Merge pull request #4441 from trentxintong/EnumProj

Small comment fix in FSO

Author: eeckstein

include/swift/SIL/Projection.h

@@ -870,6 +870,9 @@ class ProjectionTree {
   bool isSingleton() const {
     // If we only have one root node, there is no interesting explosion
     // here. Exit early.
+    //
+    // NOTE: In case of a type unable to be exploded, e.g. enum, we treated it
+    // as a singleton.
     if (ProjectionTreeNodes.size() == 1)
       return true;
 

include/swift/SILOptimizer/Utils/FunctionSignatureOptUtils.h

@@ -107,6 +107,9 @@ struct ArgumentDescriptor {
     // See if the projection tree consists of potentially multiple levels of
     // structs containing one field. In such a case, there is no point in
     // exploding the argument.
+    //
+    // Also, in case of a type can not be exploded, e.g an enum, we treat it
+    // as a singleton.
     if (ProjTree.isSingleton())
       return false;