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Subject[PATCH 1/1] minmax.h: Slightly relax the type checking done by min() and max().
Date
Slightly relax the type checking done by min() and max().
- Promote signed/unsiged char/short to int prior to the type test.
This matches what the compiler does before doing the comparison.
- Skip the type test if either argument is a positive 'int' constant.
Instead cast the constant to 'int', the compiler may promote it
back to 'unsigned int' when doing the test.

Reduces the need to use min_t/max_t() and the possibly unwanted
side effects if a type that is too small is specified.

Signed-off-by: David Laight <David.Laight@aculab.com>
---
include/linux/minmax.h | 24 +++++++++++++++++++-----
1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index 5433c08fcc68..77de234cf502 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -9,15 +9,28 @@
*
* - avoid multiple evaluations of the arguments (so side-effects like
* "x++" happen only once) when non-constant.
- * - perform strict type-checking (to generate warnings instead of
- * nasty runtime surprises). See the "unnecessary" pointer comparison
- * in __typecheck().
+ * - perform type-checking (to generate warnings instead of nasty runtime
+ * surprises). See the "unnecessary" pointer comparison in __typecheck().
* - retain result as a constant expressions when called with only
* constant expressions (to avoid tripping VLA warnings in stack
* allocation usage).
+ *
+ * The type-check need not be strict in all cases:
+ * - char and short can be promoted to int.
+ * - comparisons against non-negative constant integers can be done by
+ * casting the constant to (int).
*/
+#define __is_constzero(x) \
+ (sizeof(*(1 ? ((void *)((long)(x))) : (int *)0)) == sizeof(void *))
+
+#define __is_positive_int(x) __is_constzero((long)(x) >> 31)
+#define __maybe_int_cast(x) __builtin_choose_expr(__is_positive_int(x), (int)(long)(x), x)
+
+#define __typecheck_ptr_type(x, y) \
+ (1 ? (void *)!(__is_positive_int(x) || __is_positive_int(y)) : (typeof((x) + 0) *)0)
+
#define __typecheck(x, y) \
- (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
+ (!!(sizeof(__typecheck_ptr_type(x, y) == __typecheck_ptr_type(y, x))))

#define __no_side_effects(x, y) \
(__is_constexpr(x) && __is_constexpr(y))
@@ -25,7 +38,8 @@
#define __safe_cmp(x, y) \
(__typecheck(x, y) && __no_side_effects(x, y))

-#define __cmp(x, y, op) ((x) op (y) ? (x) : (y))
+#define __cmp_maybe_int(x, y, op) ((x) op (y) ? (x) : (y))
+#define __cmp(x, y, op) __cmp_maybe_int(__maybe_int_cast(x), __maybe_int_cast(y), op)

#define __cmp_once(x, y, unique_x, unique_y, op) ({ \
typeof(x) unique_x = (x); \
--
2.17.1
-
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