[PATCH v2] minmax: clamp more efficiently by avoiding extra comparison

Jason A. Donenfeld
2022-09-23T15:42:10Z

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 | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index 5433c08fcc68..30e2e2cd0f44 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -37,6 +37,27 @@
__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 __clamp_input_check(lo, hi) \
+ (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
+ __is_constexpr((lo) > (hi)), (lo) > (hi), false)))
+
+#define __careful_clamp(val, lo, hi) ({ \
+ __clamp_input_check(lo, hi) + \
+ __builtin_choose_expr(__typecheck(val, lo) && __typecheck(val, hi) && \
+ __typecheck(hi, lo) && __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
@@ -86,7 +107,7 @@
* 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
@@ -121,7 +142,7 @@
* 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
--
2.37.3