diff --git a/include/llvm/Target/TargetInstrInfo.h b/include/llvm/Target/TargetInstrInfo.h
index 44953a287ad..d7cc1cf45a6 100644
--- a/include/llvm/Target/TargetInstrInfo.h
+++ b/include/llvm/Target/TargetInstrInfo.h
@@ -459,6 +459,13 @@ public:
   }
 
   /// copyPhysReg - Emit instructions to copy a pair of physical registers.
+  ///
+  /// This function should support copies within any legal register class as
+  /// well as any cross-class copies created during instruction selection.
+  ///
+  /// The source and destination registers may overlap, which may require a
+  /// careful implementation when multiple copy instructions are required for
+  /// large registers. See for example the ARM target.
   virtual void copyPhysReg(MachineBasicBlock &MBB,
                            MachineBasicBlock::iterator MI, DebugLoc DL,
                            unsigned DestReg, unsigned SrcReg,