Document an implementation detail about EXTRACT_SUBREG and INSERT_SUBREG sub-register...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48283 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/CodeGen/SelectionDAGNodes.h b/include/llvm/CodeGen/SelectionDAGNodes.h
index b2c354dae1d56c3674e559533f154b5c5f776095..49012b4b0e758a1953308bf8d83625871b1f2dd5 100644 (file)
--- a/include/llvm/CodeGen/SelectionDAGNodes.h
+++ b/include/llvm/CodeGen/SelectionDAGNodes.h
@@ -320,6 +320,9 @@ namespace ISD {
     
     // EXTRACT_SUBREG - This node is used to extract a sub-register value. 
     // This node takes a superreg and a constant sub-register index as operands.
+    // Note sub-register indices must be increasing. That is, if the
+    // sub-register index of a 8-bit sub-register is N, then the index for a
+    // 16-bit sub-register must be at least N+1.
     EXTRACT_SUBREG,
     
     // INSERT_SUBREG - This node is used to insert a sub-register value.