Add a top-level comment about DAGCombiner's role in the compiler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@70052 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 72b3e3627ed53133104d703c13d81b3a83fca66c..b937eaff757cd2a271c980f614aa04c576d84325 100644 (file)
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -10,6 +10,10 @@
 // This pass combines dag nodes to form fewer, simpler DAG nodes.  It can be run
 // both before and after the DAG is legalized.
 //
+// This pass is not a substitute for the LLVM IR instcombine pass. This pass is
+// primarily intended to handle simplification opportunities that are implicit
+// in the LLVM IR and exposed by the various codegen lowering phases.
+//
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "dagcombine"