linux-kernel - Re: [PATCH v4 2/5] m68k/bitops: use __builtin_{clz,ctzl,ffs} to evaluate constant expressions

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Date: Mon, 29 Jan 2024 09:34:56 +1100 (AEDT)
From: Finn Thain <fthain@...ux-m68k.org>
To: Vincent MAILHOL <mailhol.vincent@...adoo.fr>
cc: David Laight <David.Laight@...lab.com>, 
    Andrew Morton <akpm@...ux-foundation.org>, 
    "linux-kernel@...r.kernel.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>, 
    Geert Uytterhoeven <geert@...ux-m68k.org>, 
    Matthew Wilcox <willy@...radead.org>, 
    "Paul E . McKenney" <paulmck@...nel.org>, 
    "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH v4 2/5] m68k/bitops: use __builtin_{clz,ctzl,ffs} to
 evaluate constant expressions


On Sun, 28 Jan 2024, Vincent MAILHOL wrote:

> > > The asm is meant to produce better results when the argument is not 
> > > a constant expression.

Is that because gcc's implementation has to satisfy requirements that are 
excessively stringent for the kernel's purposes? Or is it a compiler 
deficiency only affecting certain architectures?

> ... The only thing I am not ready to do is to compare the produced 
> assembly code and confirm whether or not it is better to remove asm 
> code.
> 

If you do the comparison and find no change, you get to say so in the 
commit log, and everyone is happy.