linux-kernel - Re: [PATCH v3 04/11] linkmode: convert linkmode_{test,set,clear,mod}

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Message-ID: <33967a01-9ed0-43db-a615-907abab989b7@lunn.ch>
Date:   Mon, 13 Nov 2023 20:08:40 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>
Cc:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexander Potapenko <glider@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Przemek Kitszel <przemyslaw.kitszel@...el.com>,
        netdev@...r.kernel.org, linux-btrfs@...r.kernel.org,
        dm-devel@...hat.com, ntfs3@...ts.linux.dev,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/11] linkmode: convert
 linkmode_{test,set,clear,mod}_bit() to macros

On Mon, Nov 13, 2023 at 06:37:10PM +0100, Alexander Lobakin wrote:
> Since commit b03fc1173c0c ("bitops: let optimize out non-atomic bitops
> on compile-time constants"), the non-atomic bitops are macros which can
> be expanded by the compilers into compile-time expressions, which will
> result in better optimized object code. Unfortunately, turned out that
> passing `volatile` to those macros discards any possibility of
> optimization, as the compilers then don't even try to look whether
> the passed bitmap is known at compilation time. In addition to that,
> the mentioned linkmode helpers are marked with `inline`, not
> `__always_inline`, meaning that it's not guaranteed some compiler won't
> uninline them for no reason, which will also effectively prevent them
> from being optimized (it's a well-known thing the compilers sometimes
> uninline `2 + 2`).
> Convert linkmode_*_bit() from inlines to macros. Their calling
> convention are 1:1 with the corresponding bitops, so that it's not even
> needed to enumerate and map the arguments, only the names. No changes in
> vmlinux' object code (compiled by LLVM for x86_64) whatsoever, but that
> doesn't necessarily means the change is meaningless.
> 
> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> Acked-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew