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Date:   Wed, 29 May 2019 18:07:31 +0200
Message-ID: <20190529180731.Horde.NGHeOXuCgw23pVdGqjc0fw9@messagerie.si.c-s.fr>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
To:     Pascal Van Leeuwen <pvanleeuwen@insidesecure.com>
Cc:     linux-crypto@vger.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: Re: Conding style question regarding configuration
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Pascal Van Leeuwen <pvanleeuwen@insidesecure.com> a écrit :

>> > Quick question regarding how to configure out code depending on a
>> CONFIG_xxx
>> > switch. As far as I understood so far, the proper way to do this is
>> not by
>> > doing an #ifdef but by using a regular if with IS_ENABLED like so:
>> >
>> > if (IS_ENABLED(CONFIG_PCI)) {
>> > }
>> >
>> > Such that the compiler can still check the code even if the switch is
>> > disabled. Now that all works fine and dandy for statements within a
>> > function, but how do you configure out, say, global variable
>> definitions
>> > referencing types that are tied to this configuration switch? Or
>> should
>> > I just leave them in, depending on the compiler to optimize them away?
>> >
>> > Obviously the code depends on those variables again, so if it's not
>> > done consistently the compiler will complain somehow if the switch is
>> not
>> > defined ...
>> >
>> > Also, with if (IS_ENABLED()) I cannot remove my function prototypes,
>> > just the function body. Is that really how it's supposed to be done?
>> >
>>
>> Yes. Code and data with static linkage will just be optimized away by
>> the compiler if the CONFIG_xx option is not enabled, so all you need
>> to guard are the actual statements, function calls etc.
>>
> Ok, makes sense. Then I'll just config out the relevant function bodies
> and assume the compiler will do the rest ...
>

No need to config out function bodies when they are static.
If not, it's better to group then in a C file and associate that file  
to the config symbol through Makefile

Christophe

> Thanks,
> Pascal van Leeuwen
> Silicon IP Architect, Multi-Protocol Engines @ Inside Secure
> www.insidesecure.com