diff --git a/libcxx/include/__config b/libcxx/include/__config
index 8e39ad60726b..e50325a4ef42 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -96,7 +96,7 @@
 # endif
 #endif  // !defined(_LIBCPP_LITTLE_ENDIAN) || !defined(_LIBCPP_BIG_ENDIAN)
 
-#if _WIN32
+#ifdef _WIN32
 
 // only really useful for a DLL
 #ifdef _LIBCPP_DLL // this should be a compiler builtin define ideally...
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
index cfa3fda7a1b0..1c093f69a8e3 100644
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -19,7 +19,7 @@
 #include <cstdint>
 #include <cctype>
 #include <locale.h>
-#if _WIN32
+#ifdef _WIN32
 # include <support/win32/locale_win32.h>
 #elif (__GLIBC__ || __APPLE__ || __FreeBSD__ || __sun__)
 # include <xlocale.h>
@@ -327,7 +327,7 @@ public:
     static const mask punct  = _ISpunct;
     static const mask xdigit = _ISxdigit;
     static const mask blank  = _ISblank;
-#elif _WIN32
+#elif defined(_WIN32)
     typedef unsigned short mask;
     static const mask space  = _SPACE;
     static const mask print  = _BLANK|_PUNCT|_ALPHA|_DIGIT;
diff --git a/libcxx/include/cwchar b/libcxx/include/cwchar
index eed6de18f1a3..caed08de3fa0 100644
--- a/libcxx/include/cwchar
+++ b/libcxx/include/cwchar
@@ -106,7 +106,7 @@ size_t wcsrtombs(char* restrict dst, const wchar_t** restrict src, size_t len,
 #include <__config>
 #include <cwctype>
 #include <wchar.h>
-#if _WIN32
+#ifdef _WIN32
 #include <support/win32/support.h> // pull in *swprintf defines
 #endif // _WIN32
 
diff --git a/libcxx/include/locale b/libcxx/include/locale
index 8f9a3b696404..a6f2e35bc29a 100644
--- a/libcxx/include/locale
+++ b/libcxx/include/locale
@@ -186,7 +186,7 @@ template <class charT> class messages_byname;
 #endif
 #include <cstdlib>
 #include <ctime>
-#if _WIN32
+#ifdef _WIN32
 #include <support/win32/locale_win32.h>
 #else // _WIN32
 #include <nl_types.h>
@@ -3799,7 +3799,7 @@ template <class _CharT>
 typename messages<_CharT>::catalog
 messages<_CharT>::do_open(const basic_string<char>& __nm, const locale&) const
 {
-#if _WIN32
+#ifdef _WIN32
     return -1;
 #else // _WIN32
     catalog __cat = (catalog)catopen(__nm.c_str(), NL_CAT_LOCALE);
@@ -3814,7 +3814,7 @@ typename messages<_CharT>::string_type
 messages<_CharT>::do_get(catalog __c, int __set, int __msgid,
                          const string_type& __dflt) const
 {
-#if _WIN32
+#ifdef _WIN32
     return __dflt;
 #else // _WIN32
     string __ndflt;
@@ -3836,7 +3836,7 @@ template <class _CharT>
 void
 messages<_CharT>::do_close(catalog __c) const
 {
-#if !_WIN32
+#if !defined(_WIN32)
     if (__c != -1)
         __c <<= 1;
     nl_catd __cat = (nl_catd)__c;
diff --git a/libcxx/src/locale.cpp b/libcxx/src/locale.cpp
index 214979036ccb..b7a47843b3da 100644
--- a/libcxx/src/locale.cpp
+++ b/libcxx/src/locale.cpp
@@ -25,7 +25,7 @@
 #include "cstring"
 #include "cwctype"
 #include "__sso_allocator"
-#if _WIN32
+#ifdef _WIN32
 #include <support/win32/locale_win32.h>
 #else // _WIN32
 #include <langinfo.h>
@@ -987,7 +987,7 @@ ctype<char>::classic_table()  _NOEXCEPT
     return __cloc()->__ctype_b;
 #elif __sun__
     return __ctype_mask;
-#elif _WIN32
+#elif defined(_WIN32)
     return _ctype+1; // internal ctype mask table defined in msvcrt.dll
 // This is assumed to be safe, which is a nonsense assumption because we're
 // going to end up dereferencing it later...
@@ -5779,7 +5779,7 @@ moneypunct_byname<char, true>::init(const char* nm)
         __frac_digits_ = lc->int_frac_digits;
     else
         __frac_digits_ = base::do_frac_digits();
-#if _WIN32
+#ifdef _WIN32
     if (lc->p_sign_posn == 0)
 #else // _WIN32
     if (lc->int_p_sign_posn == 0)
@@ -5787,7 +5787,7 @@ moneypunct_byname<char, true>::init(const char* nm)
         __positive_sign_ = "()";
     else
         __positive_sign_ = lc->positive_sign;
-#if _WIN32
+#ifdef _WIN32
     if(lc->n_sign_posn == 0)
 #else // _WIN32
     if (lc->int_n_sign_posn == 0)
@@ -5799,7 +5799,7 @@ moneypunct_byname<char, true>::init(const char* nm)
     // the same places in curr_symbol since there's no way to
     // represent anything else.
     string_type __dummy_curr_symbol = __curr_symbol_;
-#if _WIN32
+#ifdef _WIN32
     __init_pat(__pos_format_, __dummy_curr_symbol, true,
                lc->p_cs_precedes, lc->p_sep_by_space, lc->p_sign_posn, ' ');
     __init_pat(__neg_format_, __curr_symbol_, true,
@@ -5938,7 +5938,7 @@ moneypunct_byname<wchar_t, true>::init(const char* nm)
         __frac_digits_ = lc->int_frac_digits;
     else
         __frac_digits_ = base::do_frac_digits();
-#if _WIN32
+#ifdef _WIN32
     if (lc->p_sign_posn == 0)
 #else // _WIN32
     if (lc->int_p_sign_posn == 0)
@@ -5958,7 +5958,7 @@ moneypunct_byname<wchar_t, true>::init(const char* nm)
         wbe = wbuf + j;
         __positive_sign_.assign(wbuf, wbe);
     }
-#if _WIN32
+#ifdef _WIN32
     if (lc->n_sign_posn == 0)
 #else // _WIN32
     if (lc->int_n_sign_posn == 0)
@@ -5982,7 +5982,7 @@ moneypunct_byname<wchar_t, true>::init(const char* nm)
     // the same places in curr_symbol since there's no way to
     // represent anything else.
     string_type __dummy_curr_symbol = __curr_symbol_;
-#if _WIN32
+#ifdef _WIN32
     __init_pat(__pos_format_, __dummy_curr_symbol, true,
                lc->p_cs_precedes, lc->p_sep_by_space, lc->p_sign_posn, L' ');
     __init_pat(__neg_format_, __curr_symbol_, true,
diff --git a/libcxx/src/string.cpp b/libcxx/src/string.cpp
index 40723e74a156..c71af4feb944 100644
--- a/libcxx/src/string.cpp
+++ b/libcxx/src/string.cpp
@@ -11,7 +11,7 @@
 #include "cstdlib"
 #include "cwchar"
 #include "cerrno"
-#if _WIN32
+#ifdef _WIN32
 #include "support/win32/support.h"
 #endif // _WIN32
 
diff --git a/libcxx/src/thread.cpp b/libcxx/src/thread.cpp
index 447eca7b0193..1acb13108e88 100644
--- a/libcxx/src/thread.cpp
+++ b/libcxx/src/thread.cpp
@@ -13,7 +13,7 @@
 #include "future"
 #include "limits"
 #include <sys/types.h>
-#if !_WIN32
+#if !defined(_WIN32)
 #if !__sun__ && !__linux__
 #include <sys/sysctl.h>
 #else
diff --git a/libcxx/test/support/platform_support.h b/libcxx/test/support/platform_support.h
index eff5af072c51..0035975019a8 100644
--- a/libcxx/test/support/platform_support.h
+++ b/libcxx/test/support/platform_support.h
@@ -16,7 +16,7 @@
 #define PLATFORM_SUPPORT_H
 
 // locale names
-#if _WIN32
+#ifdef _WIN32
 // WARNING: Windows does not support UTF-8 codepages.
 // Locales are "converted" using http://docs.moodle.org/dev/Table_of_locales
 #define LOCALE_en_US_UTF_8     "English_United States.1252"