[llvm-commits] [llvm] r130992 - /llvm/trunk/lib/CodeGen/PostRASchedulerList.cpp

[Andrew Trick]

Author: atrick
Date: Fri May  6 12:09:08 2011
New Revision: 130992

URL: http://llvm.org/viewvc/llvm-project?rev=130992&view=rev
Log:
Post-RA scheduler compile time fix. Quadratic computation of DAG node depth.

The post-ra scheduler was explicitly updating the depth of a node's
successors after scheduling it, regardless of whether the successor
was ready. This is quadratic for DAGs with transitively redundant
edges. I simply removed the useless update of depth, which is lazilly
computed later.
Fixes <rdar://problem/9044332> compiler takes way too long to build TextInput.

Modified:
    llvm/trunk/lib/CodeGen/PostRASchedulerList.cpp

Modified: llvm/trunk/lib/CodeGen/PostRASchedulerList.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/PostRASchedulerList.cpp?rev=130992&r1=130991&r2=130992&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/PostRASchedulerList.cpp (original)
+++ llvm/trunk/lib/CodeGen/PostRASchedulerList.cpp Fri May  6 12:09:08 2011
@@ -540,10 +540,16 @@
 #endif
   --SuccSU->NumPredsLeft;
 
-  // Compute how many cycles it will be before this actually becomes
-  // available.  This is the max of the start time of all predecessors plus
-  // their latencies.
-  SuccSU->setDepthToAtLeast(SU->getDepth() + SuccEdge->getLatency());
+  // Standard scheduler algorithms will recomute the depth of the successor
+  // here as such:
+  //   SuccSU->setDepthToAtLeast(SU->getDepth() + SuccEdge->getLatency());
+  //
+  // However, we lazily compute node depth instead. Note that
+  // ScheduleNodeTopDown has already updated the depth of this node which causes
+  // all descendents to be marked dirty. Setting the successor depth explicitly
+  // here would cause depth to be recomputed for all its ancestors. If the
+  // successor is not yet ready (because of a transitively redundant edge) then
+  // this causes depth computation to be quadratic in the size of the DAG.
 
   // If all the node's predecessors are scheduled, this node is ready
   // to be scheduled. Ignore the special ExitSU node.