Messages in this thread | | | From | Masahiro Yamada <> | Date | Wed, 7 Aug 2019 00:19:36 +0900 | Subject | Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs |
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Hi Rikard,
On Tue, Aug 6, 2019 at 4:55 AM Rikard Falkeborn <rikard.falkeborn@gmail.com> wrote: > > On Sun, Aug 04, 2019 at 03:45:16PM +0900, Masahiro Yamada wrote: > > On Sun, Aug 4, 2019 at 3:36 AM Rikard Falkeborn > > <rikard.falkeborn@gmail.com> wrote: > > > > > > On Sat, Aug 03, 2019 at 12:12:46PM +0900, Masahiro Yamada wrote: > > > > On Sat, Aug 3, 2019 at 12:03 PM Masahiro Yamada > > > > <yamada.masahiro@socionext.com> wrote: > > > > > > > > > > > > > > BTW, v2 is already inconsistent. > > > > > If you wanted GENMASK_INPUT_CHECK() to return 'unsigned long',, > > > > > you would have to cast (low) > (high) as well: > > > > > > > > > > (unsigned long)((low) > (high)), UL(0)))) > > > > > > > > > > This is totally redundant, and weird. > > > > > > > > I take back this comment. > > > > You added (unsigned long) to the beginning of this macro. > > > > So, the type is consistent, but I believe all casts should be removed. > > > > > > Maybe you're right. BUILD_BUG_ON_ZERO returns size_t regardless of > > > inputs. I was worried that on some platform, size_t would be larger than > > > unsigned long (as far as I could see, the standard does not give any > > > guarantees), and thus all of a sudden GENMASK would be 8 bytes instead > > > of 4, but perhaps that is not a problem? > > > > > > How about adding (int) cast to BUILD_BUG_ON_ZERO() ? > > I'll have a look.
I found a more important problem in this patch.
You used __is_constexpr(), which is defined in <linux/kernel.h>.
This header does not include <linux/kernel.h>, so this header is not self-contained anymore.
The following test code fails to build:
#include <linux/bits.h> unsigned long foo(unsigned long in_bits) { return in_bits & GENMASK(5, 3); }
However, you cannot include <linux/kernel.h> from <linux/bits.h>. See the log of 8bd9cb51daac89337295b6f037b0486911e1b408
This header was split out to not pull in <linux/bitops.h> Including <linux/kernel.h> pulls in <linux/bitops.h> again.
In summary, please use __builtin_constant_p() instead of __is_constexpr().
You can shorten __builtin_constant_p(high) && __builtin_constant_p(low) into __builtin_constant_p((low) > (high)).
How about this?
#define GENMASK_INPUT_CHECK(high, low) \ BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ __builtin_constant_p((low) > (high)), (low) > (high), 0))
-- Best Regards Masahiro Yamada
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