Define a popcount util

add include

Define a popcount util

Update MathUtils.h

include/swift/Basic/MathUtils.h

@@ -19,6 +19,10 @@
 
 #include <cstddef>
 
+#if SWIFT_COMPILER_IS_MSVC
+#include <intrin.h>
+#endif
+
 namespace swift {
 
 /// Round the given value up to the given alignment, as a power of two.
@@ -33,6 +37,15 @@ static inline size_t roundUpToAlignMask(size_t size, size_t alignMask) {
   return (size + alignMask) & ~alignMask;
 }
 
+static inline unsigned popcount(unsigned value) {
+#if SWIFT_COMPILER_IS_MSVC
+  return __popcnt(value);
+#else
+  // Assume we have a compiler with this intrinsic.
+  return __builtin_popcount(value);
+#endif
+}
+
 } // namespace swift
 
 #endif // #ifndef SWIFT_BASIC_MATH_UTILS_H