Add MSVC support for pg_leftmost_one_pos32() and friends

To allow testing for general support for fast bitscan intrinsics,
add symbols HAVE_BITSCAN_REVERSE and HAVE_BITSCAN_FORWARD.

Also do related cleanup in AllocSetFreeIndex(): Previously, we
tested for HAVE__BUILTIN_CLZ and copied the relevant internals of
pg_leftmost_one_pos32(), with a special fallback that does less
work than the general fallback for that function. Now that we have
a more general test, we just call pg_leftmost_one_pos32() directly
for platforms with intrinsic support. On gcc at least, there is no
difference in the binary for non-assert builds.

Discussion: https://www.postgresql.org/message-id/CAFBsxsEPc%2BFnX_0vmmQ5DHv60sk4rL_RZJ%2BMD6ei%3D76L0kFMvA%40mail.gmail.com

src/backend/utils/mmgr/aset.c

@@ -289,7 +289,7 @@ AllocSetFreeIndex(Size size)
 		 * or equivalently
 		 *		pg_leftmost_one_pos32(size - 1) - ALLOC_MINBITS + 1
 		 *
-		 * However, rather than just calling that function, we duplicate the
+		 * However, for platforms without intrinsic support, we duplicate the
 		 * logic here, allowing an additional optimization.  It's reasonable
 		 * to assume that ALLOC_CHUNK_LIMIT fits in 16 bits, so we can unroll
 		 * the byte-at-a-time loop in pg_leftmost_one_pos32 and just handle
@@ -299,8 +299,8 @@ AllocSetFreeIndex(Size size)
 		 * much trouble.
 		 *----------
 		 */
-#ifdef HAVE__BUILTIN_CLZ
+#ifdef HAVE_BITSCAN_REVERSE
-		idx = 31 - __builtin_clz((uint32) size - 1) - ALLOC_MINBITS + 1;
+		idx = pg_leftmost_one_pos32((uint32) size - 1) - ALLOC_MINBITS + 1;
 #else
 		uint32		t,
 					tsize;

src/include/port/pg_bitutils.h

@@ -13,6 +13,21 @@
 #ifndef PG_BITUTILS_H
 #define PG_BITUTILS_H
 
+#ifdef _MSC_VER
+#include <intrin.h>
+#define HAVE_BITSCAN_FORWARD
+#define HAVE_BITSCAN_REVERSE
+
+#else
+#if defined(HAVE__BUILTIN_CTZ)
+#define HAVE_BITSCAN_FORWARD
+#endif
+
+#if defined(HAVE__BUILTIN_CLZ)
+#define HAVE_BITSCAN_REVERSE
+#endif
+#endif							/* _MSC_VER */
+
 extern PGDLLIMPORT const uint8 pg_leftmost_one_pos[256];
 extern PGDLLIMPORT const uint8 pg_rightmost_one_pos[256];
 extern PGDLLIMPORT const uint8 pg_number_of_ones[256];
@@ -27,9 +42,12 @@ pg_leftmost_one_pos32(uint32 word)
 {
 #ifdef HAVE__BUILTIN_CLZ
 	int			bitscan_result;
+#elif defined(_MSC_VER)
+	unsigned long bitscan_result;
+	bool		non_zero;
 #endif
 
-#if !defined(HAVE__BUILTIN_CLZ) || defined(USE_ASSERT_CHECKING)
+#if !defined(HAVE_BITSCAN_REVERSE) || defined(USE_ASSERT_CHECKING)
 	int			result;
 	int			shift = 32 - 8;
 
@@ -41,13 +59,20 @@ pg_leftmost_one_pos32(uint32 word)
 	result = shift + pg_leftmost_one_pos[(word >> shift) & 255];
 #endif
 
+#ifdef HAVE_BITSCAN_REVERSE
+
 #if defined(HAVE__BUILTIN_CLZ)
 	bitscan_result = 31 - __builtin_clz(word);
+#elif defined(_MSC_VER)
+	non_zero = _BitScanReverse(&bitscan_result, word);
+	Assert(non_zero);
+#endif
 	Assert(bitscan_result == result);
 	return bitscan_result;
+
 #else
 	return result;
-#endif							/* HAVE__BUILTIN_CLZ */
+#endif							/* HAVE_BITSCAN_REVERSE */
 }
 
 /*
@@ -59,9 +84,12 @@ pg_leftmost_one_pos64(uint64 word)
 {
 #ifdef HAVE__BUILTIN_CLZ
 	int			bitscan_result;
+#elif defined(_MSC_VER)
+	unsigned long bitscan_result;
+	bool		non_zero;
 #endif
 
-#if !defined(HAVE__BUILTIN_CLZ) || defined(USE_ASSERT_CHECKING)
+#if !defined(HAVE_BITSCAN_REVERSE) || defined(USE_ASSERT_CHECKING)
 	int			result;
 	int			shift = 64 - 8;
 
@@ -73,6 +101,8 @@ pg_leftmost_one_pos64(uint64 word)
 	result = shift + pg_leftmost_one_pos[(word >> shift) & 255];
 #endif
 
+#ifdef HAVE_BITSCAN_REVERSE
+
 #if defined(HAVE__BUILTIN_CLZ)
 #if defined(HAVE_LONG_INT_64)
 	bitscan_result = 63 - __builtin_clzl(word);
@@ -81,11 +111,17 @@ pg_leftmost_one_pos64(uint64 word)
 #else
 #error must have a working 64-bit integer datatype
 #endif							/* HAVE_LONG_INT_64 */
+
+#elif defined(_MSC_VER)
+	non_zero = _BitScanReverse64(&bitscan_result, word);
+	Assert(non_zero);
+#endif							/* HAVE__BUILTIN_CLZ */
 	Assert(bitscan_result == result);
 	return bitscan_result;
+
 #else
 	return result;
-#endif							/* HAVE__BUILTIN_CLZ */
+#endif							/* HAVE_BITSCAN_REVERSE */
 }
 
 /*
@@ -99,9 +135,13 @@ pg_rightmost_one_pos32(uint32 word)
 #ifdef HAVE__BUILTIN_CTZ
 	const uint32 orig_word = word;
 	int			bitscan_result;
+#elif defined(_MSC_VER)
+	const unsigned long orig_word = word;
+	unsigned long bitscan_result;
+	bool		non_zero;
 #endif
 
-#if !defined(HAVE__BUILTIN_CTZ) || defined(USE_ASSERT_CHECKING)
+#if !defined(HAVE_BITSCAN_FORWARD) || defined(USE_ASSERT_CHECKING)
 	int			result = 0;
 
 	Assert(word != 0);
@@ -114,13 +154,20 @@ pg_rightmost_one_pos32(uint32 word)
 	result += pg_rightmost_one_pos[word & 255];
 #endif
 
+#ifdef HAVE_BITSCAN_FORWARD
+
 #if defined(HAVE__BUILTIN_CTZ)
 	bitscan_result = __builtin_ctz(orig_word);
+#elif defined(_MSC_VER)
+	non_zero = _BitScanForward(&bitscan_result, orig_word);
+	Assert(non_zero);
+#endif
 	Assert(bitscan_result == result);
 	return bitscan_result;
+
 #else
 	return result;
-#endif							/* HAVE__BUILTIN_CTZ */
+#endif							/* HAVE_BITSCAN_FORWARD */
 }
 
 /*
@@ -133,9 +180,13 @@ pg_rightmost_one_pos64(uint64 word)
 #ifdef HAVE__BUILTIN_CTZ
 	const uint64 orig_word = word;
 	int			bitscan_result;
+#elif defined(_MSC_VER)
+	const unsigned __int64 orig_word = word;
+	unsigned long bitscan_result;
+	bool		non_zero;
 #endif
 
-#if !defined(HAVE__BUILTIN_CTZ) || defined(USE_ASSERT_CHECKING)
+#if !defined(HAVE_BITSCAN_FORWARD) || defined(USE_ASSERT_CHECKING)
 	int			result = 0;
 
 	Assert(word != 0);
@@ -148,6 +199,8 @@ pg_rightmost_one_pos64(uint64 word)
 	result += pg_rightmost_one_pos[word & 255];
 #endif
 
+#ifdef HAVE_BITSCAN_FORWARD
+
 #if defined(HAVE__BUILTIN_CTZ)
 #if defined(HAVE_LONG_INT_64)
 	bitscan_result = __builtin_ctzl(orig_word);
@@ -156,11 +209,17 @@ pg_rightmost_one_pos64(uint64 word)
 #else
 #error must have a working 64-bit integer datatype
 #endif							/* HAVE_LONG_INT_64 */
+
+#elif defined(_MSC_VER)
+	non_zero = _BitScanForward64(&bitscan_result, orig_word);
+	Assert(non_zero);
+#endif							/* HAVE__BUILTIN_CTZ */
 	Assert(bitscan_result == result);
 	return bitscan_result;
+
 #else
 	return result;
-#endif							/* HAVE__BUILTIN_CTZ */
+#endif							/* HAVE_BITSCAN_FORWARD */
 }
 
 /*