[PATCH] minmax: clamp more efficiently by avoiding extra comparison

Jason A. Donenfeld
2022-09-23T10:06:42Z

Currently the clamp algorithm does:

if (val > hi)
val = hi;
if (val < lo)
val = lo;

But since hi > lo by definition, this can be made more efficient with:

if (val > hi)
val = hi;
else if (val < lo)
val = lo;

So fix up the clamp and clamp_t functions to do this, adding the same
argument checking as for min and min_t.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
include/linux/minmax.h | 20 ++++++++++++++++++--
1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index 5433c08fcc68..0153c9eba013 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -19,42 +19,58 @@
#define __typecheck(x, y) \
(!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))

#define __no_side_effects(x, y) \
(__is_constexpr(x) && __is_constexpr(y))

#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_once(x, y, unique_x, unique_y, op) ({ \
typeof(x) unique_x = (x); \
typeof(y) unique_y = (y); \
__cmp(unique_x, unique_y, op); })

#define __careful_cmp(x, y, op) \
__builtin_choose_expr(__safe_cmp(x, y), \
__cmp(x, y, op), \
__cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))

+#define __clamp(val, lo, hi) \
+ ((val) >= (hi) ? (hi) : ((val) <= (lo) ? (lo) : (val)))
+
+#define __clamp_once(val, lo, hi, unique_val, unique_lo, unique_hi) ({ \
+ typeof(val) unique_val = (val); \
+ typeof(lo) unique_lo = (lo); \
+ typeof(hi) unique_hi = (hi); \
+ __clamp(unique_val, unique_lo, unique_hi); })
+
+#define __careful_clamp(val, lo, hi) \
+ __builtin_choose_expr(__typecheck(val, lo) && __typecheck(val, hi) && \
+ __is_constexpr(val) && \
+ __is_constexpr(lo) && __is_constexpr(hi), \
+ __clamp(val, lo, hi), \
+ __clamp_once(val, lo, hi, __UNIQUE_ID(__val), __UNIQUE_ID(__lo), __UNIQUE_ID(__hi)))
+
/**
* min - return minimum of two values of the same or compatible types
* @x: first value
* @y: second value
*/
#define min(x, y) __careful_cmp(x, y, <)

/**
* max - return maximum of two values of the same or compatible types
* @x: first value
* @y: second value
*/
#define max(x, y) __careful_cmp(x, y, >)

/**
* min3 - return minimum of three values
* @x: first value
* @y: second value
* @z: third value
*/
#define min3(x, y, z) min((typeof(x))min(x, y), z)
@@ -68,78 +84,78 @@
#define max3(x, y, z) max((typeof(x))max(x, y), z)

/**
* min_not_zero - return the minimum that is _not_ zero, unless both are zero
* @x: value1
* @y: value2
*/
#define min_not_zero(x, y) ({ \
typeof(x) __x = (x); \
typeof(y) __y = (y); \
__x == 0 ? __y : ((__y == 0) ? __x : min(__x, __y)); })

/**
* clamp - return a value clamped to a given range with strict typechecking
* @val: current value
* @lo: lowest allowable value
* @hi: highest allowable value
*
* This macro does strict typechecking of @lo/@hi to make sure they are of the
* same type as @val. See the unnecessary pointer comparisons.
*/
-#define clamp(val, lo, hi) min((typeof(val))max(val, lo), hi)
+#define clamp(val, lo, hi) __careful_clamp(val, lo, hi)

/*
* ..and if you can't take the strict
* types, you can specify one yourself.
*
* Or not use min/max/clamp at all, of course.
*/

/**
* min_t - return minimum of two values, using the specified type
* @type: data type to use
* @x: first value
* @y: second value
*/
#define min_t(type, x, y) __careful_cmp((type)(x), (type)(y), <)

/**
* max_t - return maximum of two values, using the specified type
* @type: data type to use
* @x: first value
* @y: second value
*/
#define max_t(type, x, y) __careful_cmp((type)(x), (type)(y), >)

/**
* clamp_t - return a value clamped to a given range using a given type
* @type: the type of variable to use
* @val: current value
* @lo: minimum allowable value
* @hi: maximum allowable value
*
* This macro does no typechecking and uses temporary variables of type
* @type to make all the comparisons.
*/
-#define clamp_t(type, val, lo, hi) min_t(type, max_t(type, val, lo), hi)
+#define clamp_t(type, val, lo, hi) __careful_clamp((type)(val), (type)(lo), (type)(hi))

/**
* clamp_val - return a value clamped to a given range using val's type
* @val: current value
* @lo: minimum allowable value
* @hi: maximum allowable value
*
* This macro does no typechecking and uses temporary variables of whatever
* type the input argument @val is. This is useful when @val is an unsigned
* type and @lo and @hi are literals that will otherwise be assigned a signed
* integer type.
*/
#define clamp_val(val, lo, hi) clamp_t(typeof(val), val, lo, hi)

/**
* swap - swap values of @a and @b
* @a: first value
* @b: second value
*/
#define swap(a, b) \
do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
--
2.37.3