Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness

[Ard Biesheuvel]

On 1 February 2017 at 20:34, Joe Perches <joe at perches.com> wrote:
> On Wed, 2017-02-01 at 19:53 +0000, Ard Biesheuvel wrote:
>> On 1 February 2017 at 19:49, Joe Perches <joe at perches.com> wrote:
> []
>> > Or maybe add a BUILD_BUG_ON something like:
>> >
>> > #define order_base_2(n)                                                 \
>> > ({                                                                      \
>> >         typeof(n) _n = n;                                               \
>> >         BUILD_BUG_ON(__builtin_constant_p(_n) && _n < 0);               \
>> >         __builtin_constant_p(_n) ? (_n < 2 ? _n : ilog2((_n) - 1) + 1)) \
>> >                                  : __order_base_2(_n);                  \
>> > })
>> >
>>
>> This would interfere with the ability to use order_base_2() in
>> initializers for global variables.
>
> There aren't any as far as I can tell and would using
> order_base_2() for a global initializer make sense?
>

Why wouldn't it make sense?

In any case, we could also solve this by doing this instead

#define order_base_2(n)                        \
(                                              \
       __builtin_constant_p(n) ? (             \
               ((n) == 0 || (n) == 1) ? 0 :    \
               ilog2((n) - 1) + 1) :           \
       __order_base_2(n)                       \
)

which will emit the usual unresolveable __ilog2_NaN reference when
constants < 0 are passed to order_base_2()