Index: include/llvm/MC/MCSchedule.h
===================================================================
--- include/llvm/MC/MCSchedule.h
+++ include/llvm/MC/MCSchedule.h
@@ -202,6 +202,18 @@
 struct MCSchedModel {
   // IssueWidth is the maximum number of instructions that may be scheduled in
   // the same per-cycle group.
+  //
+  // The abstract pipeline is built around the notion of an "issue point". This
+  // is merely a reference point for counting machine cycles.  IssueWidth is
+  // meant to be a hard in-order constraint (we sometimes call this kind of
+  // constraint a "hazard"). In the GenericScheduler strategy, no more than
+  // IssueWidth micro-ops can ever be scheduled in a particular cycle.
+  //
+  // In practice, IssueWidth is useful to model to the bottleneck between the
+  // decoder (after micro-op expansion) and the out-of-order reservation
+  // stations. If the total number of reservation stations is also a bottleneck,
+  // or if any other pipeline stage has a bandwidth limitation, then that can be
+  // naturally modeled by adding an out-of-order processor resource.
   unsigned IssueWidth;
   static const unsigned DefaultIssueWidth = 1;