[libc][stdbit] implement stdc_first_leading_zero (C23) (#81340)

libc/config/linux/x86_64/entrypoints.txt

@@ -112,6 +112,11 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdbit.stdc_trailing_ones_ui
     libc.src.stdbit.stdc_trailing_ones_ul
     libc.src.stdbit.stdc_trailing_ones_ull
+    libc.src.stdbit.stdc_first_leading_zero_uc
+    libc.src.stdbit.stdc_first_leading_zero_us
+    libc.src.stdbit.stdc_first_leading_zero_ui
+    libc.src.stdbit.stdc_first_leading_zero_ul
+    libc.src.stdbit.stdc_first_leading_zero_ull
 
     # stdlib.h entrypoints
     libc.src.stdlib.abs

libc/include/llvm-libc-macros/stdbit-macros.h

@@ -71,6 +71,21 @@ inline unsigned stdc_trailing_ones(unsigned long x) {
 inline unsigned stdc_trailing_ones(unsigned long long x) {
   return stdc_trailing_ones_ull(x);
 }
+inline unsigned stdc_first_leading_zero(unsigned char x) {
+  return stdc_first_leading_zero_uc(x);
+}
+inline unsigned stdc_first_leading_zero(unsigned short x) {
+  return stdc_first_leading_zero_us(x);
+}
+inline unsigned stdc_first_leading_zero(unsigned x) {
+  return stdc_first_leading_zero_ui(x);
+}
+inline unsigned stdc_first_leading_zero(unsigned long x) {
+  return stdc_first_leading_zero_ul(x);
+}
+inline unsigned stdc_first_leading_zero(unsigned long long x) {
+  return stdc_first_leading_zero_ull(x);
+}
 #else
 #define stdc_leading_zeros(x)                                                  \
   _Generic((x),                                                                \
@@ -100,6 +115,13 @@ inline unsigned stdc_trailing_ones(unsigned long long x) {
       unsigned: stdc_trailing_ones_ui,                                         \
       unsigned long: stdc_trailing_ones_ul,                                    \
       unsigned long long: stdc_trailing_ones_ull)(x)
+#define stdc_first_leading_zero(x)                                             \
+  _Generic((x),                                                                \
+      unsigned char: stdc_first_leading_zero_uc,                               \
+      unsigned short: stdc_first_leading_zero_us,                              \
+      unsigned: stdc_first_leading_zero_ui,                                    \
+      unsigned long: stdc_first_leading_zero_ul,                               \
+      unsigned long long: stdc_first_leading_zero_ull)(x)
 #endif // __cplusplus
 
 #endif // __LLVM_LIBC_MACROS_STDBIT_MACROS_H

libc/spec/stdc.td

@@ -780,7 +780,8 @@ def StdC : StandardSpec<"stdc"> {
         Macro<"stdc_leading_zeros">,
         Macro<"stdc_leading_ones">,
         Macro<"stdc_trailing_zeros">,
-        Macro<"stdc_trailing_ones">
+        Macro<"stdc_trailing_ones">,
+        Macro<"stdc_first_leading_zero">
       ], // Macros
       [], // Types
       [], // Enumerations
@@ -804,7 +805,12 @@ def StdC : StandardSpec<"stdc"> {
           FunctionSpec<"stdc_trailing_ones_us", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedShortType>]>,
           FunctionSpec<"stdc_trailing_ones_ui", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedIntType>]>,
           FunctionSpec<"stdc_trailing_ones_ul", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongType>]>,
-          FunctionSpec<"stdc_trailing_ones_ull", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongLongType>]>
+          FunctionSpec<"stdc_trailing_ones_ull", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongLongType>]>,
+          FunctionSpec<"stdc_first_leading_zero_uc", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedCharType>]>,
+          FunctionSpec<"stdc_first_leading_zero_us", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedShortType>]>,
+          FunctionSpec<"stdc_first_leading_zero_ui", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedIntType>]>,
+          FunctionSpec<"stdc_first_leading_zero_ul", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongType>]>,
+          FunctionSpec<"stdc_first_leading_zero_ull", RetValSpec<UnsignedIntType>, [ArgSpec<UnsignedLongLongType>]>
       ] // Functions
   >;
 

libc/src/__support/CPP/bit.h

@@ -238,6 +238,36 @@ LIBC_INLINE constexpr To bit_or_static_cast(const From &from) {
   }
 }
 
+#define SPECIALIZE_FLZ(NAME, TYPE, BUILTIN)                                    \
+  template <> [[nodiscard]] LIBC_INLINE constexpr int NAME<TYPE>(TYPE value) { \
+    static_assert(cpp::is_unsigned_v<TYPE>);                                   \
+    return value == cpp::numeric_limits<TYPE>::max()                           \
+               ? 0                                                             \
+               : BUILTIN(static_cast<TYPE>(~value)) + 1;                       \
+  }
+
+template <typename T, typename = cpp::enable_if_t<cpp::is_unsigned_v<T>>>
+[[nodiscard]] LIBC_INLINE constexpr int first_leading_zero(T value) {
+  return value == cpp::numeric_limits<T>::max()
+             ? 0
+             : countl_zero(static_cast<T>(~value)) + 1;
+}
+
+#if LIBC_HAS_BUILTIN(__builtin_clzs)
+SPECIALIZE_FLZ(first_leading_zero, unsigned short, __builtin_clzs)
+#endif
+#if LIBC_HAS_BUILTIN(__builtin_clz)
+SPECIALIZE_FLZ(first_leading_zero, unsigned int, __builtin_clz)
+#endif
+#if LIBC_HAS_BUILTIN(__builtin_clzl)
+SPECIALIZE_FLZ(first_leading_zero, unsigned long, __builtin_clzl)
+#endif
+#if LIBC_HAS_BUILTIN(__builtin_clzll)
+SPECIALIZE_FLZ(first_leading_zero, unsigned long long, __builtin_clzll)
+#endif
+
+#undef SPECIALIZE_FLZ
+
 } // namespace LIBC_NAMESPACE::cpp
 
 #endif // LLVM_LIBC_SRC___SUPPORT_CPP_BIT_H

libc/src/stdbit/CMakeLists.txt

@@ -3,6 +3,7 @@ set(prefixes
   leading_ones
   trailing_zeros
   trailing_ones
+  first_leading_zero
 )
 set(suffixes c s i l ll)
 foreach(prefix IN LISTS prefixes)

libc/src/stdbit/stdc_first_leading_zero_uc.cpp

@@ -0,0 +1,21 @@
+//===-- Implementation of stdc_first_leading_zero_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_leading_zero_uc.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_leading_zero_uc,
+                   (unsigned char value)) {
+  return static_cast<unsigned>(cpp::first_leading_zero(value));
+}
+
+} // namespace LIBC_NAMESPACE

libc/src/stdbit/stdc_first_leading_zero_uc.h

@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_leading_zero_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_LEADING_ZERO_UC_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_UC_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_leading_zero_uc(unsigned char value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_UC_H

libc/src/stdbit/stdc_first_leading_zero_ui.cpp

@@ -0,0 +1,20 @@
+//===-- Implementation of stdc_first_leading_zero_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_leading_zero_ui.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_leading_zero_ui, (unsigned value)) {
+  return static_cast<unsigned>(cpp::first_leading_zero(value));
+}
+
+} // namespace LIBC_NAMESPACE

libc/src/stdbit/stdc_first_leading_zero_ui.h

@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_leading_zero_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_LEADING_ZERO_UI_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_UI_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_leading_zero_ui(unsigned value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_UI_H

libc/src/stdbit/stdc_first_leading_zero_ul.cpp

@@ -0,0 +1,21 @@
+//===-- Implementation of stdc_first_leading_zero_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_leading_zero_ul.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_leading_zero_ul,
+                   (unsigned long value)) {
+  return static_cast<unsigned>(cpp::first_leading_zero(value));
+}
+
+} // namespace LIBC_NAMESPACE

libc/src/stdbit/stdc_first_leading_zero_ul.h

@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_leading_zero_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_LEADING_ZERO_UL_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_UL_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_leading_zero_ul(unsigned long value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_UL_H

libc/src/stdbit/stdc_first_leading_zero_ull.cpp

@@ -0,0 +1,21 @@
+//===-- Implementation of stdc_first_leading_zero_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_leading_zero_ull.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_leading_zero_ull,
+                   (unsigned long long value)) {
+  return static_cast<unsigned>(cpp::first_leading_zero(value));
+}
+
+} // namespace LIBC_NAMESPACE

libc/src/stdbit/stdc_first_leading_zero_ull.h

@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_leading_zero_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_LEADING_ZERO_ULL_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_ULL_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_leading_zero_ull(unsigned long long value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_ULL_H

libc/src/stdbit/stdc_first_leading_zero_us.cpp

@@ -0,0 +1,21 @@
+//===-- Implementation of stdc_first_leading_zero_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_leading_zero_us.h"
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(unsigned, stdc_first_leading_zero_us,
+                   (unsigned short value)) {
+  return static_cast<unsigned>(cpp::first_leading_zero(value));
+}
+
+} // namespace LIBC_NAMESPACE

libc/src/stdbit/stdc_first_leading_zero_us.h

@@ -0,0 +1,18 @@
+//===-- Implementation header for stdc_first_leading_zero_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_LEADING_ZERO_US_H
+#define LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_US_H
+
+namespace LIBC_NAMESPACE {
+
+unsigned stdc_first_leading_zero_us(unsigned short value);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDBIT_STDC_FIRST_LEADING_ZERO_US_H

libc/test/include/stdbit_test.cpp

@@ -43,6 +43,13 @@ unsigned stdc_trailing_ones_us(unsigned short) noexcept { return 0xDBU; }
 unsigned stdc_trailing_ones_ui(unsigned) noexcept { return 0xDCU; }
 unsigned stdc_trailing_ones_ul(unsigned long) noexcept { return 0xDDU; }
 unsigned stdc_trailing_ones_ull(unsigned long long) noexcept { return 0xDFU; }
+unsigned stdc_first_leading_zero_uc(unsigned char) noexcept { return 0xEAU; }
+unsigned stdc_first_leading_zero_us(unsigned short) noexcept { return 0xEBU; }
+unsigned stdc_first_leading_zero_ui(unsigned) noexcept { return 0xECU; }
+unsigned stdc_first_leading_zero_ul(unsigned long) noexcept { return 0xEDU; }
+unsigned stdc_first_leading_zero_ull(unsigned long long) noexcept {
+  return 0xEFU;
+}
 }
 
 #include "include/llvm-libc-macros/stdbit-macros.h"
@@ -78,3 +85,11 @@ TEST(LlvmLibcStdbitTest, TypeGenericMacroTrailingOnes) {
   EXPECT_EQ(stdc_trailing_ones(0UL), 0xDDU);
   EXPECT_EQ(stdc_trailing_ones(0ULL), 0xDFU);
 }
+
+TEST(LlvmLibcStdbitTest, TypeGenericMacroFirstLeadingZero) {
+  EXPECT_EQ(stdc_first_leading_zero(static_cast<unsigned char>(0U)), 0xEAU);
+  EXPECT_EQ(stdc_first_leading_zero(static_cast<unsigned short>(0U)), 0xEBU);
+  EXPECT_EQ(stdc_first_leading_zero(0U), 0xECU);
+  EXPECT_EQ(stdc_first_leading_zero(0UL), 0xEDU);
+  EXPECT_EQ(stdc_first_leading_zero(0ULL), 0xEFU);
+}

libc/test/src/__support/CPP/bit_test.cpp

@@ -206,4 +206,11 @@ TEST(LlvmLibcBitTest, Rotr) {
             rotr<uint64_t>(0x12345678deadbeefULL, -19));
 }
 
+TYPED_TEST(LlvmLibcBitTest, FirstLeadingZero, UnsignedTypes) {
+  EXPECT_EQ(first_leading_zero<T>(cpp::numeric_limits<T>::max()), 0);
+  for (int i = 0U; i != cpp::numeric_limits<T>::digits; ++i)
+    EXPECT_EQ(first_leading_zero<T>(~(T(1) << i)),
+              cpp::numeric_limits<T>::digits - i);
+}
+
 } // namespace LIBC_NAMESPACE::cpp

libc/test/src/stdbit/CMakeLists.txt

@@ -5,6 +5,7 @@ set(prefixes
   leading_ones
   trailing_zeros
   trailing_ones
+  first_leading_zero
 )
 set(suffixes c s i l ll)
 foreach(prefix IN LISTS prefixes)

libc/test/src/stdbit/stdc_first_leading_zero_uc_test.cpp

@@ -0,0 +1,21 @@
+//===-- Unittests for stdc_first_leading_zero_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_leading_zero_uc.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdcFirstLeadingZeroUcTest, ALL) {
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_leading_zero_uc(UCHAR_MAX), 0U);
+}
+
+TEST(LlvmLibcStdcFirstLeadingZeroUcTest, ZeroHot) {
+  for (unsigned i = 0U; i != UCHAR_WIDTH; ++i)
+    EXPECT_EQ(LIBC_NAMESPACE::stdc_first_leading_zero_uc(~(1U << i)),
+              UCHAR_WIDTH - i);
+}