From: Rikard Falkeborn <rikard.falkeborn@gmail.com>
To: Ard Biesheuvel <ardb@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>
Cc: Rikard Falkeborn <rikard.falkeborn@gmail.com>,
Arnd Bergmann <arnd@arndb.de>,
Andrew Morton <akpm@linux-foundation.org>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
Yury Norov <yury.norov@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] linux/bits.h: Fix compilation error with GENMASK
Date: Thu, 20 May 2021 20:46:40 +0200 [thread overview]
Message-ID: <YKaukFKw4KTmnOVd@rikard> (raw)
In-Reply-To: <CAMj1kXF0rMwjgm27=i3XkrXJ=21C_x4he5Ls+7FSKUhsva970Q@mail.gmail.com>
On Wed, May 12, 2021 at 02:53:27PM +0200, Ard Biesheuvel wrote:
> (+ Arnd)
>
> On Tue, 11 May 2021 at 22:37, Rikard Falkeborn
> <rikard.falkeborn@gmail.com> wrote:
> >
> > GENMASK() has an input check which uses __builtin_choose_expr() to enable
> > a compile time sanity check of its inputs if they are known at compile
> > time. However, it turns out that __builtin_constant_p() does not always
> > return a compile time constant [0]. It was thought this problem was fixed
> > with gcc 4.9 [1], but apparently this is not the case [2].
> >
> > Switch to use __is_constexpr() instead which always returns a compile
> > time constant, regardless of its inputs.
> >
> > [0]: https://lore.kernel.org/lkml/42b4342b-aefc-a16a-0d43-9f9c0d63ba7a@rasmusvillemoes.dk
> > [1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449
> > [2]: https://lore.kernel.org/lkml/1ac7bbc2-45d9-26ed-0b33-bf382b8d858b@I-love.SAKURA.ne.jp
> >
> > Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
> > ---
> > Feedback on placing __is_constexpr() in const.h is welcome, at least the
> > name is appropriate...
> >
> > include/linux/bits.h | 2 +-
> > include/linux/const.h | 8 ++++++++
> > include/linux/minmax.h | 10 ++--------
> > tools/include/linux/bits.h | 2 +-
> > tools/include/linux/const.h | 8 ++++++++
> > 5 files changed, 20 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/linux/bits.h b/include/linux/bits.h
> > index 7f475d59a097..87d112650dfb 100644
> > --- a/include/linux/bits.h
> > +++ b/include/linux/bits.h
> > @@ -22,7 +22,7 @@
> > #include <linux/build_bug.h>
> > #define GENMASK_INPUT_CHECK(h, l) \
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > - __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> > + __is_constexpr((l) > (h)), (l) > (h), 0)))
> > #else
> > /*
> > * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
> > diff --git a/include/linux/const.h b/include/linux/const.h
> > index 81b8aae5a855..435ddd72d2c4 100644
> > --- a/include/linux/const.h
> > +++ b/include/linux/const.h
> > @@ -3,4 +3,12 @@
> >
> > #include <vdso/const.h>
> >
> > +/*
> > + * This returns a constant expression while determining if an argument is
> > + * a constant expression, most importantly without evaluating the argument.
> > + * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
> > + */
> > +#define __is_constexpr(x) \
> > + (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
> > +
> > #endif /* _LINUX_CONST_H */
> > diff --git a/include/linux/minmax.h b/include/linux/minmax.h
> > index c0f57b0c64d9..5433c08fcc68 100644
> > --- a/include/linux/minmax.h
> > +++ b/include/linux/minmax.h
> > @@ -2,6 +2,8 @@
> > #ifndef _LINUX_MINMAX_H
> > #define _LINUX_MINMAX_H
> >
> > +#include <linux/const.h>
> > +
> > /*
> > * min()/max()/clamp() macros must accomplish three things:
> > *
> > @@ -17,14 +19,6 @@
> > #define __typecheck(x, y) \
> > (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
> >
> > -/*
> > - * This returns a constant expression while determining if an argument is
> > - * a constant expression, most importantly without evaluating the argument.
> > - * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
> > - */
> > -#define __is_constexpr(x) \
> > - (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
> > -
> > #define __no_side_effects(x, y) \
> > (__is_constexpr(x) && __is_constexpr(y))
> >
> > diff --git a/tools/include/linux/bits.h b/tools/include/linux/bits.h
> > index 7f475d59a097..87d112650dfb 100644
> > --- a/tools/include/linux/bits.h
> > +++ b/tools/include/linux/bits.h
> > @@ -22,7 +22,7 @@
> > #include <linux/build_bug.h>
> > #define GENMASK_INPUT_CHECK(h, l) \
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > - __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> > + __is_constexpr((l) > (h)), (l) > (h), 0)))
> > #else
> > /*
> > * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
> > diff --git a/tools/include/linux/const.h b/tools/include/linux/const.h
> > index 81b8aae5a855..435ddd72d2c4 100644
> > --- a/tools/include/linux/const.h
> > +++ b/tools/include/linux/const.h
> > @@ -3,4 +3,12 @@
> >
> > #include <vdso/const.h>
> >
> > +/*
> > + * This returns a constant expression while determining if an argument is
> > + * a constant expression, most importantly without evaluating the argument.
> > + * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
> > + */
> > +#define __is_constexpr(x) \
> > + (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
> > +
> > #endif /* _LINUX_CONST_H */
> > --
> > 2.31.1
> >
Friendly ping.
Rikard
next prev parent reply other threads:[~2021-05-20 18:46 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-11 20:37 [PATCH] linux/bits.h: Fix compilation error with GENMASK Rikard Falkeborn
2021-05-12 12:53 ` Ard Biesheuvel
2021-05-20 18:46 ` Rikard Falkeborn [this message]
2021-05-20 19:44 ` Yury Norov
2021-05-20 20:15 ` Arnd Bergmann
2021-05-20 20:41 ` Andrew Morton
2021-05-21 11:59 ` Andy Shevchenko
2021-05-21 10:15 ` Andy Shevchenko
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