path: root/src/port/pg_bitutils.c

/*-------------------------------------------------------------------------
 *
 * pg_bitutils.c
 *	  Miscellaneous functions for bit-wise operations.
 *
 * Copyright (c) 2019-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *	  src/port/pg_bitutils.c
 *
 *-------------------------------------------------------------------------
 */
#include "c.h"

#ifdef HAVE__GET_CPUID
#include <cpuid.h>
#endif
#ifdef HAVE__CPUID
#include <intrin.h>
#endif

#include "port/pg_bitutils.h"


/*
 * Array giving the position of the left-most set bit for each possible
 * byte value.  We count the right-most position as the 0th bit, and the
 * left-most the 7th bit.  The 0th entry of the array should not be used.
 *
 * Note: this is not used by the functions in pg_bitutils.h when
 * HAVE__BUILTIN_CLZ is defined, but we provide it anyway, so that
 * extensions possibly compiled with a different compiler can use it.
 */
const uint8 pg_leftmost_one_pos[256] = {
	0, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
	4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
	5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
	5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
	6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
	6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
	6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
	6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7
};

/*
 * Array giving the position of the right-most set bit for each possible
 * byte value.  We count the right-most position as the 0th bit, and the
 * left-most the 7th bit.  The 0th entry of the array should not be used.
 *
 * Note: this is not used by the functions in pg_bitutils.h when
 * HAVE__BUILTIN_CTZ is defined, but we provide it anyway, so that
 * extensions possibly compiled with a different compiler can use it.
 */
const uint8 pg_rightmost_one_pos[256] = {
	0, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	7, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
	4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
};

/*
 * Array giving the number of 1-bits in each possible byte value.
 *
 * Note: we export this for use by functions in which explicit use
 * of the popcount functions seems unlikely to be a win.
 */
const uint8 pg_number_of_ones[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
};

/*
 * On x86_64, we can use the hardware popcount instruction, but only if
 * we can verify that the CPU supports it via the cpuid instruction.
 *
 * Otherwise, we fall back to __builtin_popcount if the compiler has that,
 * or a hand-rolled implementation if not.
 */
#ifdef HAVE_X86_64_POPCNTQ
#if defined(HAVE__GET_CPUID) || defined(HAVE__CPUID)
#define USE_POPCNT_ASM 1
#endif
#endif

static int	pg_popcount32_slow(uint32 word);
static int	pg_popcount64_slow(uint64 word);

#ifdef USE_POPCNT_ASM
static bool pg_popcount_available(void);
static int	pg_popcount32_choose(uint32 word);
static int	pg_popcount64_choose(uint64 word);
static int	pg_popcount32_asm(uint32 word);
static int	pg_popcount64_asm(uint64 word);

int			(*pg_popcount32) (uint32 word) = pg_popcount32_choose;
int			(*pg_popcount64) (uint64 word) = pg_popcount64_choose;
#else
int			(*pg_popcount32) (uint32 word) = pg_popcount32_slow;
int			(*pg_popcount64) (uint64 word) = pg_popcount64_slow;
#endif							/* USE_POPCNT_ASM */

#ifdef USE_POPCNT_ASM

/*
 * Return true if CPUID indicates that the POPCNT instruction is available.
 */
static bool
pg_popcount_available(void)
{
	unsigned int exx[4] = {0, 0, 0, 0};

#if defined(HAVE__GET_CPUID)
	__get_cpuid(1, &exx[0], &exx[1], &exx[2], &exx[3]);
#elif defined(HAVE__CPUID)
	__cpuid(exx, 1);
#else
#error cpuid instruction not available
#endif

	return (exx[2] & (1 << 23)) != 0;	/* POPCNT */
}

/*
 * These functions get called on the first call to pg_popcount32 etc.
 * They detect whether we can use the asm implementations, and replace
 * the function pointers so that subsequent calls are routed directly to
 * the chosen implementation.
 */
static int
pg_popcount32_choose(uint32 word)
{
	if (pg_popcount_available())
	{
		pg_popcount32 = pg_popcount32_asm;
		pg_popcount64 = pg_popcount64_asm;
	}
	else
	{
		pg_popcount32 = pg_popcount32_slow;
		pg_popcount64 = pg_popcount64_slow;
	}

	return pg_popcount32(word);
}

static int
pg_popcount64_choose(uint64 word)
{
	if (pg_popcount_available())
	{
		pg_popcount32 = pg_popcount32_asm;
		pg_popcount64 = pg_popcount64_asm;
	}
	else
	{
		pg_popcount32 = pg_popcount32_slow;
		pg_popcount64 = pg_popcount64_slow;
	}

	return pg_popcount64(word);
}

/*
 * pg_popcount32_asm
 *		Return the number of 1 bits set in word
 */
static int
pg_popcount32_asm(uint32 word)
{
	uint32		res;

__asm__ __volatile__(" popcntl %1,%0\n":"=q"(res):"rm"(word):"cc");
	return (int) res;
}

/*
 * pg_popcount64_asm
 *		Return the number of 1 bits set in word
 */
static int
pg_popcount64_asm(uint64 word)
{
	uint64		res;

__asm__ __volatile__(" popcntq %1,%0\n":"=q"(res):"rm"(word):"cc");
	return (int) res;
}

#endif							/* USE_POPCNT_ASM */


/*
 * pg_popcount32_slow
 *		Return the number of 1 bits set in word
 */
static int
pg_popcount32_slow(uint32 word)
{
#ifdef HAVE__BUILTIN_POPCOUNT
	return __builtin_popcount(word);
#else							/* !HAVE__BUILTIN_POPCOUNT */
	int			result = 0;

	while (word != 0)
	{
		result += pg_number_of_ones[word & 255];
		word >>= 8;
	}

	return result;
#endif							/* HAVE__BUILTIN_POPCOUNT */
}

/*
 * pg_popcount64_slow
 *		Return the number of 1 bits set in word
 */
static int
pg_popcount64_slow(uint64 word)
{
#ifdef HAVE__BUILTIN_POPCOUNT
#if defined(HAVE_LONG_INT_64)
	return __builtin_popcountl(word);
#elif defined(HAVE_LONG_LONG_INT_64)
	return __builtin_popcountll(word);
#else
#error must have a working 64-bit integer datatype
#endif
#else							/* !HAVE__BUILTIN_POPCOUNT */
	int			result = 0;

	while (word != 0)
	{
		result += pg_number_of_ones[word & 255];
		word >>= 8;
	}

	return result;
#endif							/* HAVE__BUILTIN_POPCOUNT */
}


/*
 * pg_popcount
 *		Returns the number of 1-bits in buf
 */
uint64
pg_popcount(const char *buf, int bytes)
{
	uint64		popcnt = 0;

#if SIZEOF_VOID_P >= 8
	/* Process in 64-bit chunks if the buffer is aligned. */
	if (buf == (const char *) TYPEALIGN(8, buf))
	{
		const uint64 *words = (const uint64 *) buf;

		while (bytes >= 8)
		{
			popcnt += pg_popcount64(*words++);
			bytes -= 8;
		}

		buf = (const char *) words;
	}
#else
	/* Process in 32-bit chunks if the buffer is aligned. */
	if (buf == (const char *) TYPEALIGN(4, buf))
	{
		const uint32 *words = (const uint32 *) buf;

		while (bytes >= 4)
		{
			popcnt += pg_popcount32(*words++);
			bytes -= 4;
		}

		buf = (const char *) words;
	}
#endif

	/* Process any remaining bytes */
	while (bytes--)
		popcnt += pg_number_of_ones[(unsigned char) *buf++];

	return popcnt;
}