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From: David Laight <David.Laight@ACULAB.COM>
To: "'linux-kernel@vger.kernel.org'" <linux-kernel@vger.kernel.org>, "'Linus
 Torvalds'" <torvalds@linux-foundation.org>, 'Netdev'
	<netdev@vger.kernel.org>, "'dri-devel@lists.freedesktop.org'"
	<dri-devel@lists.freedesktop.org>
CC: 'Andy Shevchenko' <andriy.shevchenko@linux.intel.com>, 'Andrew Morton'
	<akpm@linux-foundation.org>, "'Matthew Wilcox (Oracle)'"
	<willy@infradead.org>, 'Christoph Hellwig' <hch@infradead.org>, "'Dan
 Carpenter'" <dan.carpenter@linaro.org>, 'Linus Walleij'
	<linus.walleij@linaro.org>, "'David S . Miller'" <davem@davemloft.net>,
	"'linux-btrfs@vger.kernel.org'" <linux-btrfs@vger.kernel.org>, 'Jens Axboe'
	<axboe@kernel.dk>
Subject: RE: [PATCH next 03/11] minmax: Simplify signedness check
Thread-Topic: [PATCH next 03/11] minmax: Simplify signedness check
Thread-Index: AdpSIB/Zp5Jfu0DYRYSLz+RLoj6R6Q==
Date: Sun, 28 Jan 2024 19:27:51 +0000
Message-ID: <760fed5020ac42419cb0c3579ba1ed6c@AcuMS.aculab.com>
References: <0ca26166dd2a4ff5a674b84704ff1517@AcuMS.aculab.com>
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It is enough to check that both 'x' and 'y' are valid for either
a signed compare or an unsigned compare.
For unsigned they must be an unsigned type or a positive constant.
For signed they must be signed after unsigned char/short are promoted.

The predicate for _Static_assert() only needs to be a compile-time
constant not a constant integeger expression.
In particular the short-circuit evaluation of || && ?: can be used
to avoid the non-constantness of (pointer_type)1 in is_signed_type().

The '+ 0' in '(x) + 0 > =3D 0' is there to convert 'bool' to 'int'
and avoid a compiler warning because max() gets used for 'bool'
in one place (a very expensive 'or').
(The code is optimised away by two earlier checks - but the compiler
still bleats.)

Signed-off-by: David Laight <david.laight@aculab.com>
---
 include/linux/minmax.h | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index 900eec7a28e5..c32b4b40ce01 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -8,7 +8,7 @@
 #include <linux/types.h>
=20
 /*
- * min()/max()/clamp() macros must accomplish three things:
+ * min()/max()/clamp() macros must accomplish several things:
  *
  * - Avoid multiple evaluations of the arguments (so side-effects like
  *   "x++" happen only once) when non-constant.
@@ -26,19 +26,17 @@
 #define __typecheck(x, y) \
 =09(!!(sizeof((typeof(x) *)1 =3D=3D (typeof(y) *)1)))
=20
-/* is_signed_type() isn't a constexpr for pointer types */
-#define __is_signed(x) =09=09=09=09=09=09=09=09\
-=09__builtin_choose_expr(__is_constexpr(is_signed_type(typeof(x))),=09\
-=09=09is_signed_type(typeof(x)), 0)
+/* Allow unsigned compares against non-negative signed constants. */
+#define __is_ok_unsigned(x) \
+=09(is_unsigned_type(typeof(x)) || (__is_constexpr(x) ? (x) + 0 >=3D 0 : 0=
))
=20
-/* True for a non-negative signed int constant */
-#define __is_noneg_int(x)=09\
-=09(__builtin_choose_expr(__is_constexpr(x) && __is_signed(x), x, -1) >=3D=
 0)
+/* Check for signed after promoting unsigned char/short to int */
+#define __is_ok_signed(x) is_signed_type(typeof((x) + 0))
=20
-#define __types_ok(x, y) =09=09=09=09=09\
-=09(__is_signed(x) =3D=3D __is_signed(y) ||=09=09=09\
-=09=09__is_signed((x) + 0) =3D=3D __is_signed((y) + 0) ||=09\
-=09=09__is_noneg_int(x) || __is_noneg_int(y))
+/* Allow if both x and y are valid for either signed or unsigned compares.=
 */
+#define __types_ok(x, y)=09=09=09=09\
+=09((__is_ok_signed(x) && __is_ok_signed(y)) ||=09\
+=09 (__is_ok_unsigned(x) && __is_ok_unsigned(y)))
=20
 #define __cmp_op_min <
 #define __cmp_op_max >
--=20
2.17.1

-
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