linux-kernel - Re: [PATCH] asm-generic: Force inlining of get_order() to work around gcc10 poor decision

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Message-ID: <20201019083225.GN2672@gate.crashing.org>
Date:   Mon, 19 Oct 2020 03:32:25 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Joel Stanley <joel@....id.au>, Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arch@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] asm-generic: Force inlining of get_order() to work around gcc10 poor decision

On Mon, Oct 19, 2020 at 07:50:41AM +0200, Christophe Leroy wrote:
> Le 19/10/2020 à 06:55, Joel Stanley a écrit :
> >>In the old days, marking a function 'static inline' was forcing
> >>GCC to inline, but since commit ac7c3e4ff401 ("compiler: enable
> >>CONFIG_OPTIMIZE_INLINING forcibly") GCC may decide to not inline
> >>a function.
> >>
> >>It looks like GCC 10 is taking poor decisions on this.

> >1952 bytes smaller with your patch applied. Did you raise this with
> >anyone from GCC?
> 
> Yes I did, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445
> 
> For the time being, it's at a standstill.

The kernel should just use __always_inline if that is what it *wants*;
that is true here most likely.  GCC could perhaps improve its heuristics
so that it no longer thinks these functions are often too big for
inlining (they *are* pretty big, but not after basic optimisations with
constant integer arguments).


Segher