Index: lib/Target/X86/X86TargetTransformInfo.cpp
===================================================================
--- lib/Target/X86/X86TargetTransformInfo.cpp
+++ lib/Target/X86/X86TargetTransformInfo.cpp
@@ -13,6 +13,16 @@
 /// independent and default TTI implementations handle the rest.
 ///
 //===----------------------------------------------------------------------===//
+/// About Cost Model numbers used below it's necessary to say the folowing:
+/// the numbers correspond to some "generic" X86 CPU instead of usage of
+/// concrete CPU model. Usually the numbers correspond to CPU where the feature
+/// apeared at the first time. For example, if we do Subtarget.hasSSE42() in
+/// the lookups below the cost is based on Nehalem as that was the first CPU
+/// to support that feature level and thus has most likely the worst case cost.
+/// We're going to introduce the target dependent cost model soon as well as we
+/// are going to specialize cost numbers for different Cost Model Targets such
+/// as throughput, code size, latency and uop count.
+//===----------------------------------------------------------------------===//
 
 #include "X86TargetTransformInfo.h"
 #include "llvm/Analysis/TargetTransformInfo.h"