diff --git a/libc/include/sys/param.h b/libc/include/sys/param.h
index 5cde4b7..16fed86 100644
--- a/libc/include/sys/param.h
+++ b/libc/include/sys/param.h
@@ -51,8 +51,17 @@
 #endif
 #define roundup(x, y)   ((((x)+((y)-1))/(y))*(y))
 
-/** Returns true if the argument is a power of two. */
-#define powerof2(x) ((((x)-1)&(x))==0)
+/*
+ * Returns true if the binary representation of the argument is all zeros
+ * or has exactly one bit set. Contrary to the macro name, this macro
+ * DOES NOT determine if the provided value is a power of 2. In particular,
+ * this function falsely returns true for powerof2(0) and some negative
+ * numbers.
+ */
+#define powerof2(x) \
+  ({ __typeof__(x) _x = (x); \
+     __typeof__(x) _x2; \
+     __builtin_add_overflow(_x, -1, &_x2) ? 1 : ((_x2&_x) == 0 ); })
 
 /** Returns the lesser of its two arguments. */
 #define MIN(a,b) (((a)<(b))?(a):(b))