Merge pull request rust-lang#104 from vext01/tidy

Tidy up IR serialiser a bit

Author: ltratt

llvm/lib/YkIR/YkIRWriter.cpp

@@ -169,10 +169,6 @@ class YkIRWriter {
     return getIndex(&M, F);
   }
 
-public:
-  YkIRWriter(Module &M, MCStreamer &OutStreamer)
-      : M(M), OutStreamer(OutStreamer) {}
-
   // Serialises a null-terminated string.
   void serialiseString(StringRef S) {
     OutStreamer.emitBinaryData(S);
@@ -489,6 +485,18 @@ class YkIRWriter {
     }
   }
 
+public:
+  YkIRWriter(Module &M, MCStreamer &OutStreamer)
+      : M(M), OutStreamer(OutStreamer) {}
+
+  // Entry point for IR serialisation.
+  //
+  // The order of serialisation matters.
+  //
+  // - Serialising functions can introduce new types and constants.
+  // - Serialising constants can introduce new types.
+  //
+  // So we must serialise functions, then constants, then types.
   void serialise() {
     // header:
     OutStreamer.emitInt32(Magic);