ProjectExplorer: Update known predefined macros for MSVC

...and add a comment explaining what we do and why we do it this way.

The added macros were collected as stated in the comment for MSVC
versions (x86, amd64) 2003 - 2017. Note that for Visual Studio 2017
there is no "Predefined macros" page yet.

Change-Id: I7601be08d7a0bbb63077abe9657d203661f76cb9
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/msvctoolchain.cpp

@@ -262,28 +262,51 @@ static QByteArray msvcCompilationFile()
 {
     static const char* macros[] = {
         "_ATL_VER",
+        "__ATOM__",
+        "__AVX__",
+        "__AVX2__",
         "_CHAR_UNSIGNED",
         "__CLR_VER",
+        "_CMMN_INTRIN_FUNC",
+        "_CONTROL_FLOW_GUARD",
         "__COUNTER__",
         "__cplusplus",
         "__cplusplus_cli",
+        "__cplusplus_winrt",
         "_CPPLIB_VER",
         "_CPPRTTI",
         "_CPPUNWIND",
         "__DATE__",
         "_DEBUG",
         "_DLL",
+        "__FILE__",
+        "__func__",
         "__FUNCDNAME__",
         "__FUNCSIG__",
         "__FUNCTION__",
         "_INTEGRAL_MAX_BITS",
+        "__INTELLISENSE__",
+        "_ISO_VOLATILE",
+        "_KERNEL_MODE",
+        "__LINE__",
         "_M_AAMD64",
         "_M_ALPHA",
+        "_M_AMD64",
         "_MANAGED",
+        "_M_ARM",
+        "_M_ARM64",
+        "_M_ARM_ARMV7VE",
+        "_M_ARM_FP",
+        "_M_ARM_NT",
+        "_M_ARMT",
         "_M_CEE",
         "_M_CEE_PURE",
         "_M_CEE_SAFE",
         "_MFC_VER",
+        "_M_FP_EXCEPT",
+        "_M_FP_FAST",
+        "_M_FP_PRECISE",
+        "_M_FP_STRICT",
         "_M_IA64",
         "_M_IX86",
         "_M_IX86_FP",
@@ -294,11 +317,17 @@ static QByteArray msvcCompilationFile()
         "_MSC_EXTENSIONS",
         "_MSC_FULL_VER",
         "_MSC_VER",
+        "_MSVC_LANG",
         "__MSVC_RUNTIME_CHECKS",
         "_MT",
+        "_M_THUMB",
         "_M_X64",
         "_NATIVE_WCHAR_T_DEFINED",
         "_OPENMP",
+        "_PREFAST_",
+        "__STDC__",
+        "__STDC_HOSTED__",
+        "__STDCPP_THREADS__",
         "__TIME__",
         "__TIMESTAMP__",
         "_VC_NODEFAULTLIB",
@@ -306,6 +335,7 @@ static QByteArray msvcCompilationFile()
         "_WIN32",
         "_WIN32_WCE",
         "_WIN64",
+        "_WINRT_DLL",
         "_Wp64",
         0
     };
@@ -320,7 +350,62 @@ static QByteArray msvcCompilationFile()
 }
 
 // Run MSVC 'cl' compiler to obtain #defines.
-// Function must be thread-safe!
+// This function must be thread-safe!
+//
+// Some notes regarding the used approach:
+//
+// It seems that there is no reliable way to get all the
+// predefined macros for a cl invocation. The following two
+// approaches are unfortunately limited since both lead to an
+// incomplete list of actually predefined macros and come with
+// other problems, too.
+//
+// 1) Maintain a list of predefined macros from the official
+//    documentation (for MSVC2015, e.g. [1]). Feed cl with a
+//    temporary file that queries the values of those macros.
+//
+//    Problems:
+//     * Maintaining that list.
+//     * The documentation is incomplete, we do not get all
+//       predefined macros. E.g. the cl from MSVC2015, set up
+//       with "vcvars.bat x86_arm", predefines among others
+//       _M_ARMT, but that's not reflected in the
+//       documentation.
+//
+// 2) Run cl with the undocumented options /B1 and /Bx, as
+//    described in [2].
+//
+//    Note: With qmake from Qt >= 5.8 it's possible to print
+//    the macros formatted as preprocessor code in an easy to
+//    read/compare/diff way:
+//
+//      > cl /nologo /c /TC /B1 qmake NUL
+//      > cl /nologo /c /TP /Bx qmake NUL
+//
+//    Problems:
+//     * Using undocumented options.
+//     * Resulting macros are incomplete.
+//       For example, the nowadays default option /Zc:wchar_t
+//       predefines _WCHAR_T_DEFINED, but this is not reflected
+//       with this approach.
+//
+//       To work around this we would need extra cl invocations
+//       to get the actual values of the missing macros
+//       (approach 1).
+//
+// Currently we combine both approaches in this way:
+//  * As base, maintain the list from the documentation and
+//    update it once a new MSVC version is released.
+//  * Enrich it with macros that we discover with approach 2
+//    once a new MSVC version is released.
+//  * Enrich it further with macros that are not covered with
+//    the above points.
+//
+// TODO: Update the predefined macros for MSVC 2017 once the
+//       page exists.
+//
+// [1] https://msdn.microsoft.com/en-us/library/b0084kay.aspx
+// [2] http://stackoverflow.com/questions/3665537/how-to-find-out-cl-exes-built-in-macros
 QByteArray MsvcToolChain::msvcPredefinedMacros(const QStringList cxxflags,
                                                const Utils::Environment &env) const
 {