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SubjectRe: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness
On 1 February 2017 at 20:34, Joe Perches <joe@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@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()

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