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[98.128.228.193]) by smtp.gmail.com with ESMTPSA id m13sm375534lfr.23.2021.05.20.11.46.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 20 May 2021 11:46:43 -0700 (PDT) From: Rikard Falkeborn X-Google-Original-From: Rikard Falkeborn Date: Thu, 20 May 2021 20:46:40 +0200 To: Ard Biesheuvel , Andrew Morton Cc: Rikard Falkeborn , Arnd Bergmann , Andrew Morton , Andy Shevchenko , Tetsuo Handa , Yury Norov , Linux Kernel Mailing List , Peter Zijlstra Subject: Re: [PATCH] linux/bits.h: Fix compilation error with GENMASK Message-ID: References: <20210511203716.117010-1-rikard.falkeborn@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 12, 2021 at 02:53:27PM +0200, Ard Biesheuvel wrote: > (+ Arnd) > > On Tue, 11 May 2021 at 22:37, Rikard Falkeborn > 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 > > Signed-off-by: Rikard Falkeborn > > --- > > 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 > > #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 > > > > +/* > > + * This returns a constant expression while determining if an argument is > > + * a constant expression, most importantly without evaluating the argument. > > + * Glory to Martin Uecker > > + */ > > +#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 > > + > > /* > > * 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 > > - */ > > -#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 > > #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 > > > > +/* > > + * This returns a constant expression while determining if an argument is > > + * a constant expression, most importantly without evaluating the argument. > > + * Glory to Martin Uecker > > + */ > > +#define __is_constexpr(x) \ > > + (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8))) > > + > > #endif /* _LINUX_CONST_H */ > > -- > > 2.31.1 > > Friendly ping. Rikard