Guard against UB in lsb/msb

[stockfish] / src / bitboard.h

diff --git a/src/bitboard.h b/src/bitboard.h
index 967a68f59f8f8247397bb62e24eb8a7a3a569b7a..77a824ba62106965ba9513803a19c650c092bb4c 100644 (file)
--- a/src/bitboard.h
+++ b/src/bitboard.h
@@ -259,54 +259,37 @@ inline Bitboard attacks_bb(Piece pc, Square s, Bitboard occupied) {
 
 /// lsb() and msb() return the least/most significant bit in a non-zero bitboard
 
-#ifdef USE_BSFQ
-
-#  if defined(_MSC_VER) && !defined(__INTEL_COMPILER)
+#if defined(__GNUC__)
 
 inline Square lsb(Bitboard b) {
-  unsigned long idx;
-  _BitScanForward64(&idx, b);
-  return (Square) idx;
+  assert(b);
+  return Square(__builtin_ctzll(b));
 }
 
 inline Square msb(Bitboard b) {
-  unsigned long idx;
-  _BitScanReverse64(&idx, b);
-  return (Square) idx;
+  assert(b);
+  return Square(63 - __builtin_clzll(b));
 }
 
-#  elif defined(__arm__)
-
-inline int lsb32(uint32_t v) {
-  __asm__("rbit %0, %1" : "=r"(v) : "r"(v));
-  return __builtin_clz(v);
-}
-
-inline Square msb(Bitboard b) {
-  return (Square) (63 - __builtin_clzll(b));
-}
+#elif defined(_WIN64) && defined(_MSC_VER)
 
 inline Square lsb(Bitboard b) {
-  return (Square) (uint32_t(b) ? lsb32(uint32_t(b)) : 32 + lsb32(uint32_t(b >> 32)));
-}
-
-#  else // Assumed gcc or compatible compiler
-
-inline Square lsb(Bitboard b) { // Assembly code by Heinz van Saanen
-  Bitboard idx;
-  __asm__("bsfq %1, %0": "=r"(idx): "rm"(b) );
+  assert(b);
+  unsigned long idx;
+  _BitScanForward64(&idx, b);
   return (Square) idx;
 }
 
 inline Square msb(Bitboard b) {
-  Bitboard idx;
-  __asm__("bsrq %1, %0": "=r"(idx): "rm"(b) );
+  assert(b);
+  unsigned long idx;
+  _BitScanReverse64(&idx, b);
   return (Square) idx;
 }
 
-#  endif
+#else
 
-#else // ifdef(USE_BSFQ)
+#define NO_BSF // Fallback on software implementation for other cases
 
 Square lsb(Bitboard b);
 Square msb(Bitboard b);