Win32 does not have popcnt64 function and fix bug with defines in ctz function

[EricWF]

LGTM other than inline nits.

include/bit

92

This check seems redundant since _LIBCPP_HAS_BITSCAN is defined by those very same conditions.

[danlark]

In D58021#1392322, @EricWF wrote:

LGTM other than inline nits.

I don't have commit rights, I believe

[mclow.lists]

include/bit

150

We like our own defines in our files; we try to avoid using things like _M_IX86 (what does that even mean?)

If we need a new macro that says "We are a 32 bit windows environment", then we should define one.

[mclow.lists]

This is all pretty much a lost cause on Windows anyway, since none of the compiler intrinsics are constexpr, and with the (upcoming) adoption of P0553 and P0556 these routines will have to be constexpr.

include/bit

 int __ctz(unsigned long __x) {
     static_assert(sizeof(unsigned long) == sizeof(unsigned), "");
     return __ctz(static_cast<unsigned>(__x));
 }
 
 inline _LIBCPP_INLINE_VISIBILITY
 int __ctz(unsigned long long __x) {
     unsigned long __where;
-#if defined(_LIBCPP_HAS_BITSCAN64)
+#if defined(_LIBCPP_HAS_BITSCAN64) && \
     (defined(_M_AMD64) || defined(__x86_64__))
   if (_BitScanForward64(&__where, __x))
     return static_cast<int>(__where);
 #else
   // Win32 doesn't have _BitScanForward64 so emulate it with two 32 bit calls.
   if (_BitScanForward(&__where, static_cast<unsigned long>(__x)))
     return static_cast<int>(__where);
   if (_BitScanForward(&__where, static_cast<unsigned long>(__x >> 32)))
     return static_cast<int>(__where + 32);

 
 inline _LIBCPP_INLINE_VISIBILITY int __popcount(unsigned long __x) {
   static_assert(sizeof(unsigned long) == 4, "");
   return __popcnt(__x);
 }
 
 inline _LIBCPP_INLINE_VISIBILITY int __popcount(unsigned long long __x) {
   static_assert(sizeof(unsigned long long) == 8, "");
+#if defined(_M_IX86)
+  // Win32 doesn't have __popcnt64 so emulate it with two 32 bit calls.
+  return __popcnt(__x) + __popcnt(__x >> 32);
+#else
   return __popcnt64(__x);
+#endif
 }
 
 #endif // _LIBCPP_COMPILER_MSVC
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_BIT