libc/src/__support/math_extras.h

   1 //===-- Mimics llvm/Support/MathExtras.h ------------------------*- C++ -*-===//
   2 // Provides useful math functions.
   3 //
   4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   5 // See https://llvm.org/LICENSE.txt for license information.
   6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   7 //
   8 //===----------------------------------------------------------------------===//
   9
  10 #ifndef LLVM_LIBC_SRC___SUPPORT_MATH_EXTRAS_H
  11 #define LLVM_LIBC_SRC___SUPPORT_MATH_EXTRAS_H
  12
  13 #include "src/__support/CPP/bit.h"         // countl_one, countr_zero
  14 #include "src/__support/CPP/limits.h"      // CHAR_BIT, numeric_limits
  15 #include "src/__support/CPP/type_traits.h" // is_unsigned_v, is_constant_evaluated
  16 #include "src/__support/macros/attributes.h" // LIBC_INLINE
  17 #include "src/__support/macros/config.h"
  18
  19 namespace LIBC_NAMESPACE_DECL {
  20
  21 // Create a bitmask with the count right-most bits set to 1, and all other bits
  22 // set to 0.  Only unsigned types are allowed.
  23 template <typename T, size_t count>
  24 LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, T>
  25 mask_trailing_ones() {
  26   constexpr unsigned T_BITS = CHAR_BIT * sizeof(T);
  27   static_assert(count <= T_BITS && "Invalid bit index");
  28   return count == 0 ? 0 : (T(-1) >> (T_BITS - count));
  29 }
  30
  31 // Create a bitmask with the count left-most bits set to 1, and all other bits
  32 // set to 0.  Only unsigned types are allowed.
  33 template <typename T, size_t count>
  34 LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, T>
  35 mask_leading_ones() {
  36   return T(~mask_trailing_ones<T, CHAR_BIT * sizeof(T) - count>());
  37 }
  38
  39 // Create a bitmask with the count right-most bits set to 0, and all other bits
  40 // set to 1.  Only unsigned types are allowed.
  41 template <typename T, size_t count>
  42 LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, T>
  43 mask_trailing_zeros() {
  44   return mask_leading_ones<T, CHAR_BIT * sizeof(T) - count>();
  45 }
  46
  47 // Create a bitmask with the count left-most bits set to 0, and all other bits
  48 // set to 1.  Only unsigned types are allowed.
  49 template <typename T, size_t count>
  50 LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, T>
  51 mask_leading_zeros() {
  52   return mask_trailing_ones<T, CHAR_BIT * sizeof(T) - count>();
  53 }
  54
  55 // Returns whether 'a + b' overflows, the result is stored in 'res'.
  56 template <typename T>
  57 [[nodiscard]] LIBC_INLINE constexpr bool add_overflow(T a, T b, T &res) {
  58   return __builtin_add_overflow(a, b, &res);
  59 }
  60
  61 // Returns whether 'a - b' overflows, the result is stored in 'res'.
  62 template <typename T>
  63 [[nodiscard]] LIBC_INLINE constexpr bool sub_overflow(T a, T b, T &res) {
  64   return __builtin_sub_overflow(a, b, &res);
  65 }
  66
  67 #define RETURN_IF(TYPE, BUILTIN)                                               \
  68   if constexpr (cpp::is_same_v<T, TYPE>)                                       \
  69     return BUILTIN(a, b, carry_in, carry_out);
  70
  71 // Returns the result of 'a + b' taking into account 'carry_in'.
  72 // The carry out is stored in 'carry_out' it not 'nullptr', dropped otherwise.
  73 // We keep the pass by pointer interface for consistency with the intrinsic.
  74 template <typename T>
  75 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, T>
  76 add_with_carry(T a, T b, T carry_in, T &carry_out) {
  77   if constexpr (!cpp::is_constant_evaluated()) {
  78 #if __has_builtin(__builtin_addcb)
  79     RETURN_IF(unsigned char, __builtin_addcb)
  80 #elif __has_builtin(__builtin_addcs)
  81     RETURN_IF(unsigned short, __builtin_addcs)
  82 #elif __has_builtin(__builtin_addc)
  83     RETURN_IF(unsigned int, __builtin_addc)
  84 #elif __has_builtin(__builtin_addcl)
  85     RETURN_IF(unsigned long, __builtin_addcl)
  86 #elif __has_builtin(__builtin_addcll)
  87     RETURN_IF(unsigned long long, __builtin_addcll)
  88 #endif
  89   }
  90   T sum = {};
  91   T carry1 = add_overflow(a, b, sum);
  92   T carry2 = add_overflow(sum, carry_in, sum);
  93   carry_out = carry1 | carry2;
  94   return sum;
  95 }
  96
  97 // Returns the result of 'a - b' taking into account 'carry_in'.
  98 // The carry out is stored in 'carry_out' it not 'nullptr', dropped otherwise.
  99 // We keep the pass by pointer interface for consistency with the intrinsic.
 100 template <typename T>
 101 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, T>
 102 sub_with_borrow(T a, T b, T carry_in, T &carry_out) {
 103   if constexpr (!cpp::is_constant_evaluated()) {
 104 #if __has_builtin(__builtin_subcb)
 105     RETURN_IF(unsigned char, __builtin_subcb)
 106 #elif __has_builtin(__builtin_subcs)
 107     RETURN_IF(unsigned short, __builtin_subcs)
 108 #elif __has_builtin(__builtin_subc)
 109     RETURN_IF(unsigned int, __builtin_subc)
 110 #elif __has_builtin(__builtin_subcl)
 111     RETURN_IF(unsigned long, __builtin_subcl)
 112 #elif __has_builtin(__builtin_subcll)
 113     RETURN_IF(unsigned long long, __builtin_subcll)
 114 #endif
 115   }
 116   T sub = {};
 117   T carry1 = sub_overflow(a, b, sub);
 118   T carry2 = sub_overflow(sub, carry_in, sub);
 119   carry_out = carry1 | carry2;
 120   return sub;
 121 }
 122
 123 #undef RETURN_IF
 124
 125 template <typename T>
 126 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, int>
 127 first_leading_zero(T value) {
 128   return value == cpp::numeric_limits<T>::max() ? 0
 129                                                 : cpp::countl_one(value) + 1;
 130 }
 131
 132 template <typename T>
 133 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, int>
 134 first_leading_one(T value) {
 135   return first_leading_zero(static_cast<T>(~value));
 136 }
 137
 138 template <typename T>
 139 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, int>
 140 first_trailing_zero(T value) {
 141   return value == cpp::numeric_limits<T>::max()
 142              ? 0
 143              : cpp::countr_zero(static_cast<T>(~value)) + 1;
 144 }
 145
 146 template <typename T>
 147 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, int>
 148 first_trailing_one(T value) {
 149   return value == cpp::numeric_limits<T>::max() ? 0
 150                                                 : cpp::countr_zero(value) + 1;
 151 }
 152
 153 template <typename T>
 154 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, int>
 155 count_zeros(T value) {
 156   return cpp::popcount<T>(static_cast<T>(~value));
 157 }
 158
 159 } // namespace LIBC_NAMESPACE_DECL
 160
 161 #endif // LLVM_LIBC_SRC___SUPPORT_MATH_EXTRAS_H