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transmission/libtransmission/tr-popcount.h

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// This file Copyright © 2022 Mnemosyne LLC.
// It may be used under GPLv2 (SPDX: GPL-2.0-only), GPLv3 (SPDX: GPL-3.0-only),
// or any future license endorsed by Mnemosyne LLC.
// License text can be found in the licenses/ folder.
#ifndef __TRANSMISSION__
#error only libtransmission should #include this header.
#endif
#ifndef TR_POPCNT_H
#define TR_POPCNT_H
#include <cstdint>
#include <type_traits>
/* Avoid defining irrelevant helpers that might interfere with other
* preprocessor logic. */
#if (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L) || __cplusplus >= 202002L
#define TR_HAVE_STD_POPCOUNT
#endif
#if (defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 2))) || \
(defined(__clang__) && (__clang_major__ > 3 || (__clang_major__ == 3 && __clang_minor__ >= 0)))
#ifdef __has_builtin
#if __has_builtin(__builtin_popcount)
#define TR_HAVE_BUILTIN_POPCOUNT
#endif
#endif
#if defined(__x86_64__) && defined(__SSE__)
#define TR_HAVE_ASM_POPCNT
#endif
#endif
#if defined(_MSC_VER)
#if defined(_M_X64) || defined(_M_IX86)
#include <nmmintrin.h>
#endif
#endif
#if defined(TR_HAVE_STD_POPCOUNT)
#include <bit>
#endif
template<typename T>
struct tr_popcnt
{
static_assert(std::is_integral<T>::value != 0, "Can only popcnt integral types");
static_assert(sizeof(T) <= sizeof(uint64_t), "Unsupported size");
/* Needed regularly to avoid sign extension / get unsigned shift behavior.
*/
using unsigned_T = typename std::make_unsigned<T>::type;
/* Sanity tests. */
static_assert(sizeof(unsigned_T) == sizeof(T), "Unsigned type somehow smaller than signed type");
static_assert(std::is_integral<unsigned_T>::value != 0, "Unsigned type somehow non integral");
#if defined(TR_HAVE_STD_POPCOUNT)
/* If we have std::popcount just use that. */
static constexpr inline unsigned count(T v)
{
unsigned_T unsigned_v = static_cast<unsigned_T>(v);
return static_cast<unsigned>(std::popcount(unsigned_v));
}
#elif defined(TR_HAVE_BUILTIN_POPCOUNT) && defined(TR_HAVE_ASM_POPCNT)
/* Use __builtin as opposed to straight assembly to avoid any
strain the latter puts on the optimized. */
static inline unsigned count(T v)
{
/* Make sure we know how to find that right __builtin_popcount. */
static_assert(
sizeof(T) == sizeof(long long) || sizeof(T) == sizeof(long) || sizeof(T) <= sizeof(int),
"Unknown type size!");
if constexpr (sizeof(T) == sizeof(long long))
{
return __builtin_popcountll(v);
}
else if constexpr (sizeof(T) == sizeof(long))
{
return __builtin_popcountl(v);
}
else if constexpr (sizeof(T) == sizeof(int))
{
return __builtin_popcount(v);
}
else
{
static_assert(sizeof(T) < sizeof(int));
/* Need to avoid sign extension. */
unsigned_T unsigned_v = static_cast<unsigned_T>(v);
return __builtin_popcount(unsigned_v);
}
}
#elif defined(_MSC_VER) && defined(_M_X64)
/* 64-bit x86 intrinsics. */
static inline unsigned count(T v)
{
if constexpr (sizeof(T) == sizeof(uint64_t))
{
return _mm_popcnt_u64(v);
}
else if constexpr (sizeof(T) == sizeof(uint32_t))
{
return _mm_popcnt_u32(v);
}
else
{
static_assert(sizeof(T) < sizeof(uint32_t));
/* Need to avoid sign extension. */
unsigned_T unsigned_v = static_cast<unsigned_T>(v);
return _mm_popcnt_u32(unsigned_v);
}
}
#elif defined(_MSC_VER) && defined(_M_IX86)
/* 32-bit x86 intrinsics. */
static inline unsigned count(T v)
{
if constexpr (sizeof(T) == sizeof(uint64_t))
{
/* Avoid signed shift. */
unsigned_T unsigned_v = static_cast<unsigned_T>(v);
return _mm_popcnt_u32(static_cast<uint32_t>(unsigned_v)) + _mm_popcnt_u32(static_cast<uint32_t>(unsigned_v >> 32));
}
else if constexpr (sizeof(T) == sizeof(uint32_t))
{
return _mm_popcnt_u32(v);
}
else
{
static_assert(sizeof(T) < sizeof(uint32_t));
/* Need to avoid sign extension. */
unsigned_T unsigned_v = static_cast<unsigned_T>(v);
return _mm_popcnt_u32(unsigned_v);
}
}
#elif defined(TR_HAVE_ASM_POPCNT)
/* raw assembly. This prohibits constexpr so may be worth dropping if there
* is need for count() to be constexpr. */
static inline unsigned count(T v)
{
unsigned_T unsigned_v = static_cast<unsigned_T>(v);
if constexpr (sizeof(T) >= sizeof(uint32_t))
{
/* Make sure dst == src to avoid false dependency on many x86
* implementations. */
__asm__ __volatile__("popcnt %1, %0" : "=r"(unsigned_v) : "0"(unsigned_v) : "cc");
return unsigned_v;
}
else
{
/* No popcnt instruct for register size < 32. */
uint32_t unsigned_v32 = static_cast<uint32_t>(unsigned_v);
/* Make sure dst == src to avoid false dependency on many x86
* implementations. */
__asm__ __volatile__("popcnt %1, %0" : "=r"(unsigned_v32) : "0"(unsigned_v32) : "cc");
return unsigned_v32;
}
}
#else
/* Generic implementation. */
static inline unsigned count(T v)
{
/* To avoid signed shifts. */
unsigned_T unsigned_v = static_cast<unsigned_T>(v);
if constexpr (sizeof(T) <= sizeof(uint16_t))
{
/* Use LUT for small sizes. In reality we only need half a
* byte for each value if ever hit a case where perf is
* limited by severe bottleneck on L1D this can be
* optimized. */
static constexpr uint8_t popcnt_lut[256] = {
0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 1, 2, 2, 3, 2,
3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3,
3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5,
6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4,
3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4,
5, 5, 6, 5, 6, 6, 7, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6,
6, 7, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8
};
if constexpr (sizeof(T) == sizeof(uint8_t))
{
return popcnt_lut[unsigned_v];
}
else
{
return popcnt_lut[unsigned_v & 0xFF] + popcnt_lut[unsigned_v >> 8];
}
}
else
{
/* for larger sizes use implementation described here:
* http://en.wikipedia.org/wiki/Hamming_weight#Efficient_implementation
*/
static constexpr unsigned_T m1 = static_cast<unsigned_T>(0x5555555555555555ll);
static constexpr unsigned_T m2 = static_cast<unsigned_T>(0x3333333333333333ll);
static constexpr unsigned_T m4 = static_cast<unsigned_T>(0x0F0F0F0F0F0F0F0Fll);
static constexpr unsigned_T h01 = static_cast<unsigned_T>(0x0101010101010101ll);
unsigned_v = unsigned_v - ((unsigned_v >> 1) & m1);
unsigned_v = (unsigned_v & m2) + ((unsigned_v >> 2) & m2);
unsigned_v = (unsigned_v + (unsigned_v >> 4)) & m4;
unsigned_v = unsigned_v * h01;
return unsigned_v >> (8 * sizeof(T) - 8);
}
}
#endif
};
#endif
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