Replace __builtin_popcount with std::bitset<>::count (#1336)

pdq/cpp/common/pdqhamming.h

@@ -5,16 +5,20 @@
 #ifndef PDQHAMMING_H
 #define PDQHAMMING_H
 
+#include <bitset>
+
 #include <pdq/cpp/common/pdqbasetypes.h>
 
+// OLD NOTICE:
 // If your compiler doesn't support __builtin_popcount then feel free to
 // undefine this. (Experiments have shown that using builtin popcount helps
 // performance by a few percent -- worth using but OK to live without.)
-#if !defined(_MSC_VER) && !defined(WIN32) && !defined(_WIN32) &&  \
-    !defined(__WIN32__) && !defined(WIN64) && !defined(_WIN64) && \
-    !defined(__WIN64__)
+//
+// NOTICE:
+// __builin_popcount was replaced by std::bitset<>::count()
+// The define is left here for backwards compatibility
+// if you want to use the lookup table.
 #define USE_BUILTIN_POPCOUNT
-#endif
 
 namespace facebook {
 namespace pdq {
@@ -23,16 +27,16 @@ namespace hashing {
 #ifdef USE_BUILTIN_POPCOUNT
 // Inlined right here
 inline int hammingNorm8(Hash8 h) {
-  return __builtin_popcount((unsigned)h);
+  return std::bitset<8>(h).count();
 }
 inline int hammingNorm16(Hash16 h) {
-  return __builtin_popcount((unsigned)h);
+  return std::bitset<16>(h).count();
 }
 inline int hammingDistance8(Hash8 a, Hash8 b) {
-  return __builtin_popcount((unsigned)(a ^ b));
+  return std::bitset<8>(a ^ b).count();
 }
 inline int hammingDistance16(Hash16 a, Hash16 b) {
-  return __builtin_popcount((unsigned)(a ^ b));
+  return std::bitset<16>(a ^ b).count();
 }
 #else
 // Implemented in pdqhamming.cpp with lookup tables.