EightBit/inc/EightBitCompilerDefinitions.h

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#pragma once
#ifdef _MSC_VER
# include <intrin.h>
#endif
#ifdef __GNUG__
# include <x86intrin.h>
#endif
namespace EightBit {
int countBits(uint8_t value);
bool oddParity(uint8_t value);
int findFirstSet(int value);
constexpr void assume(int expression);
}
inline int EightBit::countBits(const uint8_t value) {
#ifdef _MSC_VER
return __popcnt(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) {
return countBits(value) % 2;
}
inline int EightBit::findFirstSet(const int value) {
#ifdef _MSC_VER
unsigned long index;
if (_BitScanForward(&index, value))
return index + 1;
return 0;
#elif defined(__GNUG__)
return __builtin_ffs(value);
#else
# error Find first set not implemented
#endif
}
inline constexpr void EightBit::assume(const int expression) {
#ifdef _MSC_VER
__assume(expression);
#elif defined(__GNUG__)
if (!expression)
__builtin_unreachable();
#else
assert(expression);
#endif
}
#define ASSUME(x) EightBit::assume(x)
#ifdef _MSC_VER
# define LIKELY(x) (x)
# define UNLIKELY(x) (x)
# define PARITY(x) EightBit::oddParity(x)
# define UNREACHABLE { ASSUME(0); throw std::exception("unreachable"); }
#elif defined(__GNUG__)
# define LIKELY(x) __builtin_expect(!!(x), 1)
# define UNLIKELY(x) __builtin_expect(!!(x), 0)
# define PARITY(x) __builtin_parity(x)
# define UNREACHABLE __builtin_unreachable();
#else
# define LIKELY(x) (x)
# define UNLIKELY(x) (x)
# define PARITY(x) EightBit::oddParity(x)
# define UNREACHABLE ASSUME(0)
#endif