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Date:   Wed, 18 Jan 2023 17:49:07 +0100
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Subject: Re: [PATCH] bitfield: add FIELD_PREP_CONST()
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To:     Johannes Berg <johannes@sipsolutions.net>
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From:   Alexander Lobakin <alexandr.lobakin@intel.com>
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From: Johannes Berg <johannes@sipsolutions.net>
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