[libc][stdbit] implement stdc_first_trailing_one (C23) (#81768)

24 files changed, 362 insertions, 12 deletions

diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
index d53b225e4d87..57b4a1e0f93d 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -127,6 +127,11 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdbit.stdc_first_trailing_zero_ui
     libc.src.stdbit.stdc_first_trailing_zero_ul
     libc.src.stdbit.stdc_first_trailing_zero_ull
+    libc.src.stdbit.stdc_first_trailing_one_uc
+    libc.src.stdbit.stdc_first_trailing_one_us
+    libc.src.stdbit.stdc_first_trailing_one_ui
+    libc.src.stdbit.stdc_first_trailing_one_ul
+    libc.src.stdbit.stdc_first_trailing_one_ull
 
     # stdlib.h entrypoints
     libc.src.stdlib.abs
diff --git a/libc/docs/stdbit.rst b/libc/docs/stdbit.rst
index 790a747baaa2..3bd83ff70c89 100644
--- a/libc/docs/stdbit.rst
+++ b/libc/docs/stdbit.rst
@@ -66,11 +66,11 @@ stdc_first_trailing_zero_us  |check|
 stdc_first_trailing_zero_ui  |check|
 stdc_first_trailing_zero_ul  |check|
 stdc_first_trailing_zero_ull |check|
-stdc_first_trailing_one_uc
-stdc_first_trailing_one_us
-stdc_first_trailing_one_ui
-stdc_first_trailing_one_ul
-stdc_first_trailing_one_ull
+stdc_first_trailing_one_uc   |check|
+stdc_first_trailing_one_us   |check|
+stdc_first_trailing_one_ui   |check|
+stdc_first_trailing_one_ul   |check|
+stdc_first_trailing_one_ull  |check|
 stdc_count_zeros_uc
 stdc_count_zeros_us
 stdc_count_zeros_ui
@@ -121,7 +121,7 @@ stdc_trailing_ones         |check|
 stdc_first_leading_zero    |check|
 stdc_first_leading_one     |check|
 stdc_first_trailing_zero   |check|
-stdc_first_trailing_one
+stdc_first_trailing_one    |check|
 stdc_count_zeros
 stdc_count_ones
 stdc_has_single_bit
diff --git a/libc/include/llvm-libc-macros/stdbit-macros.h b/libc/include/llvm-libc-macros/stdbit-macros.h
index 0ec8aef10d3e..0c97da96ebba 100644
--- a/libc/include/llvm-libc-macros/stdbit-macros.h
+++ b/libc/include/llvm-libc-macros/stdbit-macros.h
@@ -116,6 +116,21 @@ inline unsigned stdc_first_trailing_zero(unsigned long x) {
 inline unsigned stdc_first_trailing_zero(unsigned long long x) {
   return stdc_first_trailing_zero_ull(x);
 }
+inline unsigned stdc_first_trailing_one(unsigned char x) {
+  return stdc_first_trailing_one_uc(x);
+}
+inline unsigned stdc_first_trailing_one(unsigned short x) {
+  return stdc_first_trailing_one_us(x);
+}
+inline unsigned stdc_first_trailing_one(unsigned x) {
+  return stdc_first_trailing_one_ui(x);
+}
+inline unsigned stdc_first_trailing_one(unsigned long x) {
+  return stdc_first_trailing_one_ul(x);
+}
+inline unsigned stdc_first_trailing_one(unsigned long long x) {
+  return stdc_first_trailing_one_ull(x);
+}
 #else
 #define stdc_leading_zeros(x)                                                  \
   _Generic((x),                                                                \
@@ -166,6 +181,13 @@ inline unsigned stdc_first_trailing_zero(unsigned long long x) {
       unsigned: stdc_first_trailing_zero_ui,                                   \
       unsigned long: stdc_first_trailing_zero_ul,                              \
       unsigned long long: stdc_first_trailing_zero_ull)(x)
+#define stdc_first_trailing_one(x)                                             \
+  _Generic((x),                                                                \
+      unsigned char: stdc_first_trailing_one_uc,                               \
+      unsigned short: stdc_first_trailing_one_us,                              \
+      unsigned: stdc_first_trailing_one_ui,                                    \
+      unsigned long: stdc_first_trailing_one_ul,                               \
+      unsigned long long: stdc_first_trailing_one_ull)(x)
 #endif // __cplusplus
 
 #endif // __LLVM_LIBC_MACROS_STDBIT_MACROS_H
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
index b97e90d1a8ae..9ed94638f522 100644
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -783,7 +783,8 @@ def StdC : StandardSpec<"stdc"> {
         Macro<"stdc_trailing_ones">,
         Macro<"stdc_first_leading_zero">,
         Macro<"stdc_first_leading_one">,
-        Macro<"stdc_first_trailing_zero">
+        Macro<"stdc_first_trailing_zero">,
+        Macro<"stdc_first_trailing_one">
       ], // Macros
       [], // Types
       [], // Enumerations
@@ -818,11 +819,11 @@ def StdC : StandardSpec<"stdc"> {
           FunctionSpec<"stdc_first_leading_one_ui", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedIntType>]>,
           FunctionSpec<"stdc_first_leading_one_ul", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongType>]>,
           FunctionSpec<"stdc_first_leading_one_ull", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongLongType>]>,
-          FunctionSpec<"stdc_first_trailing_zero_uc", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedCharType>]>,
-          FunctionSpec<"stdc_first_trailing_zero_us", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedShortType>]>,
-          FunctionSpec<"stdc_first_trailing_zero_ui", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedIntType>]>,
-          FunctionSpec<"stdc_first_trailing_zero_ul", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongType>]>,
-          FunctionSpec<"stdc_first_trailing_zero_ull", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongLongType>]>,
+          FunctionSpec<"stdc_first_trailing_one_uc", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedCharType>]>,
+          FunctionSpec<"stdc_first_trailing_one_us", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedShortType>]>,
+          FunctionSpec<"stdc_first_trailing_one_ui", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedIntType>]>,
+          FunctionSpec<"stdc_first_trailing_one_ul", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongType>]>,
+          FunctionSpec<"stdc_first_trailing_one_ull", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongLongType>]>
       ] // Functions
   >;
 
diff --git a/libc/src/__support/CPP/bit.h b/libc/src/__support/CPP/bit.h
index 5afcc875752c..f5e50262371f 100644
--- a/libc/src/__support/CPP/bit.h
+++ b/libc/src/__support/CPP/bit.h
@@ -243,6 +243,11 @@ template <typename T, typename = cpp::enable_if_t<cpp::is_unsigned_v<T>>>
              : countr_zero(static_cast<T>(~value)) + 1;
 }
 
+template <typename T, typename = cpp::enable_if_t<cpp::is_unsigned_v<T>>>
+[[nodiscard]] LIBC_INLINE constexpr int first_trailing_one(T value) {
+  return value == cpp::numeric_limits<T>::max() ? 0 : countr_zero(value) + 1;
+}
+
 } // namespace LIBC_NAMESPACE::cpp
 
 #endif // LLVM_LIBC_SRC___SUPPORT_CPP_BIT_H
diff --git a/libc/src/stdbit/CMakeLists.txt b/libc/src/stdbit/CMakeLists.txt
index fa68d4a90dbb..14cc26e206e0 100644
--- a/libc/src/stdbit/CMakeLists.txt
+++ b/libc/src/stdbit/CMakeLists.txt
@@ -6,6 +6,7 @@ set(prefixes
   first_leading_zero
   first_leading_one
   first_trailing_zero
+  first_trailing_one
 )
 set(suffixes c s i l ll)
 foreach(prefix IN LISTS prefixes)
diff --git a/libc/src/stdbit/stdc_first_trailing_one_uc.cpp b/libc/src/stdbit/stdc_first_trailing_one_uc.cpp
new file mode 100644
index 000000000000..6ed35966be61
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_uc.cpp
@@ -0,0 +1,21 @@
+//===-- Implementation of stdc_first_trailing_one_uc ----------------------===//
+//
+// 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/stdbit/stdc_first_trailing_one_uc.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_trailing_one_uc,
+                   (unsigned char value)) {
+  return static_cast<unsigned>(cpp::first_trailing_one(value));
+}
+
+} // namespace LIBC_NAMESPACE
diff --git a/libc/src/stdbit/stdc_first_trailing_one_uc.h b/libc/src/stdbit/stdc_first_trailing_one_uc.h
new file mode 100644
index 000000000000..d733ce850de0
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_uc.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_trailing_one_uc ---*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_UC_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_UC_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_trailing_one_uc(unsigned char value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_UC_H
diff --git a/libc/src/stdbit/stdc_first_trailing_one_ui.cpp b/libc/src/stdbit/stdc_first_trailing_one_ui.cpp
new file mode 100644
index 000000000000..a89083bd4950
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_ui.cpp
@@ -0,0 +1,20 @@
+//===-- Implementation of stdc_first_trailing_one_ui ----------------------===//
+//
+// 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/stdbit/stdc_first_trailing_one_ui.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_trailing_one_ui, (unsigned value)) {
+  return static_cast<unsigned>(cpp::first_trailing_one(value));
+}
+
+} // namespace LIBC_NAMESPACE
diff --git a/libc/src/stdbit/stdc_first_trailing_one_ui.h b/libc/src/stdbit/stdc_first_trailing_one_ui.h
new file mode 100644
index 000000000000..6a6a5046709a
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_ui.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_trailing_one_ui ---*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_UI_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_UI_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_trailing_one_ui(unsigned value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_UI_H
diff --git a/libc/src/stdbit/stdc_first_trailing_one_ul.cpp b/libc/src/stdbit/stdc_first_trailing_one_ul.cpp
new file mode 100644
index 000000000000..f30078d0f5ff
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_ul.cpp
@@ -0,0 +1,21 @@
+//===-- Implementation of stdc_first_trailing_one_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/stdbit/stdc_first_trailing_one_ul.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_trailing_one_ul,
+                   (unsigned long value)) {
+  return static_cast<unsigned>(cpp::first_trailing_one(value));
+}
+
+} // namespace LIBC_NAMESPACE
diff --git a/libc/src/stdbit/stdc_first_trailing_one_ul.h b/libc/src/stdbit/stdc_first_trailing_one_ul.h
new file mode 100644
index 000000000000..09b6a9bbbe34
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_ul.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_trailing_one_ul ---*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_UL_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_UL_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_trailing_one_ul(unsigned long value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_UL_H
diff --git a/libc/src/stdbit/stdc_first_trailing_one_ull.cpp b/libc/src/stdbit/stdc_first_trailing_one_ull.cpp
new file mode 100644
index 000000000000..2e526a890cda
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_ull.cpp
@@ -0,0 +1,21 @@
+//===-- Implementation of stdc_first_trailing_one_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/stdbit/stdc_first_trailing_one_ull.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_trailing_one_ull,
+                   (unsigned long long value)) {
+  return static_cast<unsigned>(cpp::first_trailing_one(value));
+}
+
+} // namespace LIBC_NAMESPACE
diff --git a/libc/src/stdbit/stdc_first_trailing_one_ull.h b/libc/src/stdbit/stdc_first_trailing_one_ull.h
new file mode 100644
index 000000000000..3e12a1d74566
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_ull.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_trailing_one_ull --*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_ULL_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_ULL_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_trailing_one_ull(unsigned long long value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_ULL_H
diff --git a/libc/src/stdbit/stdc_first_trailing_one_us.cpp b/libc/src/stdbit/stdc_first_trailing_one_us.cpp
new file mode 100644
index 000000000000..e4c88e0d7906
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_us.cpp
@@ -0,0 +1,21 @@
+//===-- Implementation of stdc_first_trailing_one_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/stdbit/stdc_first_trailing_one_us.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_trailing_one_us,
+                   (unsigned short value)) {
+  return static_cast<unsigned>(cpp::first_trailing_one(value));
+}
+
+} // namespace LIBC_NAMESPACE
diff --git a/libc/src/stdbit/stdc_first_trailing_one_us.h b/libc/src/stdbit/stdc_first_trailing_one_us.h
new file mode 100644
index 000000000000..f380898fc68c
--- /dev/null
+++ b/libc/src/stdbit/stdc_first_trailing_one_us.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_trailing_one_us ---*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_US_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_US_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_trailing_one_us(unsigned short value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_TRAILING_ONE_US_H
diff --git a/libc/test/include/stdbit_test.cpp b/libc/test/include/stdbit_test.cpp
index 238c2d44fac4..22d5533df1e8 100644
--- a/libc/test/include/stdbit_test.cpp
+++ b/libc/test/include/stdbit_test.cpp
@@ -64,6 +64,13 @@ unsigned stdc_first_trailing_zero_ul(unsigned long) noexcept { return 0x0DU; }
 unsigned stdc_first_trailing_zero_ull(unsigned long long) noexcept {
   return 0x0FU;
 }
+unsigned stdc_first_trailing_one_uc(unsigned char) noexcept { return 0x1AU; }
+unsigned stdc_first_trailing_one_us(unsigned short) noexcept { return 0x1BU; }
+unsigned stdc_first_trailing_one_ui(unsigned) noexcept { return 0x1CU; }
+unsigned stdc_first_trailing_one_ul(unsigned long) noexcept { return 0x1DU; }
+unsigned stdc_first_trailing_one_ull(unsigned long long) noexcept {
+  return 0x1FU;
+}
 }
 
 #include "include/llvm-libc-macros/stdbit-macros.h"
@@ -123,3 +130,11 @@ TEST(LlvmLibcStdbitTest, TypeGenericMacroFirstTrailingZero) {
   EXPECT_EQ(stdc_first_trailing_zero(0UL), 0x0DU);
   EXPECT_EQ(stdc_first_trailing_zero(0ULL), 0x0FU);
 }
+
+TEST(LlvmLibcStdbitTest, TypeGenericMacroFirstTrailingOne) {
+  EXPECT_EQ(stdc_first_trailing_one(static_cast<unsigned char>(0U)), 0x1AU);
+  EXPECT_EQ(stdc_first_trailing_one(static_cast<unsigned short>(0U)), 0x1BU);
+  EXPECT_EQ(stdc_first_trailing_one(0U), 0x1CU);
+  EXPECT_EQ(stdc_first_trailing_one(0UL), 0x1DU);
+  EXPECT_EQ(stdc_first_trailing_one(0ULL), 0x1FU);
+}
diff --git a/libc/test/src/__support/CPP/bit_test.cpp b/libc/test/src/__support/CPP/bit_test.cpp
index 914c6ca823c8..5d1f451776a5 100644
--- a/libc/test/src/__support/CPP/bit_test.cpp
+++ b/libc/test/src/__support/CPP/bit_test.cpp
@@ -226,4 +226,10 @@ TYPED_TEST(LlvmLibcBitTest, FirstTrailingZero, UnsignedTypes) {
     EXPECT_EQ(first_trailing_zero<T>(~(T(1) << i)), i + 1);
 }
 
+TYPED_TEST(LlvmLibcBitTest, FirstTrailingOne, UnsignedTypes) {
+  EXPECT_EQ(first_trailing_one<T>(cpp::numeric_limits<T>::max()), 0);
+  for (int i = 0U; i != cpp::numeric_limits<T>::digits; ++i)
+    EXPECT_EQ(first_trailing_one<T>(T(1) << i), i + 1);
+}
+
 } // namespace LIBC_NAMESPACE::cpp
diff --git a/libc/test/src/stdbit/CMakeLists.txt b/libc/test/src/stdbit/CMakeLists.txt
index d5896ad83dbf..203f48bda99a 100644
--- a/libc/test/src/stdbit/CMakeLists.txt
+++ b/libc/test/src/stdbit/CMakeLists.txt
@@ -8,6 +8,7 @@ set(prefixes
   first_leading_zero
   first_leading_one
   first_trailing_zero
+  first_trailing_one
 )
 set(suffixes c s i l ll)
 foreach(prefix IN LISTS prefixes)
diff --git a/libc/test/src/stdbit/stdc_first_trailing_one_uc_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_uc_test.cpp
new file mode 100644
index 000000000000..ed2b4921cdad
--- /dev/null
+++ b/libc/test/src/stdbit/stdc_first_trailing_one_uc_test.cpp
@@ -0,0 +1,20 @@
+//===-- Unittests for stdc_first_trailing_one_uc -------------------------===//
+//
+// 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_first_trailing_one_uc.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdcFirstTrailingOneUcTest, ALL) {
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_uc(UCHAR_MAX), 0U);
+}
+
+TEST(LlvmLibcStdcFirstTrailingOneUcTest, OneHot) {
+  for (unsigned i = 0U; i != UCHAR_WIDTH; ++i)
+    EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_uc(1U << i), i + 1);
+}
diff --git a/libc/test/src/stdbit/stdc_first_trailing_one_ui_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_ui_test.cpp
new file mode 100644
index 000000000000..137c8a42e407
--- /dev/null
+++ b/libc/test/src/stdbit/stdc_first_trailing_one_ui_test.cpp
@@ -0,0 +1,20 @@
+//===-- Unittests for stdc_first_trailing_one_ui -------------------------===//
+//
+// 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_first_trailing_one_ui.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdcFirstTrailingOneUiTest, ALL) {
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ui(UINT_MAX), 0U);
+}
+
+TEST(LlvmLibcStdcFirstTrailingOneUiTest, OneHot) {
+  for (unsigned i = 0U; i != UINT_WIDTH; ++i)
+    EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ui(1U << i), i + 1);
+}
diff --git a/libc/test/src/stdbit/stdc_first_trailing_one_ul_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_ul_test.cpp
new file mode 100644
index 000000000000..3fc1f3f16c60
--- /dev/null
+++ b/libc/test/src/stdbit/stdc_first_trailing_one_ul_test.cpp
@@ -0,0 +1,20 @@
+//===-- Unittests for stdc_first_trailing_one_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_first_trailing_one_ul.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdcFirstTrailingOneUlTest, ALL) {
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ul(ULONG_MAX), 0U);
+}
+
+TEST(LlvmLibcStdcFirstTrailingOneUlTest, OneHot) {
+  for (unsigned i = 0U; i != ULONG_WIDTH; ++i)
+    EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ul(1UL << i), i + 1);
+}
diff --git a/libc/test/src/stdbit/stdc_first_trailing_one_ull_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_ull_test.cpp
new file mode 100644
index 000000000000..5719e09a5120
--- /dev/null
+++ b/libc/test/src/stdbit/stdc_first_trailing_one_ull_test.cpp
@@ -0,0 +1,20 @@
+//===-- Unittests for stdc_first_trailing_one_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_first_trailing_one_ull.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdcFirstTrailingOneUllTest, ALL) {
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ull(ULLONG_MAX), 0U);
+}
+
+TEST(LlvmLibcStdcFirstTrailingOneUllTest, OneHot) {
+  for (unsigned i = 0U; i != ULLONG_WIDTH; ++i)
+    EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ull(1ULL << i), i + 1);
+}
diff --git a/libc/test/src/stdbit/stdc_first_trailing_one_us_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_us_test.cpp
new file mode 100644
index 000000000000..60021552310b
--- /dev/null
+++ b/libc/test/src/stdbit/stdc_first_trailing_one_us_test.cpp
@@ -0,0 +1,20 @@
+//===-- Unittests for stdc_first_trailing_one_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_first_trailing_one_us.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdcFirstTrailingOneUsTest, ALL) {
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_us(USHRT_MAX), 0U);
+}
+
+TEST(LlvmLibcStdcFirstTrailingOneUsTest, OneHot) {
+  for (unsigned i = 0U; i != USHRT_WIDTH; ++i)
+    EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_us(1U << i), i + 1);
+}