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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130992 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/PostRASchedulerList.cpp b/lib/CodeGen/PostRASchedulerList.cpp
index 60c24b7..848af2a 100644
--- a/lib/CodeGen/PostRASchedulerList.cpp
+++ b/lib/CodeGen/PostRASchedulerList.cpp
@@ -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.