diff --git a/libc/test/src/stdbit/stdc_count_ones_ul_test.cpp b/libc/test/src/stdbit/stdc_count_ones_ul_test.cpp
@@ -0,0 +1,21 @@
+//===-- Unittests for stdc_count_ones_ul ----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/CPP/limits.h"
+#include "src/stdbit/stdc_count_ones_ul.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdcCountOnesUlTest, Zero) {
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_count_ones_ul(0UL), 0U);
+}
+
+TEST(LlvmLibcStdcCountOnesUlTest, Ones) {
+  for (unsigned i = 0U; i != ULONG_WIDTH; ++i)
+    EXPECT_EQ(LIBC_NAMESPACE::stdc_count_ones_ul(ULONG_MAX >> i),
+              ULONG_WIDTH - i);
+}
diff --git a/libc/test/src/stdbit/stdc_count_ones_ull_test.cpp b/libc/test/src/stdbit/stdc_count_ones_ull_test.cpp
@@ -0,0 +1,21 @@
+//===-- Unittests for stdc_count_ones_ull ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/CPP/limits.h"
+#include "src/stdbit/stdc_count_ones_ull.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdcCountOnesUllTest, Zero) {
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_count_ones_ull(0ULL), 0U);
+}
+
+TEST(LlvmLibcStdcCountOnesUllTest, Ones) {
+  for (unsigned i = 0U; i != ULLONG_WIDTH; ++i)
+    EXPECT_EQ(LIBC_NAMESPACE::stdc_count_ones_ull(ULLONG_MAX >> i),
+              ULLONG_WIDTH - i);
+}
diff --git a/libc/test/src/stdbit/stdc_count_ones_us_test.cpp b/libc/test/src/stdbit/stdc_count_ones_us_test.cpp
@@ -0,0 +1,21 @@
+//===-- Unittests for stdc_count_ones_us ----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/CPP/limits.h"
+#include "src/stdbit/stdc_count_ones_us.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdcCountOnesUiTest, Zero) {
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_count_ones_us(0), 0U);
+}
+
+TEST(LlvmLibcStdcCountOnesUsTest, Ones) {
+  for (unsigned i = 0U; i != USHRT_WIDTH; ++i)
+    EXPECT_EQ(LIBC_NAMESPACE::stdc_count_ones_us(USHRT_MAX >> i),
+              USHRT_WIDTH - i);
+}