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

[Ard Biesheuvel]

On 19 October 2016 at 16:56, Markus Trippelsdorf <markus at trippelsdorf.de> wrote:
> On 2016.10.19 at 08:55 -0700, Linus Torvalds wrote:
>> On Wed, Oct 19, 2016 at 8:37 AM, Markus Trippelsdorf
>> <markus at trippelsdorf.de> wrote:
>> >
>> > This is a gcc bug, see:
>> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785
>>
>> Well, in the meantime we apparently have to live with it. Unless Will
>> is using some unreleased gcc version that nobody else is using and we
>> can just ignore it?
>
> Yes, he is using gcc-7 that is unreleased. (It will be released April
> next year.)
>

order_base_2() is still broken though, given that it is documented as

 * The first few values calculated by this routine:
 *  ob2(0) = 0
 *  ob2(1) = 0
 *  ob2(2) = 1
 *  ob2(3) = 2
 *  ob2(4) = 2
 *  ob2(5) = 3

whereas order_base_2(0) actually ends up invoking
roundup_pow_of_two(0), which is documented as being undefined.