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

On 1 February 2017 at 20:34, Joe Perches [off-list ref] wrote:

On Wed, 2017-02-01 at 19:53 +0000, Ard Biesheuvel wrote:

quoted

On 1 February 2017 at 19:49, Joe Perches [off-list ref] wrote:

[]

quoted

quoted

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()