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Use direct function calls for pg_popcount{32,64} on non-x86 platforms

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Previously, all pg_popcount{32,64} calls were indirected through
a function pointer, even though we had no fast implementation for
non-x86 platforms. Instead, for those platforms use wrappers around
the pg_popcount{32,64}_slow functions.

Review and additional hacking by David Rowley
Reviewed by Álvaro Herrera

Discussion: https://www.postgresql.org/message-id/flat/CAFBsxsE7otwnfA36Ly44zZO%2Bb7AEWHRFANxR1h1kxveEV%3DghLQ%40mail.gmail.com
This commit is contained in:
John Naylor 2021-08-16 11:45:21 -04:00
parent ea499f3d28
commit 4864c8e8f1
2 changed files with 53 additions and 27 deletions
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32
src/include/port/pg_bitutils.h
View File

@ -253,10 +253,40 @@ pg_ceil_log2_64(uint64 num)
return pg_leftmost_one_pos64(num - 1) + 1;
}
/* Count the number of one-bits in a uint32 or uint64 */
/*
* With MSVC on x86_64 builds, try using native popcnt instructions via the
* __popcnt and __popcnt64 intrinsics. These don't work the same as GCC's
* __builtin_popcount* intrinsic functions as they always emit popcnt
* instructions.
*/
#if defined(_MSC_VER) && defined(_M_AMD64)
#define HAVE_X86_64_POPCNTQ
#endif
/*
* 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 a hand-rolled implementation.
*/
#ifdef HAVE_X86_64_POPCNTQ
#if defined(HAVE__GET_CPUID) || defined(HAVE__CPUID)
#define TRY_POPCNT_FAST 1
#endif
#endif
#ifdef TRY_POPCNT_FAST
/* Attempt to use the POPCNT instruction, but perform a runtime check first */
extern int (*pg_popcount32) (uint32 word);
extern int (*pg_popcount64) (uint64 word);
#else
/* Use a portable implementation -- no need for a function pointer. */
extern int pg_popcount32(uint32 word);
extern int pg_popcount64(uint64 word);
#endif /* TRY_POPCNT_FAST */
/* Count the number of one-bits in a byte array */
extern uint64 pg_popcount(const char *buf, int bytes);

48
src/port/pg_bitutils.c
View File

@ -103,29 +103,6 @@ const uint8 pg_number_of_ones[256] = {
4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8
};
/*
* With MSVC on x86_64 builds, try using native popcnt instructions via the
* __popcnt and __popcnt64 intrinsics. These don't work the same as GCC's
* __builtin_popcount* intrinsic functions as they always emit popcnt
* instructions.
*/
#if defined(_MSC_VER) && defined(_M_AMD64)
#define HAVE_X86_64_POPCNTQ
#endif
/*
* 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 TRY_POPCNT_FAST 1
#endif
#endif
static int pg_popcount32_slow(uint32 word);
static int pg_popcount64_slow(uint64 word);
@ -138,9 +115,6 @@ static int pg_popcount64_fast(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 /* TRY_POPCNT_FAST */
#ifdef TRY_POPCNT_FAST
@ -291,6 +265,28 @@ pg_popcount64_slow(uint64 word)
#endif /* HAVE__BUILTIN_POPCOUNT */
}
#ifndef TRY_POPCNT_FAST
/*
* When the POPCNT instruction is not available, there's no point in using
* function pointers to vary the implementation between the fast and slow
* method. We instead just make these actual external functions when
* TRY_POPCNT_FAST is not defined. The compiler should be able to inline
* the slow versions here.
*/
int
pg_popcount32(uint32 word)
{
return pg_popcount32_slow(word);
}
int
pg_popcount64(uint64 word)
{
return pg_popcount64_slow(word);
}
#endif /* !TRY_POPCNT_FAST */
/*
* pg_popcount