Use intrinsics only for LSB/MSB

The NO_BSF does not cover any real life use-case today. The only compilers that
can compile SF today, with the current Makefile and no source code changes, are
either GCC compatible (define __GNUC__) or MSVC compatible (define _MSC_VER). So
they all support LSB/MSB intrinsics.

This patch simplifies away the software fall-backs of LSB/MSB that were still
in Stockfish code, but unused in any of the officially supported compilers.

Note the (legacy) MSVC/WIN32 case, where we use a 32-bit BSF/BSR solution, as
64-bit intrinsics aren't available there.

Discussed in: https://github.com/official-stockfish/Stockfish/pull/1447
and:          https://github.com/official-stockfish/Stockfish/pull/1479

No functional change.

src/bitboard.cpp

@@ -45,27 +45,11 @@ Magic BishopMagics[SQUARE_NB];
 
 namespace {
 
-  // De Bruijn sequences. See chessprogramming.wikispaces.com/BitScan
-  const uint64_t DeBruijn64 = 0x3F79D71B4CB0A89ULL;
-  const uint32_t DeBruijn32 = 0x783A9B23;
-
-  int MSBTable[256];            // To implement software msb()
-  Square BSFTable[SQUARE_NB];   // To implement software bitscan
   Bitboard RookTable[0x19000];  // To store rook attacks
   Bitboard BishopTable[0x1480]; // To store bishop attacks
 
   void init_magics(Bitboard table[], Magic magics[], Direction directions[]);
 
-  // bsf_index() returns the index into BSFTable[] to look up the bitscan. Uses
-  // Matt Taylor's folding for 32 bit case, extended to 64 bit by Kim Walisch.
-
-  unsigned bsf_index(Bitboard b) {
-    b ^= b - 1;
-    return Is64Bit ? (b * DeBruijn64) >> 58
-                   : ((unsigned(b) ^ unsigned(b >> 32)) * DeBruijn32) >> 26;
-  }
-
-
   // popcount16() counts the non-zero bits using SWAR-Popcount algorithm
 
   unsigned popcount16(unsigned u) {
@@ -76,46 +60,6 @@ namespace {
   }
 }
 
-#ifdef NO_BSF
-
-/// Software fall-back of lsb() and msb() for CPU lacking hardware support
-
-Square lsb(Bitboard b) {
-  assert(b);
-  return BSFTable[bsf_index(b)];
-}
-
-Square msb(Bitboard b) {
-
-  assert(b);
-  unsigned b32;
-  int result = 0;
-
-  if (b > 0xFFFFFFFF)
-  {
-      b >>= 32;
-      result = 32;
-  }
-
-  b32 = unsigned(b);
-
-  if (b32 > 0xFFFF)
-  {
-      b32 >>= 16;
-      result += 16;
-  }
-
-  if (b32 > 0xFF)
-  {
-      b32 >>= 8;
-      result += 8;
-  }
-
-  return Square(result + MSBTable[b32]);
-}
-
-#endif // ifdef NO_BSF
-
 
 /// Bitboards::pretty() returns an ASCII representation of a bitboard suitable
 /// to be printed to standard output. Useful for debugging.
@@ -145,13 +89,7 @@ void Bitboards::init() {
       PopCnt16[i] = (uint8_t) popcount16(i);
 
   for (Square s = SQ_A1; s <= SQ_H8; ++s)
-  {
       SquareBB[s] = 1ULL << s;
-      BSFTable[bsf_index(SquareBB[s])] = s;
-  }
-
-  for (Bitboard b = 2; b < 256; ++b)
-      MSBTable[b] = MSBTable[b - 1] + !more_than_one(b);
 
   for (File f = FILE_A; f <= FILE_H; ++f)
       FileBB[f] = f > FILE_A ? FileBB[f - 1] << 1 : FileABB;

src/bitboard.h

@@ -303,7 +303,7 @@ inline int popcount(Bitboard b) {
 
 /// lsb() and msb() return the least/most significant bit in a non-zero bitboard
 
-#if defined(__GNUC__)
+#if defined(__GNUC__)  // GCC, Clang, ICC
 
 inline Square lsb(Bitboard b) {
   assert(b);
@@ -315,7 +315,9 @@ inline Square msb(Bitboard b) {
   return Square(63 ^ __builtin_clzll(b));
 }
 
-#elif defined(_WIN64) && defined(_MSC_VER)
+#elif defined(_MSC_VER)  // MSVC
+
+#ifdef _WIN64  // MSVC, WIN64
 
 inline Square lsb(Bitboard b) {
   assert(b);
@@ -331,12 +333,39 @@ inline Square msb(Bitboard b) {
   return (Square) idx;
 }
 
-#else
+#else  // MSVC, WIN32
 
-#define NO_BSF // Fallback on software implementation for other cases
+inline Square lsb(Bitboard b) {
+  assert(b);
+  unsigned long idx;
 
-Square lsb(Bitboard b);
+  if (b & 0xffffffff) {
-Square msb(Bitboard b);
+      _BitScanForward(&idx, int32_t(b));
+      return Square(idx);
+  } else {
+      _BitScanForward(&idx, int32_t(b >> 32));
+      return Square(idx + 32);
+  }
+}
+
+inline Square msb(Bitboard b) {
+  assert(b);
+  unsigned long idx;
+
+  if (b >> 32) {
+      _BitScanReverse(&idx, int32_t(b >> 32));
+      return Square(idx + 32);
+  } else {
+      _BitScanReverse(&idx, int32_t(b));
+      return Square(idx);
+  }
+}
+
+#endif
+
+#else  // Compiler is neither GCC nor MSVC compatible
+
+#error "Compiler not supported."
 
 #endif