[wpiutil] Remove bit.hpp; replace with C++23 bit (#8792)

#7492 didn't catch all the uses and #7973 patched out byteswap in
Sleipnir for some reason (despite having upgraded compilers already)?

upstream_utils/llvm_patches/0025-Use-C-20-bit-header.patch

@@ -7,9 +7,11 @@ Subject: [PATCH 25/34] Use C++20 <bit> header
  llvm/include/llvm/ADT/DenseMap.h       |   1 +
  llvm/include/llvm/ADT/PointerUnion.h   |   2 +-
  llvm/include/llvm/ADT/SmallPtrSet.h    |   6 +-
- llvm/include/llvm/ADT/bit.h            | 286 -------------------------
- llvm/include/llvm/Support/MathExtras.h |  23 +-
- 5 files changed, 17 insertions(+), 301 deletions(-)
+ llvm/include/llvm/Support/Endian.h     | 159 +++++++++++++------------
+ llvm/include/llvm/Support/MathExtras.h |  23 ++--
+ llvm/lib/Support/ConvertUTFWrapper.cpp |   5 +-
+ llvm/unittests/Support/EndianTest.cpp  |  60 ++++------
+ 7 files changed, 125 insertions(+), 131 deletions(-)
 
 diff --git a/llvm/include/llvm/ADT/DenseMap.h b/llvm/include/llvm/ADT/DenseMap.h
 index 41199ac39da56994d9086600e6dfc1dad2c93ce0..2c23fa2d268d34429faa0ed7b1d889c2c5ea4f9c 100644
@@ -67,310 +69,412 @@ index c07de1a110479ed625b112cc5b30d0bd5d56430f..474aa405d789e8c291d3bfc16a653ad0
    /// SmallStorage - Fixed size storage used in 'small mode'.
    const void *SmallStorage[SmallSizePowTwo];
  
-diff --git a/llvm/include/llvm/ADT/bit.h b/llvm/include/llvm/ADT/bit.h
-index 5971b75045b6b5f40b46900b685faf2e475c5a7f..3426ea7602a95f7292ec262ed246527c840dc176 100644
---- a/llvm/include/llvm/ADT/bit.h
-+++ b/llvm/include/llvm/ADT/bit.h
-@@ -28,44 +28,6 @@
- #include <cstdlib>  // for _byteswap_{ushort,ulong,uint64}
- #endif
+diff --git a/llvm/include/llvm/Support/Endian.h b/llvm/include/llvm/Support/Endian.h
+index f5f4381c1003572024223da4825363d93a60aad9..1c5e447374e5dc1738a1e3fc6511395ef0829820 100644
+--- a/llvm/include/llvm/Support/Endian.h
++++ b/llvm/include/llvm/Support/Endian.h
+@@ -16,6 +16,7 @@
+ #include "llvm/ADT/bit.h"
+ #include "llvm/Support/Compiler.h"
+ #include "llvm/Support/SwapByteOrder.h"
++#include <bit>
+ #include <cassert>
+ #include <cstddef>
+ #include <cstdint>
+@@ -41,25 +42,25 @@ struct PickAlignment {
+ namespace endian {
  
--#if defined(__linux__) || defined(__GNU__) || defined(__HAIKU__) ||            \
--    defined(__Fuchsia__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__) ||  \
--    defined(__OpenBSD__) || defined(__DragonFly__) || defined(__managarm__)
--#include <endian.h>
--#elif defined(_AIX)
--#include <sys/machine.h>
--#elif defined(__sun)
--/* Solaris provides _BIG_ENDIAN/_LITTLE_ENDIAN selector in sys/types.h */
--#include <sys/types.h>
--#define BIG_ENDIAN 4321
--#define LITTLE_ENDIAN 1234
--#if defined(_BIG_ENDIAN)
--#define BYTE_ORDER BIG_ENDIAN
--#else
--#define BYTE_ORDER LITTLE_ENDIAN
--#endif
--#elif defined(__MVS__)
--#define BIG_ENDIAN 4321
--#define LITTLE_ENDIAN 1234
--#define BYTE_ORDER BIG_ENDIAN
--#else
--#if !defined(BYTE_ORDER) && !defined(_WIN32)
--#include <machine/endian.h>
--#endif
--#endif
--
--#ifdef _MSC_VER
--// Declare these intrinsics manually rather including intrin.h. It's very
--// expensive, and bit.h is popular via MathExtras.h.
--// #include <intrin.h>
--extern "C" {
--unsigned char _BitScanForward(unsigned long *_Index, unsigned long _Mask);
--unsigned char _BitScanForward64(unsigned long *_Index, unsigned __int64 _Mask);
--unsigned char _BitScanReverse(unsigned long *_Index, unsigned long _Mask);
--unsigned char _BitScanReverse64(unsigned long *_Index, unsigned __int64 _Mask);
--}
--#endif
--
- namespace llvm {
- 
- enum class endianness {
-@@ -143,254 +105,6 @@ template <typename T, typename = std::enable_if_t<std::is_integral_v<T>>>
-   }
+ template <typename value_type>
+-[[nodiscard]] inline value_type byte_swap(value_type value, endianness endian) {
+-  if (endian != llvm::endianness::native)
+-    sys::swapByteOrder(value);
++[[nodiscard]] inline value_type byte_swap(value_type value, std::endian endian) {
++  if (endian != std::endian::native)
++    return std::byteswap(value);
+   return value;
  }
  
--template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
--[[nodiscard]] constexpr inline bool has_single_bit(T Value) noexcept {
--  return (Value != 0) && ((Value & (Value - 1)) == 0);
--}
--
--/// Count the number of set bits in a value.
--/// Ex. popcount(0xF000F000) = 8
--/// Returns 0 if Value is zero.
--template <typename T> [[nodiscard]] constexpr int popcount(T Value) noexcept {
--  static_assert(std::is_unsigned_v<T>, "T must be an unsigned integer type");
--  static_assert(sizeof(T) <= 8, "T must be 8 bytes or less");
--
--  if constexpr (sizeof(T) <= 4) {
--#if defined(__GNUC__)
--    return (int)__builtin_popcount(Value);
--#else
--    uint32_t V = Value;
--    V = V - ((V >> 1) & 0x55555555);
--    V = (V & 0x33333333) + ((V >> 2) & 0x33333333);
--    return int(((V + (V >> 4) & 0xF0F0F0F) * 0x1010101) >> 24);
--#endif
--  } else {
--#if defined(__GNUC__)
--    return (int)__builtin_popcountll(Value);
--#else
--    uint64_t V = Value;
--    V = V - ((V >> 1) & 0x5555555555555555ULL);
--    V = (V & 0x3333333333333333ULL) + ((V >> 2) & 0x3333333333333333ULL);
--    V = (V + (V >> 4)) & 0x0F0F0F0F0F0F0F0FULL;
--    return int((uint64_t)(V * 0x0101010101010101ULL) >> 56);
--#endif
--  }
--}
--
--/// Count number of 0's from the least significant bit to the most
--/// stopping at the first 1.
--///
--/// A constexpr version of countr_zero.
--///
--/// Only unsigned integral types are allowed.
--///
--/// Returns std::numeric_limits<T>::digits on an input of 0.
--template <typename T> [[nodiscard]] constexpr int countr_zero_constexpr(T Val) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  // "(Val & -Val) - 1" generates a mask with all bits set up to (but not
--  // including) the least significant set bit of Val.
--  return llvm::popcount(static_cast<std::make_unsigned_t<T>>((Val & -Val) - 1));
--}
--
--/// Count number of 0's from the least significant bit to the most
--/// stopping at the first 1.
--///
--/// Only unsigned integral types are allowed.
--///
--/// Returns std::numeric_limits<T>::digits on an input of 0.
--template <typename T> [[nodiscard]] int countr_zero(T Val) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  if (!Val)
--    return std::numeric_limits<T>::digits;
--
--  // Use the intrinsic if available.
--  if constexpr (sizeof(T) <= 4) {
--#if __has_builtin(__builtin_ctz) || defined(__GNUC__)
--    return __builtin_ctz(Val);
--#elif defined(_MSC_VER)
--    unsigned long Index;
--    _BitScanForward(&Index, Val);
--    return Index;
--#endif
--  } else if constexpr (sizeof(T) == 8) {
--#if __has_builtin(__builtin_ctzll) || defined(__GNUC__)
--    return __builtin_ctzll(Val);
--#elif defined(_MSC_VER) && defined(_M_X64)
--    unsigned long Index;
--    _BitScanForward64(&Index, Val);
--    return Index;
--#endif
--  }
--
--  return countr_zero_constexpr(Val);
--}
--
--/// Count number of 0's from the most significant bit to the least
--///   stopping at the first 1.
--///
--/// Only unsigned integral types are allowed.
--///
--/// Returns std::numeric_limits<T>::digits on an input of 0.
--template <typename T> [[nodiscard]] int countl_zero(T Val) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  if (!Val)
--    return std::numeric_limits<T>::digits;
--
--  // Use the intrinsic if available.
--  if constexpr (sizeof(T) == 4) {
--#if __has_builtin(__builtin_clz) || defined(__GNUC__)
--    return __builtin_clz(Val);
--#elif defined(_MSC_VER)
--    unsigned long Index;
--    _BitScanReverse(&Index, Val);
--    return Index ^ 31;
--#endif
--  } else if constexpr (sizeof(T) == 8) {
--#if __has_builtin(__builtin_clzll) || defined(__GNUC__)
--    return __builtin_clzll(Val);
--#elif defined(_MSC_VER) && defined(_M_X64)
--    unsigned long Index;
--    _BitScanReverse64(&Index, Val);
--    return Index ^ 63;
--#endif
--  }
--
--  // Fall back to the bisection method.
--  unsigned ZeroBits = 0;
--  for (T Shift = std::numeric_limits<T>::digits >> 1; Shift; Shift >>= 1) {
--    T Tmp = Val >> Shift;
--    if (Tmp)
--      Val = Tmp;
--    else
--      ZeroBits |= Shift;
--  }
--  return ZeroBits;
--}
--
--/// Count the number of ones from the most significant bit to the first
--/// zero bit.
--///
--/// Ex. countl_one(0xFF0FFF00) == 8.
--/// Only unsigned integral types are allowed.
--///
--/// Returns std::numeric_limits<T>::digits on an input of all ones.
--template <typename T> [[nodiscard]] int countl_one(T Value) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  return llvm::countl_zero<T>(~Value);
--}
--
--/// Count the number of ones from the least significant bit to the first
--/// zero bit.
--///
--/// Ex. countr_one(0x00FF00FF) == 8.
--/// Only unsigned integral types are allowed.
--///
--/// Returns std::numeric_limits<T>::digits on an input of all ones.
--template <typename T> [[nodiscard]] int countr_one(T Value) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  return llvm::countr_zero<T>(~Value);
--}
--
--/// Returns the number of bits needed to represent Value if Value is nonzero.
--/// Returns 0 otherwise.
--///
--/// Ex. bit_width(5) == 3.
--template <typename T> [[nodiscard]] int bit_width(T Value) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  return std::numeric_limits<T>::digits - llvm::countl_zero(Value);
--}
--
--/// Returns the number of bits needed to represent Value if Value is nonzero.
--/// Returns 0 otherwise.
--///
--/// A constexpr version of bit_width.
--///
--/// Ex. bit_width_constexpr(5) == 3.
--template <typename T> [[nodiscard]] constexpr int bit_width_constexpr(T Value) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  int Width = 0;
--  while (Value > 0) {
--    Value >>= 1;
--    ++Width;
--  }
--  return Width;
--}
--
--/// Returns the largest integral power of two no greater than Value if Value is
--/// nonzero.  Returns 0 otherwise.
--///
--/// Ex. bit_floor(5) == 4.
--template <typename T> [[nodiscard]] T bit_floor(T Value) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  if (!Value)
--    return 0;
--  return T(1) << (llvm::bit_width(Value) - 1);
--}
--
--/// Returns the smallest integral power of two no smaller than Value if Value is
--/// nonzero.  Returns 1 otherwise.
--///
--/// Ex. bit_ceil(5) == 8.
--///
--/// The return value is undefined if the input is larger than the largest power
--/// of two representable in T.
--template <typename T> [[nodiscard]] T bit_ceil(T Value) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  if (Value < 2)
--    return 1;
--  return T(1) << llvm::bit_width<T>(Value - 1u);
--}
--
--/// Returns the smallest integral power of two no smaller than Value if Value is
--/// nonzero.  Returns 1 otherwise.
--///
--/// Ex. bit_ceil(5) == 8.
--///
--/// The return value is undefined if the input is larger than the largest power
--/// of two representable in T.
--template <typename T> [[nodiscard]] constexpr T bit_ceil_constexpr(T Value) {
--  static_assert(std::is_unsigned_v<T>,
--                "Only unsigned integral types are allowed.");
--  if (Value < 2)
--    return 1;
--  return T(1) << llvm::bit_width_constexpr<T>(Value - 1u);
--}
--
--template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
--[[nodiscard]] constexpr T rotl(T V, int R) {
--  constexpr unsigned N = std::numeric_limits<T>::digits;
--
--  static_assert(has_single_bit(N), "& (N - 1) is only valid for powers of two");
--  R = R & (N - 1);
--
--  if (R == 0)
--    return V;
--
--  return (V << R) | (V >> (N - R));
--}
--
--template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
--[[nodiscard]] constexpr T rotr(T V, int R) {
--  constexpr unsigned N = std::numeric_limits<T>::digits;
--
--  static_assert(has_single_bit(N), "& (N - 1) is only valid for powers of two");
--  R = R & (N - 1);
--
--  if (R == 0)
--    return V;
--
--  return (V >> R) | (V << (N - R));
--}
--
- } // namespace llvm
+ /// Swap the bytes of value to match the given endianness.
+-template <typename value_type, endianness endian>
++template <typename value_type, std::endian endian>
+ [[nodiscard]]
+ LLVM_DEPRECATED("Pass endian as a function argument instead",
+                 "byte_swap") inline value_type byte_swap(value_type value) {
+-  if constexpr (endian != llvm::endianness::native)
+-    sys::swapByteOrder(value);
++  if constexpr (endian != std::endian::native)
++    return std::byteswap(value);
+   return value;
+ }
  
- #endif
+ /// Read a value of a particular endianness from memory.
+ template <typename value_type, std::size_t alignment = unaligned>
+-[[nodiscard]] inline value_type read(const void *memory, endianness endian) {
++[[nodiscard]] inline value_type read(const void *memory, std::endian endian) {
+   value_type ret;
+ 
+   memcpy(static_cast<void *>(&ret),
+@@ -69,7 +70,7 @@ template <typename value_type, std::size_t alignment = unaligned>
+   return byte_swap<value_type>(ret, endian);
+ }
+ 
+-template <typename value_type, endianness endian, std::size_t alignment>
++template <typename value_type, std::endian endian, std::size_t alignment>
+ [[nodiscard]] LLVM_DEPRECATED("Pass endian as a function argument instead",
+                               "read") inline value_type
+     read(const void *memory) {
+@@ -81,13 +82,13 @@ template <typename value_type, endianness endian, std::size_t alignment>
+ template <typename value_type, std::size_t alignment = unaligned,
+           typename CharT>
+ [[nodiscard]] inline value_type readNext(const CharT *&memory,
+-                                         endianness endian) {
++                                         std::endian endian) {
+   value_type ret = read<value_type, alignment>(memory, endian);
+   memory += sizeof(value_type);
+   return ret;
+ }
+ 
+-template <typename value_type, endianness endian,
++template <typename value_type, std::endian endian,
+           std::size_t alignment = unaligned, typename CharT>
+ [[nodiscard]] inline value_type readNext(const CharT *&memory) {
+   return readNext<value_type, alignment, CharT>(memory, endian);
+@@ -95,14 +96,14 @@ template <typename value_type, endianness endian,
+ 
+ /// Write a value to memory with a particular endianness.
+ template <typename value_type, std::size_t alignment = unaligned>
+-inline void write(void *memory, value_type value, endianness endian) {
++inline void write(void *memory, value_type value, std::endian endian) {
+   value = byte_swap<value_type>(value, endian);
+   memcpy(LLVM_ASSUME_ALIGNED(
+              memory, (detail::PickAlignment<value_type, alignment>::value)),
+          &value, sizeof(value_type));
+ }
+ 
+-template <typename value_type, endianness endian, std::size_t alignment>
++template <typename value_type, std::endian endian, std::size_t alignment>
+ LLVM_DEPRECATED("Pass endian as a function argument instead", "write")
+ inline void write(void *memory, value_type value) {
+   write<value_type, alignment>(memory, value, endian);
+@@ -112,12 +113,12 @@ inline void write(void *memory, value_type value) {
+ /// value.
+ template <typename value_type, std::size_t alignment = unaligned,
+           typename CharT>
+-inline void writeNext(CharT *&memory, value_type value, endianness endian) {
++inline void writeNext(CharT *&memory, value_type value, std::endian endian) {
+   write(memory, value, endian);
+   memory += sizeof(value_type);
+ }
+ 
+-template <typename value_type, endianness endian,
++template <typename value_type, std::endian endian,
+           std::size_t alignment = unaligned, typename CharT>
+ inline void writeNext(CharT *&memory, value_type value) {
+   writeNext<value_type, alignment, CharT>(memory, value, endian);
+@@ -128,7 +129,7 @@ using make_unsigned_t = std::make_unsigned_t<value_type>;
+ 
+ /// Read a value of a particular endianness from memory, for a location
+ /// that starts at the given bit offset within the first byte.
+-template <typename value_type, endianness endian, std::size_t alignment>
++template <typename value_type, std::endian endian, std::size_t alignment>
+ [[nodiscard]] inline value_type readAtBitAlignment(const void *memory,
+                                                    uint64_t startBit) {
+   assert(startBit < 8);
+@@ -163,7 +164,7 @@ template <typename value_type, endianness endian, std::size_t alignment>
+ 
+ /// Write a value to memory with a particular endianness, for a location
+ /// that starts at the given bit offset within the first byte.
+-template <typename value_type, endianness endian, std::size_t alignment>
++template <typename value_type, std::endian endian, std::size_t alignment>
+ inline void writeAtBitAlignment(void *memory, value_type value,
+                                 uint64_t startBit) {
+   assert(startBit < 8);
+@@ -217,11 +218,11 @@ inline void writeAtBitAlignment(void *memory, value_type value,
+ 
+ namespace detail {
+ 
+-template <typename ValueType, endianness Endian, std::size_t Alignment,
++template <typename ValueType, std::endian Endian, std::size_t Alignment,
+           std::size_t ALIGN = PickAlignment<ValueType, Alignment>::value>
+ struct packed_endian_specific_integral {
+   using value_type = ValueType;
+-  static constexpr endianness endian = Endian;
++  static constexpr std::endian endian = Endian;
+   static constexpr std::size_t alignment = Alignment;
+ 
+   packed_endian_specific_integral() = default;
+@@ -284,210 +285,210 @@ public:
+ } // end namespace detail
+ 
+ using ulittle8_t =
+-    detail::packed_endian_specific_integral<uint8_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<uint8_t, std::endian::little,
+                                             unaligned>;
+ using ulittle16_t =
+-    detail::packed_endian_specific_integral<uint16_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<uint16_t, std::endian::little,
+                                             unaligned>;
+ using ulittle32_t =
+-    detail::packed_endian_specific_integral<uint32_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<uint32_t, std::endian::little,
+                                             unaligned>;
+ using ulittle64_t =
+-    detail::packed_endian_specific_integral<uint64_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<uint64_t, std::endian::little,
+                                             unaligned>;
+ 
+ using little16_t =
+-    detail::packed_endian_specific_integral<int16_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<int16_t, std::endian::little,
+                                             unaligned>;
+ using little32_t =
+-    detail::packed_endian_specific_integral<int32_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<int32_t, std::endian::little,
+                                             unaligned>;
+ using little64_t =
+-    detail::packed_endian_specific_integral<int64_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<int64_t, std::endian::little,
+                                             unaligned>;
+ 
+ using aligned_ulittle16_t =
+-    detail::packed_endian_specific_integral<uint16_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<uint16_t, std::endian::little,
+                                             aligned>;
+ using aligned_ulittle32_t =
+-    detail::packed_endian_specific_integral<uint32_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<uint32_t, std::endian::little,
+                                             aligned>;
+ using aligned_ulittle64_t =
+-    detail::packed_endian_specific_integral<uint64_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<uint64_t, std::endian::little,
+                                             aligned>;
+ 
+ using aligned_little16_t =
+-    detail::packed_endian_specific_integral<int16_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<int16_t, std::endian::little,
+                                             aligned>;
+ using aligned_little32_t =
+-    detail::packed_endian_specific_integral<int32_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<int32_t, std::endian::little,
+                                             aligned>;
+ using aligned_little64_t =
+-    detail::packed_endian_specific_integral<int64_t, llvm::endianness::little,
++    detail::packed_endian_specific_integral<int64_t, std::endian::little,
+                                             aligned>;
+ 
+ using ubig16_t =
+-    detail::packed_endian_specific_integral<uint16_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<uint16_t, std::endian::big,
+                                             unaligned>;
+ using ubig32_t =
+-    detail::packed_endian_specific_integral<uint32_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<uint32_t, std::endian::big,
+                                             unaligned>;
+ using ubig64_t =
+-    detail::packed_endian_specific_integral<uint64_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<uint64_t, std::endian::big,
+                                             unaligned>;
+ 
+ using big16_t =
+-    detail::packed_endian_specific_integral<int16_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<int16_t, std::endian::big,
+                                             unaligned>;
+ using big32_t =
+-    detail::packed_endian_specific_integral<int32_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<int32_t, std::endian::big,
+                                             unaligned>;
+ using big64_t =
+-    detail::packed_endian_specific_integral<int64_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<int64_t, std::endian::big,
+                                             unaligned>;
+ 
+ using aligned_ubig16_t =
+-    detail::packed_endian_specific_integral<uint16_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<uint16_t, std::endian::big,
+                                             aligned>;
+ using aligned_ubig32_t =
+-    detail::packed_endian_specific_integral<uint32_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<uint32_t, std::endian::big,
+                                             aligned>;
+ using aligned_ubig64_t =
+-    detail::packed_endian_specific_integral<uint64_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<uint64_t, std::endian::big,
+                                             aligned>;
+ 
+ using aligned_big16_t =
+-    detail::packed_endian_specific_integral<int16_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<int16_t, std::endian::big,
+                                             aligned>;
+ using aligned_big32_t =
+-    detail::packed_endian_specific_integral<int32_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<int32_t, std::endian::big,
+                                             aligned>;
+ using aligned_big64_t =
+-    detail::packed_endian_specific_integral<int64_t, llvm::endianness::big,
++    detail::packed_endian_specific_integral<int64_t, std::endian::big,
+                                             aligned>;
+ 
+ using unaligned_uint16_t =
+-    detail::packed_endian_specific_integral<uint16_t, llvm::endianness::native,
++    detail::packed_endian_specific_integral<uint16_t, std::endian::native,
+                                             unaligned>;
+ using unaligned_uint32_t =
+-    detail::packed_endian_specific_integral<uint32_t, llvm::endianness::native,
++    detail::packed_endian_specific_integral<uint32_t, std::endian::native,
+                                             unaligned>;
+ using unaligned_uint64_t =
+-    detail::packed_endian_specific_integral<uint64_t, llvm::endianness::native,
++    detail::packed_endian_specific_integral<uint64_t, std::endian::native,
+                                             unaligned>;
+ 
+ using unaligned_int16_t =
+-    detail::packed_endian_specific_integral<int16_t, llvm::endianness::native,
++    detail::packed_endian_specific_integral<int16_t, std::endian::native,
+                                             unaligned>;
+ using unaligned_int32_t =
+-    detail::packed_endian_specific_integral<int32_t, llvm::endianness::native,
++    detail::packed_endian_specific_integral<int32_t, std::endian::native,
+                                             unaligned>;
+ using unaligned_int64_t =
+-    detail::packed_endian_specific_integral<int64_t, llvm::endianness::native,
++    detail::packed_endian_specific_integral<int64_t, std::endian::native,
+                                             unaligned>;
+ 
+ template <typename T>
+ using little_t =
+-    detail::packed_endian_specific_integral<T, llvm::endianness::little,
++    detail::packed_endian_specific_integral<T, std::endian::little,
+                                             unaligned>;
+ template <typename T>
+-using big_t = detail::packed_endian_specific_integral<T, llvm::endianness::big,
++using big_t = detail::packed_endian_specific_integral<T, std::endian::big,
+                                                       unaligned>;
+ 
+ template <typename T>
+ using aligned_little_t =
+-    detail::packed_endian_specific_integral<T, llvm::endianness::little,
++    detail::packed_endian_specific_integral<T, std::endian::little,
+                                             aligned>;
+ template <typename T>
+ using aligned_big_t =
+-    detail::packed_endian_specific_integral<T, llvm::endianness::big, aligned>;
++    detail::packed_endian_specific_integral<T, std::endian::big, aligned>;
+ 
+ namespace endian {
+ 
+-template <typename T, endianness E> [[nodiscard]] inline T read(const void *P) {
++template <typename T, std::endian E> [[nodiscard]] inline T read(const void *P) {
+   return *(const detail::packed_endian_specific_integral<T, E, unaligned> *)P;
+ }
+ 
+-[[nodiscard]] inline uint16_t read16(const void *P, endianness E) {
++[[nodiscard]] inline uint16_t read16(const void *P, std::endian E) {
+   return read<uint16_t>(P, E);
+ }
+-[[nodiscard]] inline uint32_t read32(const void *P, endianness E) {
++[[nodiscard]] inline uint32_t read32(const void *P, std::endian E) {
+   return read<uint32_t>(P, E);
+ }
+-[[nodiscard]] inline uint64_t read64(const void *P, endianness E) {
++[[nodiscard]] inline uint64_t read64(const void *P, std::endian E) {
+   return read<uint64_t>(P, E);
+ }
+ 
+-template <endianness E> [[nodiscard]] inline uint16_t read16(const void *P) {
++template <std::endian E> [[nodiscard]] inline uint16_t read16(const void *P) {
+   return read<uint16_t, E>(P);
+ }
+-template <endianness E> [[nodiscard]] inline uint32_t read32(const void *P) {
++template <std::endian E> [[nodiscard]] inline uint32_t read32(const void *P) {
+   return read<uint32_t, E>(P);
+ }
+-template <endianness E> [[nodiscard]] inline uint64_t read64(const void *P) {
++template <std::endian E> [[nodiscard]] inline uint64_t read64(const void *P) {
+   return read<uint64_t, E>(P);
+ }
+ 
+ [[nodiscard]] inline uint16_t read16le(const void *P) {
+-  return read16<llvm::endianness::little>(P);
++  return read16<std::endian::little>(P);
+ }
+ [[nodiscard]] inline uint32_t read32le(const void *P) {
+-  return read32<llvm::endianness::little>(P);
++  return read32<std::endian::little>(P);
+ }
+ [[nodiscard]] inline uint64_t read64le(const void *P) {
+-  return read64<llvm::endianness::little>(P);
++  return read64<std::endian::little>(P);
+ }
+ [[nodiscard]] inline uint16_t read16be(const void *P) {
+-  return read16<llvm::endianness::big>(P);
++  return read16<std::endian::big>(P);
+ }
+ [[nodiscard]] inline uint32_t read32be(const void *P) {
+-  return read32<llvm::endianness::big>(P);
++  return read32<std::endian::big>(P);
+ }
+ [[nodiscard]] inline uint64_t read64be(const void *P) {
+-  return read64<llvm::endianness::big>(P);
++  return read64<std::endian::big>(P);
+ }
+ 
+-template <typename T, endianness E> inline void write(void *P, T V) {
++template <typename T, std::endian E> inline void write(void *P, T V) {
+   *(detail::packed_endian_specific_integral<T, E, unaligned> *)P = V;
+ }
+ 
+-inline void write16(void *P, uint16_t V, endianness E) {
++inline void write16(void *P, uint16_t V, std::endian E) {
+   write<uint16_t>(P, V, E);
+ }
+-inline void write32(void *P, uint32_t V, endianness E) {
++inline void write32(void *P, uint32_t V, std::endian E) {
+   write<uint32_t>(P, V, E);
+ }
+-inline void write64(void *P, uint64_t V, endianness E) {
++inline void write64(void *P, uint64_t V, std::endian E) {
+   write<uint64_t>(P, V, E);
+ }
+ 
+-template <endianness E> inline void write16(void *P, uint16_t V) {
++template <std::endian E> inline void write16(void *P, uint16_t V) {
+   write<uint16_t, E>(P, V);
+ }
+-template <endianness E> inline void write32(void *P, uint32_t V) {
++template <std::endian E> inline void write32(void *P, uint32_t V) {
+   write<uint32_t, E>(P, V);
+ }
+-template <endianness E> inline void write64(void *P, uint64_t V) {
++template <std::endian E> inline void write64(void *P, uint64_t V) {
+   write<uint64_t, E>(P, V);
+ }
+ 
+ inline void write16le(void *P, uint16_t V) {
+-  write16<llvm::endianness::little>(P, V);
++  write16<std::endian::little>(P, V);
+ }
+ inline void write32le(void *P, uint32_t V) {
+-  write32<llvm::endianness::little>(P, V);
++  write32<std::endian::little>(P, V);
+ }
+ inline void write64le(void *P, uint64_t V) {
+-  write64<llvm::endianness::little>(P, V);
++  write64<std::endian::little>(P, V);
+ }
+ inline void write16be(void *P, uint16_t V) {
+-  write16<llvm::endianness::big>(P, V);
++  write16<std::endian::big>(P, V);
+ }
+ inline void write32be(void *P, uint32_t V) {
+-  write32<llvm::endianness::big>(P, V);
++  write32<std::endian::big>(P, V);
+ }
+ inline void write64be(void *P, uint64_t V) {
+-  write64<llvm::endianness::big>(P, V);
++  write64<std::endian::big>(P, V);
+ }
+ 
+ } // end namespace endian
 diff --git a/llvm/include/llvm/Support/MathExtras.h b/llvm/include/llvm/Support/MathExtras.h
 index 0735d9c9f756349a19c5aa2ec34ceca5d8274f16..38b1486c54be5102025192b3b90e86e54ab347c0 100644
 --- a/llvm/include/llvm/Support/MathExtras.h
@@ -460,3 +564,227 @@ index 0735d9c9f756349a19c5aa2ec34ceca5d8274f16..38b1486c54be5102025192b3b90e86e5
  }
  
  /// A and B are either alignments or offsets. Return the minimum alignment that
+diff --git a/llvm/lib/Support/ConvertUTFWrapper.cpp b/llvm/lib/Support/ConvertUTFWrapper.cpp
+index 17b2760fe5c579a61e85188d482cbc9ec28f6c09..7f5d8958230414edf9db69ff08dd36d02c00f37d 100644
+--- a/llvm/lib/Support/ConvertUTFWrapper.cpp
++++ b/llvm/lib/Support/ConvertUTFWrapper.cpp
+@@ -9,6 +9,7 @@
+ #include "llvm/Support/ConvertUTF.h"
+ #include "llvm/Support/SmallVector.h"
+ #include "llvm/Support/ErrorHandling.h"
++#include <bit>
+ #include <span>
+ #include <string>
+ #include <string_view>
+@@ -102,7 +103,7 @@ bool convertUTF16ToUTF8String(span<const char> SrcBytes, SmallVectorImpl<char> &
+   if (Src[0] == UNI_UTF16_BYTE_ORDER_MARK_SWAPPED) {
+     ByteSwapped.insert(ByteSwapped.end(), Src, SrcEnd);
+     for (UTF16 &I : ByteSwapped)
+-      I = llvm::byteswap<uint16_t>(I);
++      I = std::byteswap(I);
+     Src = &ByteSwapped[0];
+     SrcEnd = &ByteSwapped[ByteSwapped.size() - 1] + 1;
+   }
+@@ -160,7 +161,7 @@ bool convertUTF32ToUTF8String(span<const char> SrcBytes, std::string &Out) {
+   if (Src[0] == UNI_UTF32_BYTE_ORDER_MARK_SWAPPED) {
+     ByteSwapped.insert(ByteSwapped.end(), Src, SrcEnd);
+     for (UTF32 &I : ByteSwapped)
+-      I = llvm::byteswap<uint32_t>(I);
++      I = std::byteswap(I);
+     Src = &ByteSwapped[0];
+     SrcEnd = &ByteSwapped[ByteSwapped.size() - 1] + 1;
+   }
+diff --git a/llvm/unittests/Support/EndianTest.cpp b/llvm/unittests/Support/EndianTest.cpp
+index 0ee631db74ac1f70ca5e9bb4cf6cdcb1d2205e57..d707fdb55c7c97b21310b48bf3c23e328dd644aa 100644
+--- a/llvm/unittests/Support/EndianTest.cpp
++++ b/llvm/unittests/Support/EndianTest.cpp
+@@ -24,33 +24,32 @@ TEST(Endian, Read) {
+   unsigned char littleval[] = {0x00, 0x04, 0x03, 0x02, 0x01};
+   int32_t BigAsHost = 0x00010203;
+   EXPECT_EQ(BigAsHost,
+-            (endian::read<int32_t, unaligned>(bigval, llvm::endianness::big)));
++            (endian::read<int32_t, unaligned>(bigval, std::endian::big)));
+   int32_t LittleAsHost = 0x02030400;
+   EXPECT_EQ(LittleAsHost, (endian::read<int32_t, unaligned>(
+-                              littleval, llvm::endianness::little)));
++                              littleval, std::endian::little)));
+ 
+   EXPECT_EQ(
+-      (endian::read<int32_t, unaligned>(bigval + 1, llvm::endianness::big)),
+-      (endian::read<int32_t, unaligned>(littleval + 1,
+-                                        llvm::endianness::little)));
++      (endian::read<int32_t, unaligned>(bigval + 1, std::endian::big)),
++      (endian::read<int32_t, unaligned>(littleval + 1, std::endian::little)));
+ }
+ 
+ TEST(Endian, WriteNext) {
+   unsigned char bigval[] = {0x00, 0x00}, *p = bigval;
+-  endian::writeNext<int16_t, llvm::endianness::big>(p, short(0xaabb));
++  endian::writeNext<int16_t, std::endian::big>(p, short(0xaabb));
+   EXPECT_EQ(bigval[0], 0xaa);
+   EXPECT_EQ(bigval[1], 0xbb);
+   EXPECT_EQ(p, bigval + 2);
+ 
+   char littleval[8] = {}, *q = littleval;
+-  endian::writeNext<uint32_t, llvm::endianness::little>(q, 0x44556677);
++  endian::writeNext<uint32_t, std::endian::little>(q, 0x44556677);
+   EXPECT_EQ(littleval[0], 0x77);
+   EXPECT_EQ(littleval[1], 0x66);
+   EXPECT_EQ(littleval[2], 0x55);
+   EXPECT_EQ(littleval[3], 0x44);
+   EXPECT_EQ(q, littleval + 4);
+ 
+-  endian::writeNext<uint32_t>(q, 0x11223344, llvm::endianness::little);
++  endian::writeNext<uint32_t>(q, 0x11223344, std::endian::little);
+   EXPECT_EQ(littleval[4], 0x44);
+   EXPECT_EQ(littleval[5], 0x33);
+   EXPECT_EQ(littleval[6], 0x22);
+@@ -63,10 +62,10 @@ TEST(Endian, ReadBitAligned) {
+   unsigned char littleval[] = {0x3f, 0x00, 0x00, 0x00, 0xc0, 0xff, 0xff, 0xff};
+   unsigned char bigval[] = {0x00, 0x00, 0x00, 0x3f, 0xff, 0xff, 0xff, 0xc0};
+   EXPECT_EQ(
+-      (endian::readAtBitAlignment<int, llvm::endianness::little, unaligned>(
++      (endian::readAtBitAlignment<int, std::endian::little, unaligned>(
+           &littleval[0], 6)),
+       0x0);
+-  EXPECT_EQ((endian::readAtBitAlignment<int, llvm::endianness::big, unaligned>(
++  EXPECT_EQ((endian::readAtBitAlignment<int, std::endian::big, unaligned>(
+                 &bigval[0], 6)),
+             0x0);
+   // Test to make sure that signed right shift of 0xf0000000 is masked
+@@ -74,18 +73,18 @@ TEST(Endian, ReadBitAligned) {
+   unsigned char littleval2[] = {0x00, 0x00, 0x00, 0xf0, 0x00, 0x00, 0x00, 0x00};
+   unsigned char bigval2[] = {0xf0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
+   EXPECT_EQ(
+-      (endian::readAtBitAlignment<int, llvm::endianness::little, unaligned>(
++      (endian::readAtBitAlignment<int, std::endian::little, unaligned>(
+           &littleval2[0], 4)),
+       0x0f000000);
+-  EXPECT_EQ((endian::readAtBitAlignment<int, llvm::endianness::big, unaligned>(
++  EXPECT_EQ((endian::readAtBitAlignment<int, std::endian::big, unaligned>(
+                 &bigval2[0], 4)),
+             0x0f000000);
+   // Test to make sure left shift of start bit doesn't overflow.
+   EXPECT_EQ(
+-      (endian::readAtBitAlignment<int, llvm::endianness::little, unaligned>(
++      (endian::readAtBitAlignment<int, std::endian::little, unaligned>(
+           &littleval2[0], 1)),
+       0x78000000);
+-  EXPECT_EQ((endian::readAtBitAlignment<int, llvm::endianness::big, unaligned>(
++  EXPECT_EQ((endian::readAtBitAlignment<int, std::endian::big, unaligned>(
+                 &bigval2[0], 1)),
+             0x78000000);
+   // Test to make sure 64-bit int doesn't overflow.
+@@ -94,11 +93,11 @@ TEST(Endian, ReadBitAligned) {
+   unsigned char bigval3[] = {0xf0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+                              0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
+   EXPECT_EQ(
+-      (endian::readAtBitAlignment<int64_t, llvm::endianness::little, unaligned>(
++      (endian::readAtBitAlignment<int64_t, std::endian::little, unaligned>(
+           &littleval3[0], 4)),
+       0x0f00000000000000);
+   EXPECT_EQ(
+-      (endian::readAtBitAlignment<int64_t, llvm::endianness::big, unaligned>(
++      (endian::readAtBitAlignment<int64_t, std::endian::big, unaligned>(
+           &bigval3[0], 4)),
+       0x0f00000000000000);
+ }
+@@ -107,7 +106,7 @@ TEST(Endian, WriteBitAligned) {
+   // This test ensures that signed right shift of 0xffffaa is masked
+   // properly.
+   unsigned char bigval[8] = {0x00};
+-  endian::writeAtBitAlignment<int32_t, llvm::endianness::big, unaligned>(
++  endian::writeAtBitAlignment<int32_t, std::endian::big, unaligned>(
+       bigval, (int)0xffffaaaa, 4);
+   EXPECT_EQ(bigval[0], 0xff);
+   EXPECT_EQ(bigval[1], 0xfa);
+@@ -119,7 +118,7 @@ TEST(Endian, WriteBitAligned) {
+   EXPECT_EQ(bigval[7], 0x0f);
+ 
+   unsigned char littleval[8] = {0x00};
+-  endian::writeAtBitAlignment<int32_t, llvm::endianness::little, unaligned>(
++  endian::writeAtBitAlignment<int32_t, std::endian::little, unaligned>(
+       littleval, (int)0xffffaaaa, 4);
+   EXPECT_EQ(littleval[0], 0xa0);
+   EXPECT_EQ(littleval[1], 0xaa);
+@@ -133,7 +132,7 @@ TEST(Endian, WriteBitAligned) {
+   // This test makes sure 1<<31 doesn't overflow.
+   // Test to make sure left shift of start bit doesn't overflow.
+   unsigned char bigval2[8] = {0x00};
+-  endian::writeAtBitAlignment<int32_t, llvm::endianness::big, unaligned>(
++  endian::writeAtBitAlignment<int32_t, std::endian::big, unaligned>(
+       bigval2, (int)0xffffffff, 1);
+   EXPECT_EQ(bigval2[0], 0xff);
+   EXPECT_EQ(bigval2[1], 0xff);
+@@ -145,7 +144,7 @@ TEST(Endian, WriteBitAligned) {
+   EXPECT_EQ(bigval2[7], 0x01);
+ 
+   unsigned char littleval2[8] = {0x00};
+-  endian::writeAtBitAlignment<int32_t, llvm::endianness::little, unaligned>(
++  endian::writeAtBitAlignment<int32_t, std::endian::little, unaligned>(
+       littleval2, (int)0xffffffff, 1);
+   EXPECT_EQ(littleval2[0], 0xfe);
+   EXPECT_EQ(littleval2[1], 0xff);
+@@ -158,7 +157,7 @@ TEST(Endian, WriteBitAligned) {
+ 
+   // Test to make sure 64-bit int doesn't overflow.
+   unsigned char bigval64[16] = {0x00};
+-  endian::writeAtBitAlignment<int64_t, llvm::endianness::big, unaligned>(
++  endian::writeAtBitAlignment<int64_t, std::endian::big, unaligned>(
+       bigval64, (int64_t)0xffffffffffffffff, 1);
+   EXPECT_EQ(bigval64[0], 0xff);
+   EXPECT_EQ(bigval64[1], 0xff);
+@@ -178,7 +177,7 @@ TEST(Endian, WriteBitAligned) {
+   EXPECT_EQ(bigval64[15], 0x01);
+ 
+   unsigned char littleval64[16] = {0x00};
+-  endian::writeAtBitAlignment<int64_t, llvm::endianness::little, unaligned>(
++  endian::writeAtBitAlignment<int64_t, std::endian::little, unaligned>(
+       littleval64, (int64_t)0xffffffffffffffff, 1);
+   EXPECT_EQ(littleval64[0], 0xfe);
+   EXPECT_EQ(littleval64[1], 0xff);
+@@ -200,26 +199,26 @@ TEST(Endian, WriteBitAligned) {
+ 
+ TEST(Endian, Write) {
+   unsigned char data[5];
+-  endian::write<int32_t, unaligned>(data, -1362446643, llvm::endianness::big);
++  endian::write<int32_t, unaligned>(data, -1362446643, std::endian::big);
+   EXPECT_EQ(data[0], 0xAE);
+   EXPECT_EQ(data[1], 0xCA);
+   EXPECT_EQ(data[2], 0xB6);
+   EXPECT_EQ(data[3], 0xCD);
+   endian::write<int32_t, unaligned>(data + 1, -1362446643,
+-                                    llvm::endianness::big);
++                                    std::endian::big);
+   EXPECT_EQ(data[1], 0xAE);
+   EXPECT_EQ(data[2], 0xCA);
+   EXPECT_EQ(data[3], 0xB6);
+   EXPECT_EQ(data[4], 0xCD);
+ 
+   endian::write<int32_t, unaligned>(data, -1362446643,
+-                                    llvm::endianness::little);
++                                    std::endian::little);
+   EXPECT_EQ(data[0], 0xCD);
+   EXPECT_EQ(data[1], 0xB6);
+   EXPECT_EQ(data[2], 0xCA);
+   EXPECT_EQ(data[3], 0xAE);
+   endian::write<int32_t, unaligned>(data + 1, -1362446643,
+-                                    llvm::endianness::little);
++                                    std::endian::little);
+   EXPECT_EQ(data[1], 0xCD);
+   EXPECT_EQ(data[2], 0xB6);
+   EXPECT_EQ(data[3], 0xCA);
+@@ -239,13 +238,4 @@ TEST(Endian, PackedEndianSpecificIntegral) {
+   EXPECT_EQ(big_val->value(), little_val->value());
+ }
+ 
+-TEST(Endian, PacketEndianSpecificIntegralAsEnum) {
+-  enum class Test : uint16_t { ONETWO = 0x0102, TWOONE = 0x0201 };
+-  unsigned char bytes[] = {0x01, 0x02};
+-  using LittleTest = little_t<Test>;
+-  using BigTest = big_t<Test>;
+-  EXPECT_EQ(Test::TWOONE, *reinterpret_cast<LittleTest *>(bytes));
+-  EXPECT_EQ(Test::ONETWO, *reinterpret_cast<BigTest *>(bytes));
+-}
+-
+ } // end anon namespace