Add comment explaining this mysterious macro name.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285631 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Frontend/InitPreprocessor.cpp b/lib/Frontend/InitPreprocessor.cpp
index 5da547e2b54db9865b084d7a5a4d8c7c3cad4bc5..613248a7f1c62d75a9996cc9fa8f4c5ea4112cca 100644 (file)
--- a/lib/Frontend/InitPreprocessor.cpp
+++ b/lib/Frontend/InitPreprocessor.cpp
@@ -992,6 +992,9 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
   }
 
   if (TI.hasInt128Type() && LangOpts.CPlusPlus && LangOpts.GNUMode) {
+    // For each extended integer type, g++ defines a macro mapping the
+    // index of the type (0 in this case) in some list of extended types
+    // to the type.
     Builder.defineMacro("__GLIBCXX_TYPE_INT_N_0", "__int128");
     Builder.defineMacro("__GLIBCXX_BITSIZE_INT_N_0", "128");
   }