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

David Laight
2024-01-28T19:28:18Z

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(-)

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 >