Update the docs of the cost model.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171016 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/CostModel.cpp b/lib/Analysis/CostModel.cpp
index 0d0a8933b35c61aff5b979bd597cff033e6a4178..28d6a864707798d815f4208629652da4d7bfba60 100644 (file)
--- a/lib/Analysis/CostModel.cpp
+++ b/lib/Analysis/CostModel.cpp
@@ -8,9 +8,12 @@
 //===----------------------------------------------------------------------===//
 //
 // This file defines the cost model analysis. It provides a very basic cost
-// estimation for LLVM-IR. The cost result can be thought of as cycles, but it
-// is really unit-less. The estimated cost is ment to be used for comparing
-// alternatives.
+// estimation for LLVM-IR. This analysis uses the services of the codegen
+// to approximate the cost of any IR instruction when lowered to machine
+// instructions. The cost results are unit-less and the cost number represents
+// the throughput of the machine assuming that all loads hit the cache, all
+// branches are predicted, etc. The cost numbers can be added in order to
+// compare two or more transformation alternatives.
 //
 //===----------------------------------------------------------------------===//