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

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Date:   Wed, 12 May 2021 10:33:09 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Rikard Falkeborn <rikard.falkeborn@...il.com>,
        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>,
        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 Wed, May 12, 2021 at 10:16 AM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> It's more complicated than that. __builtin_constant_p on something which
> is a bona-fide Integer Constant Expression (ICE) gets folded early to a
> 1. And then it turns out that such a __builtin_constant_p() that folds
> early to a 1 can be "stronger" than a literal 1, in the sense that when
> used as the controlling expression of a ?: with nonsense in the false
> branch, the former is OK but the latter fails:
>
> https://lore.kernel.org/lkml/c68a0f46-346c-70a0-a9b8-31747888f05f@rasmusvillemoes.dk/
>
> Now what happens when the argument to __builtin_constant_p is not an ICE
> is a lot more complicated. The argument _may_ be so obviously
> non-constant that it can be folded early to a 0, hence still be suitable
> as first argument to __b_c_e. But it is also possible that the compiler
> leaves it unevaluated, in the "hope" that a later optimization stage
> could prove the argument constant. And that's the case where __b_c_e
> will then break, because that can't be left unevaluated for very long -
> the very _type_ of the result depends on which branch is chosen.
>
> tl;dr: there's no "order in which the compiler processes those", __b_c_p
> can get evaluated (folded) early, before __b_c_e inspects it, or be left
> for later stages.

Thanks for the detailed explanation. Checking the actual behavior of
a trivial example, I find that

int f(void)
{
    const int i = 1;
    return __builtin_choose_expr(__builtin_constant_p(i), 1, 2);
}

used to return '2' with gcc-7, which is what I remembered.
With gcc-8 and up as well as any version of clang, it returns '1' now:
https://godbolt.org/z/7eKjbMocb

I have also seen a couple of cases where __builtin_constant_p()
without a __builtin_choose_expr() ended up unexpectedly
returning true when gcc found a code path that it would be constant
(e.g. conditionally initializing a variable to one of two possible
ICEs), but then later turning that back into a non-constant
expression in a later optimization stage. There is probably also
a much more detailed explanation behind those.


        Arnd