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

[Jason A. Donenfeld]

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@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 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