linux-kernel - Re: [PATCH v3 2/5] m68k/bitops: use __builtin_{clz,ctzl,ffs} to evaluate constant expressions

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Date: Tue, 2 Jan 2024 11:49:03 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	Yury Norov <yury.norov@...il.com>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Douglas Anderson <dianders@...omium.org>, Kees Cook <keescook@...omium.org>, 
	Petr Mladek <pmladek@...e.com>, Randy Dunlap <rdunlap@...radead.org>, 
	Zhaoyang Huang <zhaoyang.huang@...soc.com>, Geert Uytterhoeven <geert+renesas@...der.be>, 
	Marco Elver <elver@...gle.com>, Brian Cain <bcain@...cinc.com>, 
	Matthew Wilcox <willy@...radead.org>, "Paul E . McKenney" <paulmck@...nel.org>, linux-hexagon@...r.kernel.org, 
	linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH v3 2/5] m68k/bitops: use __builtin_{clz,ctzl,ffs} to
 evaluate constant expressions

On Sun, Dec 17, 2023 at 8:13 AM Vincent Mailhol
<mailhol.vincent@...adoo.fr> wrote:
>
> The compiler is not able to do constant folding on "asm volatile" code.
>
> Evaluate whether or not the function argument is a constant expression
> and if this is the case, return an equivalent builtin expression.
>
> On an allyesconfig, with GCC 13.2.1, it saves roughly 8 KB.
>
>   $ size --format=GNU vmlinux.before vmlinux.after
>         text       data        bss      total filename
>     60446534   70972412    2289596  133708542 vmlinux.before
>     60429746   70978876    2291676  133700298 vmlinux.after

Still a win with gcc 9.5.0-1ubuntu1~22.04:

allyesconfig:

      text       data        bss      total filename
  58882250   72419706    2284004  133585960
vmlinux-v6.7-rc8-1-m68k-bitops-force-inlining
  58863570   72419546    2285860  133568976
vmlinux-v6.7-rc8-2-m68k-bitops-use-__builtin_clz,ctzl,ffs

So an even bigger win...

atari_defconfig:

      text       data        bss      total filename
   4117008    1579350     151680    5848038
vmlinux-v6.7-rc8-1-m68k-bitops-force-inlining
   4116940    1579534     151712    5848186
vmlinux-v6.7-rc8-2-m68k-bitops-use-__builtin_clz,ctzl,ffs

... but a small size increase here.


>
> Reference: commit fdb6649ab7c1 ("x86/asm/bitops: Use __builtin_ctzl()
> to evaluate constant expressions")

Please don't split lines containing tags.

> Link: https://git.kernel.org/torvalds/c/fdb6649ab7c1
>
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>

Reviewed-by: Geert Uytterhoeven <geert@...ux-m68k.org>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds