From: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
Date: Wed, 18 Jan 2023 17:22:30 +0100
> On Wed, 2023-01-18 at 17:10 +0100, Alexander Lobakin wrote:
>>
>> Have you tried combining it with FIELD_PREP() using
>> __builtin_choose_expr() + __builtin_is_constexpr() (or
>> __builtin_constant_p() depending on which will satisfy the compiler)?
>> I'm not saying it's 100% possible, but worth trying.
>>
>
> I haven't tried it that way, but I tried rewriting FIELD_PREP() itself
> to be constant-compatible, and as soon as the compiler saw
> __builtin_constant_p() in the initializer it already complained that it
> was non-constant...
>
> I didn't think of __builtin_choose_expr, but it doesn't work either
> because it only promises that the unused expression is not *evaluated*,
> not that it's not "looked at", so it still complains both ways (in the
> constant case that you can't have ({ }) braced groups, and in the non-
> constant case that the _CONST version is bad...
Aaaah right. Seems like we can't avoid introducing a separate macro for
that. I like the idea of your patch anyways!
One note re __BF_CHECK_POW2(): can't we reuse is_power_of_2() anyhow?
Foe example, by deriving the code of the latter into a macro and then
using it in both?
>
> johannes
Thanks,
Olek
