[PowerPC] Add a FIXME.

Documenting a design choice to generate only medium model sequences for TLS
addresses at this time.  Small and large code models could be supported if
necessary.

llvm-svn: 190883

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

@@ -1408,6 +1408,10 @@ SDValue PPCTargetLowering::LowerBlockAddress(SDValue Op,
 SDValue PPCTargetLowering::LowerGlobalTLSAddress(SDValue Op,
                                               SelectionDAG &DAG) const {
 
+  // FIXME: TLS addresses currently use medium model code sequences,
+  // which is the most useful form.  Eventually support for small and
+  // large models could be added if users need it, at the cost of
+  // additional complexity.
   GlobalAddressSDNode *GA = cast<GlobalAddressSDNode>(Op);
   SDLoc dl(GA);
   const GlobalValue *GV = GA->getGlobal();