update file comment

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26573 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/SelectionDAG/ScheduleDAGList.cpp b/lib/CodeGen/SelectionDAG/ScheduleDAGList.cpp
index dfd46b7d01410bce9498ceb4272fab68a6377d8a..4d44d2d9b011499cc7e74e83e3c24c892600c570 100644 (file)
--- a/lib/CodeGen/SelectionDAG/ScheduleDAGList.cpp
+++ b/lib/CodeGen/SelectionDAG/ScheduleDAGList.cpp
@@ -7,9 +7,14 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This implements a simple two pass scheduler.  The first pass attempts to push
-// backward any lengthy instructions and critical paths.  The second pass packs
-// instructions into semi-optimal time slots.
+// This implements bottom-up and top-down list schedulers, using standard
+// algorithms.  The basic approach uses a priority queue of available nodes to
+// schedule.  One at a time, nodes are taken from the priority queue (thus in
+// priority order), checked for legality to schedule, and emitted if legal.
+//
+// Nodes may not be legal to schedule either due to structural hazards (e.g.
+// pipeline or resource constraints) or because an input to the instruction has
+// not completed execution.
 //
 //===----------------------------------------------------------------------===//