EightBit/inc/EightBitCompilerDefinitions.h

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C
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#pragma once
#ifdef _MSC_VER
# include <intrin.h>
#endif
#ifdef __GNUG__
# include <x86intrin.h>
#endif
#if !(defined(_MSC_VER) || defined(__GNUG__))
# include <cassert>
# include <bitset>
#endif
namespace EightBit {
[[nodiscard]] int countBits(uint8_t value) noexcept;
[[nodiscard]] bool oddParity(uint8_t value) noexcept;
[[nodiscard]] int findFirstSet(unsigned long value) noexcept;
}
inline int EightBit::countBits(uint8_t value) noexcept {
#if defined(_MSC_VER)
return __popcnt(value);
#elif defined(__GNUG__)
return __builtin_popcount(value);
#else
/*
Published in 1988, the C Programming Language 2nd Ed.
(by Brian W.Kernighan and Dennis M.Ritchie) mentions
this in exercise 2 - 9. On April 19, 2006 Don Knuth pointed
out to me that this method "was first published by Peter
Wegner in CACM 3 (1960), 322.
(Also discovered independently by Derrick Lehmer and published
in 1964 in a book edited by Beckenbach.)"
*/
int count; // c accumulates the total bits set in value
for (count = 0; value; ++count)
value &= value - 1; // clear the least significant bit set
return count;
#endif
}
inline bool EightBit::oddParity(const uint8_t value) noexcept {
#ifdef __GNUG__
return __builtin_parity(value)
#else
return countBits(value) % 2;
#endif
}
inline int EightBit::findFirstSet(const unsigned long value) noexcept {
#if defined(_MSC_VER)
unsigned long index = 0;
if (_BitScanForward(&index, value))
return index + 1;
return 0;
#elif defined(__GNUG__)
return __builtin_ffs(value);
#else
std::bitset<sizeof(unsigned long) * 8> bits(value);
for (size_t i = bits.size() - 1; i >= 0; --i)
if (bits.test(i))
return i + 1;
return 0;
#endif
}
#define PARITY(x) EightBit::oddParity(x)
#ifdef _MSC_VER
# define ASSUME(x) __assume(x);
# define LIKELY(x) (x)
# define UNLIKELY(x) (x)
# define UNREACHABLE { ASSUME(0); throw std::exception("unreachable"); }
#elif defined(__GNUG__)
# define ASSUME(x) { if (!x) __builtin_unreachable(); }
# define LIKELY(x) __builtin_expect(!!(x), 1)
# define UNLIKELY(x) __builtin_expect(!!(x), 0)
# define UNREACHABLE __builtin_unreachable();
#else
# define ASSUME(x) assert(x);
# define LIKELY(x) (x)
# define UNLIKELY(x) (x)
# define UNREACHABLE ASSUME(0)
#endif