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

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Message-ID: <20201019175502.GO2672@gate.crashing.org>
Date:   Mon, 19 Oct 2020 12:55:02 -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 10:54:40AM +0200, Christophe Leroy wrote:
> Le 19/10/2020 à 10:32, Segher Boessenkool a écrit :
> >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).
> 
> Yes I guess __always_inline is to be added on functions like this defined 
> in headers for exactly that, and that's the purpose of this patch.
> 
> However I find it odd that get_order() is outlined by GCC even in some 
> object files that don't use it at all, for instance in fs/pipe.o

It is (arguably) too big too always inline if you do not consider that
__builtin_constant_p will remove half of the function one way or
another.  Not sure if that is what happens here, but now we have a PR
(thanks!) and we will find out.


Segher