linux-kernel - Re: [PATCH 11/12] tools: sync lib/find

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Date:   Wed, 12 May 2021 09:48:57 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Rikard Falkeborn <rikard.falkeborn@...il.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Yury Norov <yury.norov@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linux-SH <linux-sh@...r.kernel.org>,
        Alexey Klimov <aklimov@...hat.com>,
        David Sterba <dsterba@...e.com>,
        Dennis Zhou <dennis@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Jianpeng Ma <jianpeng.ma@...el.com>,
        Joe Perches <joe@...ches.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rich Felker <dalias@...c.org>,
        Stefano Brivio <sbrivio@...hat.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 11/12] tools: sync lib/find_bit implementation

On Tue, May 11, 2021 at 10:39 PM Rikard Falkeborn
<rikard.falkeborn@...il.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