diff --git a/lib/Target/X86/X86MCAsmInfo.cpp b/lib/Target/X86/X86MCAsmInfo.cpp
index b220cff54c8..bc56e71b649 100644
--- a/lib/Target/X86/X86MCAsmInfo.cpp
+++ b/lib/Target/X86/X86MCAsmInfo.cpp
@@ -56,7 +56,10 @@ X86MCAsmInfoDarwin::X86MCAsmInfoDarwin(const Triple &Triple) {
     Data64bitsDirective = 0;       // we can't emit a 64-bit unit
 
   // Use ## as a comment string so that .s files generated by llvm can go
-  // through the GCC preprocessor without causing an error.
+  // through the GCC preprocessor without causing an error.  This is needed
+  // because "clang foo.s" runs the C preprocessor, which is usually reserved
+  // for .S files on other systems.  Perhaps this is because the file system
+  // wasn't always case preserving or something.
   CommentString = "##";
   PCSymbol = ".";