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))
diff --git a/tests/Android.bp b/tests/Android.bp
index ab11d47..408312a 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -153,6 +153,7 @@
         "sys_epoll_test.cpp",
         "sys_mman_test.cpp",
         "sys_msg_test.cpp",
+        "sys_param_test.cpp",
         "sys_personality_test.cpp",
         "sys_prctl_test.cpp",
         "sys_procfs_test.cpp",
diff --git a/tests/sys_param_test.cpp b/tests/sys_param_test.cpp
new file mode 100644
index 0000000..e4bbf42
--- /dev/null
+++ b/tests/sys_param_test.cpp
@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <gtest/gtest.h>
+#include <sys/param.h>
+
+TEST(sys_param_test, powerof2_positives) {
+  ASSERT_TRUE(powerof2(1));
+  ASSERT_TRUE(powerof2(2));
+  ASSERT_TRUE(powerof2(4));
+  ASSERT_TRUE(powerof2(8));
+  ASSERT_FALSE(powerof2(3));
+  ASSERT_FALSE(powerof2(5));
+  ASSERT_FALSE(powerof2(7));
+  ASSERT_FALSE(powerof2(9));
+  ASSERT_FALSE(powerof2(10));
+}
+
+TEST(sys_param_test, powerof2_zero) {
+  // 0 isn't a power of 2, but for compatibility, we assume it is.
+  ASSERT_TRUE(powerof2(0));
+  uint32_t zero = 0;
+  ASSERT_TRUE(powerof2(zero));
+}
+
+TEST(sys_param_test, powerof2_negatives) {
+  // negative numbers can never be a power of 2, but for compatibility,
+  // we assume they can be.
+  int32_t min32 = INT32_MIN;
+  int64_t min64 = INT64_MIN;
+  ASSERT_TRUE(powerof2(min32));
+  ASSERT_FALSE(powerof2(min32 + 1));
+  ASSERT_TRUE(powerof2(min64));
+  ASSERT_FALSE(powerof2(min64 + 1));
+}