[llvm] r185900 - X86: Add comment.

[Jim Grosbach]

Author: grosbach
Date: Mon Jul  8 21:07:28 2013
New Revision: 185900

URL: http://llvm.org/viewvc/llvm-project?rev=185900&view=rev
Log:
X86: Add comment.

Modified:
    llvm/trunk/lib/Target/X86/X86ISelDAGToDAG.cpp

Modified: llvm/trunk/lib/Target/X86/X86ISelDAGToDAG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86ISelDAGToDAG.cpp?rev=185900&r1=185899&r2=185900&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86ISelDAGToDAG.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86ISelDAGToDAG.cpp Mon Jul  8 21:07:28 2013
@@ -2531,6 +2531,11 @@ SDNode *X86DAGToDAGISel::Select(SDNode *
 
     // Prevent use of AH in a REX instruction by referencing AX instead.
     // Shift it down 8 bits.
+    //
+    // The current assumption of the register allocator is that isel
+    // won't generate explicit references to the GPR8_NOREX registers. If
+    // the allocator and/or the backend get enhanced to be more robust in
+    // that regard, this can be, and should be, removed.
     if (HiReg == X86::AH && Subtarget->is64Bit() &&
         !SDValue(Node, 1).use_empty()) {
       SDValue Result = CurDAG->getCopyFromReg(CurDAG->getEntryNode(), dl,