Enable _BitScanForward64 at runtime

Only add infrastructure, still disabled.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/bitboard.cpp

@@ -340,7 +340,7 @@ Square pop_1st_bit(Bitboard *b) {
 
 #endif
 
-#else
+#else // defined(USE_FOLDED_BITSCAN)
 
 static const int BitTable[64] = {
   0, 1, 2, 7, 3, 13, 8, 19, 4, 25, 14, 28, 9, 34, 20, 40, 5, 17, 26, 38, 15,

src/bitcount.h

@@ -66,6 +66,7 @@ inline bool cpu_has_popcnt() {
 }
 
 #define POPCNT_INTRINSIC(x) __popcnt64(x)
+#define BITSCAN_INTRINSIC(idx, x) _BitScanForward64(idx, x)
 
 #elif defined(__INTEL_COMPILER) && (defined(__x86_64) || defined(_M_X64)) // Intel compiler
 
@@ -79,12 +80,14 @@ inline bool cpu_has_popcnt() {
 }
 
 #define POPCNT_INTRINSIC(x) _mm_popcnt_u64(x)
+#define BITSCAN_INTRINSIC(idx, x) _BitScanForward64(idx, x)
 
 #else // Safe fallback for unsupported compilers
 
 inline bool cpu_has_popcnt() { return false; }
 
 #define POPCNT_INTRINSIC(x) sw_count_1s(x)
+#define BITSCAN_INTRINSIC(idx, x) sw_count_1s(x) // dummy
 
 #endif
 
@@ -183,4 +186,25 @@ const bool CpuHas64BitPath = true;
 const bool CpuHas64BitPath = false;
 #endif
 
+
+/// pop_1st_bit() finds and clears the least significant nonzero bit in a
+/// nonzero bitboard. If template parameter is true an intrinsic is called,
+/// otherwise we fallback on a software implementation.
+
+template<bool UseIntrinsic>
+inline Square pop_1st_bit(Bitboard *b) {
+
+  return pop_1st_bit(b);
+}
+
+template<>
+inline Square pop_1st_bit<true>(Bitboard *b) {
+
+  unsigned long idx;
+  Bitboard bb = *b;
+  BITSCAN_INTRINSIC(&idx, bb);
+  *b &= (bb - 1);
+  return Square(idx);
+}
+
 #endif // !defined(BITCOUNT_H_INCLUDED)

src/movegen.cpp

@@ -29,7 +29,7 @@
 
 // Simple macro to wrap a very common while loop, no facny, no flexibility,
 // hardcoded list name 'mlist' and from square 'from'.
-#define SERIALIZE_MOVES(b) while (b) (*mlist++).move = make_move(from, pop_1st_bit(&b))
+#define SERIALIZE_MOVES(b) while (b) (*mlist++).move = make_move(from, pop_1st_bit<false>(&b))
 
 ////
 //// Local definitions