linux-kernel - Re: [PATCH] asm-generic: fix -Wtype-limits compiler warnings

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Date:   Wed, 24 Jul 2019 08:49:31 +0100
From:   David Howells <dhowells@...hat.com>
To:     Qian Cai <cai@....pw>
Cc:     dhowells@...hat.com, akpm@...ux-foundation.org,
        davem@...emloft.net, arnd@...db.de, jakub@...hat.com,
        ndesaulniers@...gle.com, morbo@...gle.com, jyknight@...gle.com,
        natechancellor@...il.com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic: fix -Wtype-limits compiler warnings

Qian Cai <cai@....pw> wrote:

> Fix it by moving almost all of this multi-line macro into a proper
> function __get_order(), and leave get_order() as a single-line macro in
> order to avoid compilation errors.

The idea was that you could compile-time initialise a global variable with
get_order():

	int a = get_order(SOME_MACRO);

This is the same reason that ilog2() is a macro:

	int a = ilog2(SOME_MACRO);

See the banner comment on get_order():

 * This function may be used to initialise variables with compile time
 * evaluations of constants.

If you're moving the constant branch into __get_order(), an inline function,
then we'll no longer be able to do this and you need to modify the comment
too.  In fact, would there still be a point in having the get_order() macro?

Also, IIRC, older versions of gcc see __builtin_constant_p(n) == 0 inside an
function, inline or otherwise, even if the passed-in argument *is* constant.

David