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

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

1 files changed, 31 insertions, 1 deletions

diff --git a/src/include/port/pg_bitutils.h b/src/include/port/pg_bitutils.h
index 086bd08132f..7dd7fef4f74 100644
--- a/src/include/port/pg_bitutils.h
+++ b/src/include/port/pg_bitutils.h
@@ -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);