Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -154,11 +154,13 @@
 # define _LIBCPP_HAS_QUICK_EXIT
 #endif  // __NetBSD__
 
-#ifdef _WIN32
+#if defined(_WIN32)
+#  define _LIBCPP_WIN32API      1
 #  define _LIBCPP_LITTLE_ENDIAN 1
 #  define _LIBCPP_BIG_ENDIAN    0
+#  define _LIBCPP_SHORT_WCHAR   1
 // Compiler intrinsics (MSVC)
-#if defined(_MSC_VER) && _MSC_VER >= 1400
+#  if defined(_MSC_VER) && _MSC_VER >= 1400
 #    define _LIBCPP_HAS_IS_BASE_OF
 #  endif
 #  if defined(_MSC_VER) && !defined(__clang__)
@@ -167,11 +169,11 @@
 #    define _LIBCPP_TOSTRING(x) _LIBCPP_TOSTRING2(x)
 #    define _LIBCPP_WARNING(x) __pragma(message(__FILE__ "(" _LIBCPP_TOSTRING(__LINE__) ") : warning note: " x))
 #  endif
-#  // If mingw not explicitly detected, assume using MS C runtime only.
+// If mingw not explicitly detected, assume using MS C runtime only.
 #  ifndef __MINGW32__
 #    define _LIBCPP_MSVCRT // Using Microsoft's C Runtime library
 #  endif
-#endif  // _WIN32
+#endif // defined(_WIN32)
 
 #ifdef __sun__
 # include <sys/isa_defs.h>
@@ -195,7 +197,7 @@
   // including accesses to the special files under /dev. C++11's
   // std::random_device is instead exposed through a NaCl syscall.
 # define _LIBCPP_USING_NACL_RANDOM
-#elif defined(_WIN32)
+#elif defined(_LIBCPP_WIN32API)
 # define _LIBCPP_USING_WIN32_RANDOM
 #else
 # define _LIBCPP_USING_DEV_RANDOM
@@ -520,8 +522,15 @@
 
 #endif // __clang__ || __GNUC__ || _MSC_VER || __IBMCPP__
 
+#if defined(__ELF__)
+#define _LIBCPP_OBJECT_FORMAT_ELF   1
+#elif defined(__MACH__)
+#define _LIBCPP_OBJECT_FORMAT_MACHO 1
+#else
+#define _LIBCPP_OBJECT_FORMAT_COFF  1
+#endif
 
-#ifdef _WIN32
+#if defined(_LIBCPP_OBJECT_FORMAT_COFF)
 #if defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 # define _LIBCPP_DLL_VIS
 # define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
@@ -546,6 +555,7 @@
 #define _LIBCPP_TYPE_VIS_ONLY
 #define _LIBCPP_FUNC_VIS_ONLY
 #define _LIBCPP_ENUM_VIS
+
 #if defined(_LIBCPP_MSVC)
 # define _LIBCPP_INLINE_VISIBILITY __forceinline
 # define _LIBCPP_ALWAYS_INLINE     __forceinline
@@ -555,7 +565,7 @@
 # define _LIBCPP_ALWAYS_INLINE     __attribute__ ((__always_inline__))
 # define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY __attribute__ ((__always_inline__))
 #endif
-#endif // _WIN32
+#endif // defined(_LIBCPP_OBJECT_FORMAT_COFF)
 
 #ifndef _LIBCPP_HIDDEN
 #if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
@@ -778,14 +788,14 @@
 #define _LIBCPP_NONUNIQUE_RTTI_BIT (1ULL << 63)
 #endif
 
-#if defined(__APPLE__) || defined(__FreeBSD__) || defined(_WIN32) || \
+#if defined(__APPLE__) || defined(__FreeBSD__) || defined(_LIBCPP_MSVCRT) ||   \
     defined(__sun__) || defined(__NetBSD__) || defined(__CloudABI__)
 #define _LIBCPP_LOCALE__L_EXTENSIONS 1
 #endif
 
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 // Most unix variants have catopen.  These are the specific ones that don't.
-#if !defined(_WIN32) && !defined(__BIONIC__) && !defined(_NEWLIB_VERSION)
+#if !defined(__BIONIC__) && !defined(_NEWLIB_VERSION)
 #define _LIBCPP_HAS_CATOPEN 1
 #endif
 #endif
@@ -959,7 +969,7 @@
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_OFF_T_FUNCTIONS)
-#if defined(_WIN32) || defined(_NEWLIB_VERSION)
+#if defined(_LIBCPP_MSVCRT) || defined(_NEWLIB_VERSION)
 #define _LIBCPP_HAS_NO_OFF_T_FUNCTIONS
 #endif
 #endif
Index: include/__locale
===================================================================
--- include/__locale
+++ include/__locale
@@ -342,7 +342,7 @@
     static const mask punct  = _ISpunct;
     static const mask xdigit = _ISxdigit;
     static const mask blank  = _ISblank;
-#elif defined(_WIN32)
+#elif defined(_LIBCPP_MSVCRT)
     typedef unsigned short mask;
     static const mask space  = _SPACE;
     static const mask print  = _BLANK|_PUNCT|_ALPHA|_DIGIT;
Index: include/support/win32/support.h
===================================================================
--- include/support/win32/support.h
+++ include/support/win32/support.h
@@ -110,10 +110,11 @@
   unsigned long where;
 // Search from LSB to MSB for first set bit.
 // Returns zero if no set bit is found.
-#if defined(_WIN64)
+#if (defined(_M_ARM) || defined(__arm__)) ||                                   \
+    (defined(_M_AMD64) || defined(__x86_64__))
   if (_BitScanForward64(&where, mask))
     return static_cast<int>(where);
-#elif defined(_WIN32)
+#else
   // Win32 doesn't have _BitScanForward64 so emulate it with two 32 bit calls.
   // Scan the Low Word.
   if (_BitScanForward(&where, static_cast<unsigned long>(mask)))
@@ -121,8 +122,6 @@
   // Scan the High Word.
   if (_BitScanForward(&where, static_cast<unsigned long>(mask >> 32)))
     return static_cast<int>(where + 32); // Create a bit offset from the LSB.
-#else
-#error "Implementation of __builtin_ctzll required"
 #endif
   return 64;
 }
@@ -152,10 +151,11 @@
   unsigned long where;
 // BitScanReverse scans from MSB to LSB for first set bit.
 // Returns 0 if no set bit is found.
-#if defined(_WIN64)
+#if (defined(_M_ARM) || defined(__arm__)) ||                                   \
+    (defined(_M_AMD64) || defined(__x86_64__))
   if (_BitScanReverse64(&where, mask))
     return static_cast<int>(63 - where);
-#elif defined(_WIN32)
+#else
   // Scan the high 32 bits.
   if (_BitScanReverse(&where, static_cast<unsigned long>(mask >> 32)))
     return static_cast<int>(63 -
@@ -163,8 +163,6 @@
   // Scan the low 32 bits.
   if (_BitScanReverse(&where, static_cast<unsigned long>(mask)))
     return static_cast<int>(63 - where);
-#else
-#error "Implementation of __builtin_clzll required"
 #endif
   return 64; // Undefined Behavior.
 }
Index: include/type_traits
===================================================================
--- include/type_traits
+++ include/type_traits
@@ -1681,9 +1681,9 @@
 _CREATE_ALIGNED_STORAGE_SPECIALIZATION(0x1000);
 _CREATE_ALIGNED_STORAGE_SPECIALIZATION(0x2000);
 // PE/COFF does not support alignment beyond 8192 (=0x2000)
-#if !defined(_WIN32)
+#if !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 _CREATE_ALIGNED_STORAGE_SPECIALIZATION(0x4000);
-#endif // !_WIN32
+#endif // !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 
 #undef _CREATE_ALIGNED_STORAGE_SPECIALIZATION
 
Index: src/chrono.cpp
===================================================================
--- src/chrono.cpp
+++ src/chrono.cpp
@@ -12,7 +12,7 @@
 #include "system_error"  // __throw_system_error
 #include <time.h>        // clock_gettime, CLOCK_MONOTONIC and CLOCK_REALTIME
 
-#if defined(_WIN32)
+#if defined(_LIBCPP_WIN32API)
 #define WIN32_LEAN_AND_MEAN
 #define VC_EXTRA_LEAN
 #include <Windows.h>
@@ -22,13 +22,13 @@
 #else
 #if !defined(CLOCK_REALTIME)
 #include <sys/time.h>        // for gettimeofday and timeval
-#endif
-#endif
+#endif // !defined(CLOCK_REALTIME)
+#endif // defined(_LIBCPP_WIN32API)
 
 #if !defined(_LIBCPP_HAS_NO_MONOTONIC_CLOCK)
 #if __APPLE__
 #include <mach/mach_time.h>  // mach_absolute_time, mach_timebase_info_data_t
-#elif !defined(_WIN32) && !defined(CLOCK_MONOTONIC)
+#elif !defined(_LIBCPP_WIN32API) && !defined(CLOCK_MONOTONIC)
 #error "Monotonic clock not implemented"
 #endif
 #endif
@@ -45,7 +45,7 @@
 system_clock::time_point
 system_clock::now() _NOEXCEPT
 {
-#if defined(_WIN32)
+#if defined(_LIBCPP_WIN32API)
   // FILETIME is in 100ns units
   using filetime_duration =
       _VSTD::chrono::duration<__int64,
@@ -158,7 +158,7 @@
     return time_point(duration(fp()));
 }
 
-#elif defined(_WIN32)
+#elif defined(_LIBCPP_WIN32API)
 
 steady_clock::time_point
 steady_clock::now() _NOEXCEPT
Index: src/include/config_elast.h
===================================================================
--- src/include/config_elast.h
+++ src/include/config_elast.h
@@ -10,7 +10,7 @@
 #ifndef _LIBCPP_CONFIG_ELAST
 #define _LIBCPP_CONFIG_ELAST
 
-#if defined(_WIN32)
+#if defined(_LIBCPP_MSCVRT)
 #include <stdlib.h>
 #else
 #include <errno.h>
@@ -28,7 +28,7 @@
 // No _LIBCPP_ELAST needed on Apple
 #elif defined(__sun__)
 #define _LIBCPP_ELAST ESTALE
-#elif defined(_WIN32)
+#elif defined(_LIBCPP_MSCVRT)
 #define _LIBCPP_ELAST _sys_nerr
 #else
 // Warn here so that the person doing the libcxx port has an easier time:
Index: src/locale.cpp
===================================================================
--- src/locale.cpp
+++ src/locale.cpp
@@ -3254,7 +3254,7 @@
     const intern_type* frm, const intern_type* frm_end, const intern_type*& frm_nxt,
     extern_type* to, extern_type* to_end, extern_type*& to_nxt) const
 {
-#if _WIN32
+#if defined(_LIBCPP_SHORT_WCHAR)
     const uint16_t* _frm = reinterpret_cast<const uint16_t*>(frm);
     const uint16_t* _frm_end = reinterpret_cast<const uint16_t*>(frm_end);
     const uint16_t* _frm_nxt = _frm;
@@ -3266,7 +3266,7 @@
     uint8_t* _to = reinterpret_cast<uint8_t*>(to);
     uint8_t* _to_end = reinterpret_cast<uint8_t*>(to_end);
     uint8_t* _to_nxt = _to;
-#if _WIN32
+#if defined(_LIBCPP_SHORT_WCHAR)
     result r = ucs2_to_utf8(_frm, _frm_end, _frm_nxt, _to, _to_end, _to_nxt,
                             _Maxcode_, _Mode_);
 #else
@@ -3286,7 +3286,7 @@
     const uint8_t* _frm = reinterpret_cast<const uint8_t*>(frm);
     const uint8_t* _frm_end = reinterpret_cast<const uint8_t*>(frm_end);
     const uint8_t* _frm_nxt = _frm;
-#if _WIN32
+#if defined(_LIBCPP_SHORT_WCHAR)
     uint16_t* _to = reinterpret_cast<uint16_t*>(to);
     uint16_t* _to_end = reinterpret_cast<uint16_t*>(to_end);
     uint16_t* _to_nxt = _to;
Index: src/new.cpp
===================================================================
--- src/new.cpp
+++ src/new.cpp
@@ -72,7 +72,7 @@
     if (static_cast<size_t>(alignment) < sizeof(void*))
       alignment = std::align_val_t(sizeof(void*));
     void* p;
-#if defined(_WIN32)
+#if defined(_LIBCPP_MSVCRT)
     while ((p = _aligned_malloc(size, static_cast<size_t>(alignment))) == nullptr)
 #else
     while (::posix_memalign(&p, static_cast<size_t>(alignment), size) != 0)
Index: src/system_error.cpp
===================================================================
--- src/system_error.cpp
+++ src/system_error.cpp
@@ -65,7 +65,7 @@
 
 string do_strerror_r(int ev);
 
-#if defined(_WIN32)
+#if defined(_LIBCPP_MSVCRT)
 string do_strerror_r(int ev) {
   char buffer[strerror_buff_size];
   if (::strerror_s(buffer, strerror_buff_size, ev) == 0)
Index: src/thread.cpp
===================================================================
--- src/thread.cpp
+++ src/thread.cpp
@@ -24,16 +24,17 @@
 # endif // defined(BSD)
 #endif // defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 
-#if !defined(_WIN32)
+#if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 # include <unistd.h>
-#endif // !_WIN32
+#endif // defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 
 #if defined(__NetBSD__)
 #pragma weak pthread_create // Do not create libpthread dependency
 #endif
-#if defined(_WIN32)
+
+#if defined(_LIBCPP_WIN32API)
 #include <windows.h>
-#endif
+#endif // defined(_LIBCPP_WIN32API)
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -91,7 +92,7 @@
     if (result < 0)
         return 0;
     return static_cast<unsigned>(result);
-#elif defined(_WIN32)
+#elif defined(_LIBCPP_WIN32API)
     SYSTEM_INFO info;
     GetSystemInfo(&info);
     return info.dwNumberOfProcessors;