Add kj::popCount to abstract over compiler differences

c++/src/capnp/serialize-packed.c++

@@ -483,7 +483,7 @@ size_t computeUnpackedSizeInWords(kj::ArrayPtr<const byte> packedBytes) {
   size_t total = 0;
   while (ptr < end) {
     uint tag = *ptr;
-    size_t count = __builtin_popcount(tag);
+    size_t count = kj::popCount(tag);
     total += 1;
     KJ_REQUIRE(end - ptr >= count, "invalid packed data");
     ptr += count + 1;

c++/src/kj/common.h

@@ -85,6 +85,10 @@
 #undef _GLIBCXX_HAVE_GETS
 #endif
 
+#if defined(_MSC_VER)
+#include <intrin.h>  // __popcnt
+#endif
+
 // =======================================================================================
 
 namespace kj {
@@ -597,6 +601,15 @@ float nan();
 inline constexpr bool isNaN(float f) { return f != f; }
 inline constexpr bool isNaN(double f) { return f != f; }
 
+inline int popCount(unsigned int x) {
+#if defined(_MSC_VER)
+  return __popcnt(x);
+  // Note: __popcnt returns unsigned int, but the value is clearly guaranteed to fit into an int
+#else
+  return __builtin_popcount(x);
+#endif
+}
+
 // =======================================================================================
 // Useful fake containers