Add some comments on ISD::NodeType.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63234 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 7 insertions, 0 deletions

diff --git a/include/llvm/CodeGen/SelectionDAGNodes.h b/include/llvm/CodeGen/SelectionDAGNodes.h
index c22b90ceed..9f496d42af 100644
--- a/include/llvm/CodeGen/SelectionDAGNodes.h
+++ b/include/llvm/CodeGen/SelectionDAGNodes.h
@@ -63,6 +63,13 @@ namespace ISD {
   /// ISD::NodeType enum - This enum defines all of the operators valid in a
   /// SelectionDAG.
   ///
+  /// These are sometimes called the target-independent operators; targets
+  /// may also define target-dependent operators. For example, on x86, these
+  /// are the enum values in the X86ISD namespace. Targets should aim to use
+  /// target-independent operators to model their instruction sets as much
+  /// as possible, and only use target-dependent operators when they have
+  /// special requirements.
+  ///
   enum NodeType {
     // DELETED_NODE - This is an illegal flag value that is used to catch
     // errors.  This opcode is not a legal opcode for any node.