linux-kernel - RE: [PATCH v4 2/2] linux/bits.h: simplify GENMASK_INPUT

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Message-ID: <8bf9eb4434104a3b960d52bd1d38caea@AcuMS.aculab.com>
Date: Sun, 17 Nov 2024 17:24:54 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Vincent Mailhol' <mailhol.vincent@...adoo.fr>, Linus Torvalds
	<torvalds@...ux-foundation.org>, Yury Norov <yury.norov@...il.com>, "Rasmus
 Villemoes" <linux@...musvillemoes.dk>, Luc Van Oostenryck
	<luc.vanoostenryck@...il.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-sparse@...r.kernel.org" <linux-sparse@...r.kernel.org>, "Rikard
 Falkeborn" <rikard.falkeborn@...il.com>
Subject: RE: [PATCH v4 2/2] linux/bits.h: simplify GENMASK_INPUT_CHECK()

From: Vincent Mailhol
> Sent: 13 November 2024 17:19
> 
> In GENMASK_INPUT_CHECK(),
> 
>   __builtin_choose_expr(__is_constexpr((l) > (h)), (l) > (h), 0)
> 
> is the exact expansion of:
> 
>   const_true((l) > (h))
> 
> Apply const_true() to simplify GENMASK_INPUT_CHECK().

Wouldn't statically_true() give better coverage ?
I wouldn't have though that GENMASK() got used anywhere where a constant
integer expression was needed.

More interesting would be to get it to pass a W=1 build for
any place where 'l' is 0u.

	David

> 
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> ---
> This change passes the unit tests from CONFIG_BITS_TEST, including the
> extra negative tests provided under #ifdef TEST_GENMASK_FAILURES [1].
> 
> [1] commit 6d511020e13d ("lib/test_bits.c: add tests of GENMASK")
> Link: https://git.kernel.org/torvalds/c/6d511020e13d
> ---
>  include/linux/bits.h | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 60044b608817..61a75d3f294b 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -20,9 +20,8 @@
>   */
>  #if !defined(__ASSEMBLY__)
>  #include <linux/build_bug.h>
> -#define GENMASK_INPUT_CHECK(h, l) \
> -	(BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> -		__is_constexpr((l) > (h)), (l) > (h), 0)))
> +#include <linux/compiler.h>
> +#define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(const_true((l) > (h)))
>  #else
>  /*
>   * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
> --
> 2.45.2
> 

-
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