annotations added to MAX. *sigh*
Thus, I think I'll keep CONST_MAX, as we'd require a 3rd macro otherwise.

If you know a way to unify them all, please lemme know.

Thanks
David