Re: [PATCH 11/12] tools: sync lib/find_bit implementation

From: Arnd Bergmann <arnd@arndb.de>
To: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Andy Shevchenko <andy.shevchenko@gmail.com>,
	Yury Norov <yury.norov@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	linux-m68k <linux-m68k@lists.linux-m68k.org>,
	Linux-Arch <linux-arch@vger.kernel.org>,
	Linux-SH <linux-sh@vger.kernel.org>,
	Alexey Klimov <aklimov@redhat.com>,
	David Sterba <dsterba@suse.com>, Dennis Zhou <dennis@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Jianpeng Ma <jianpeng.ma@intel.com>,
	Joe Perches <joe@perches.com>,
	John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Rich Felker <dalias@libc.org>,
	Stefano Brivio <sbrivio@redhat.com>,
	Wei Yang <richard.weiyang@linux.alibaba.com>,
	Wolfram Sang <wsa+renesas@sang-engineering.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>
Subject: Re: [PATCH 11/12] tools: sync lib/find_bit implementation
Date: Wed, 12 May 2021 09:48:57 +0200	[thread overview]
Message-ID: <CAK8P3a02qNHcksJ8DahHgLtbM9ZOydGjE3__3GoxgJFiWrAT0w@mail.gmail.com> (raw)
In-Reply-To: <YJrrJhvwq7RUvDXD@rikard>

On Tue, May 11, 2021 at 10:39 PM Rikard Falkeborn
<rikard.falkeborn@gmail.com> wrote:
> On Tue, May 11, 2021 at 08:53:53PM +0900, Tetsuo Handa wrote:

> > #define GENMASK_INPUT_CHECK(h, l) \
> >      (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> >           __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> >
> > __GENMASK() does not need "h" and "l" being a constant.
> >
> > Yes, small_const_nbits(size) in find_next_bit() can guarantee that "size" is a
> > constant and hence "h" argument in GENMASK_INPUT_CHECK() call is also a constant.
> > But nothing can guarantee that "offset" is a constant, and hence nothing can
> > guarantee that "l" argument in GENMASK_INPUT_CHECK() call is also a constant.
> >
> > Then, how can (l) > (h) in __builtin_constant_p((l) > (h)) be evaluated at build time
> > if either l or h (i.e. "offset" and "size - 1" in find_next_bit()) lacks a guarantee of
> > being a constant?
> >
>
> So the idea is that if (l > h) is constant, __builtin_constant_p should
> evaluate that, and if it is not it should use zero instead as input to
> __builtin_chose_expr(). This works with non-const inputs in many other
> places in the kernel, but apparently in this case with a certain
> compiler, it doesn't so I guess we need to work around it.

I have a vague memory that __builtin_constant_p() inside of
__builtin_choose_expr()
always evaluates to false because of the order in which the compiler processes
those: If constant-folding only happens after __builtin_choose_expr(), then
__builtin_constant_p() has to be false.

        Arnd