[PATCH] D46841: MCSchedModel: Add comments to IssueWidth.

[Andrew Trick via Phabricator via llvm-commits]

atrick created this revision.
atrick added a reviewer: jnspaulsson.
Herald added subscribers: llvm-commits, mcrosier.

Repository:
  rL LLVM

https://reviews.llvm.org/D46841

Files:
  include/llvm/MC/MCSchedule.h


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;
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D46841.146652.patch
Type: text/x-patch
Size: 1184 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20180514/c9e3e396/attachment.bin>