[PATCH next v2 03/11] minmax: Simplify signedness check

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: 'Jens Axboe' <axboe@kernel.dk>,
	"'Matthew Wilcox (Oracle)'" <willy@infradead.org>,
	'Christoph Hellwig' <hch@infradead.org>,
	"'linux-btrfs@vger.kernel.org'" <linux-btrfs@vger.kernel.org>,
	"'Andrew Morton'" <akpm@linux-foundation.org>,
	'Andy Shevchenko' <andriy.shevchenko@linux.intel.com>,
	"'David S . Miller'" <davem@davemloft.net>,
	'Dan Carpenter' <dan.carpenter@linaro.org>,
	"'Jani Nikula'" <jani.nikula@linux.intel.com>
Subject: [PATCH next v2 03/11] minmax: Simplify signedness check
Date: Sun, 25 Feb 2024 16:49:59 +0000	[thread overview]
Message-ID: <8657dd5c2264456f8a005520a3b90e2b@AcuMS.aculab.com> (raw)
In-Reply-To: <0fff52305e584036a777f440b5f474da@AcuMS.aculab.com>

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 > = 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(-)

Changes for v2:
- Typographical and spelling corrections to the commit messages.
  Patches unchanged.

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>
 
 /*
- * 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) \
 	(!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
 
-/* is_signed_type() isn't a constexpr for pointer types */
-#define __is_signed(x) 								\
-	__builtin_choose_expr(__is_constexpr(is_signed_type(typeof(x))),	\
-		is_signed_type(typeof(x)), 0)
+/* Allow unsigned compares against non-negative signed constants. */
+#define __is_ok_unsigned(x) \
+	(is_unsigned_type(typeof(x)) || (__is_constexpr(x) ? (x) + 0 >= 0 : 0))
 
-/* True for a non-negative signed int constant */
-#define __is_noneg_int(x)	\
-	(__builtin_choose_expr(__is_constexpr(x) && __is_signed(x), x, -1) >= 0)
+/* Check for signed after promoting unsigned char/short to int */
+#define __is_ok_signed(x) is_signed_type(typeof((x) + 0))
 
-#define __types_ok(x, y) 					\
-	(__is_signed(x) == __is_signed(y) ||			\
-		__is_signed((x) + 0) == __is_signed((y) + 0) ||	\
-		__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)				\
+	((__is_ok_signed(x) && __is_ok_signed(y)) ||	\
+	 (__is_ok_unsigned(x) && __is_ok_unsigned(y)))
 
 #define __cmp_op_min <
 #define __cmp_op_max >
-- 
2.17.1

-
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