Hoist back-copies to the least busy dominator.

When a back-copy is hoisted to the nearest common dominator, keep
looking up the dominator tree for a less loopy dominator, and place the
back-copy there instead.

Don't do this when a single existing back-copy dominates all the others.
Assume the client knows what he is doing, and keep the dominating
back-copy.

This prevents us from hoisting back-copies into loops in most cases.  If
a value is defined in a loop with multiple exits, we may still hoist
back-copies into that loop.  That is the speed/size tradeoff.

llvm-svn: 139698

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SplitKit.h b/llvm/lib/CodeGen/SplitKit.h
index 5a294e50554..d8fc2122a3c 100644
--- a/llvm/lib/CodeGen/SplitKit.h
+++ b/llvm/lib/CodeGen/SplitKit.h
@@ -315,6 +315,11 @@ private:
   /// in the vector in the complement interval.
   void removeBackCopies(SmallVectorImpl<VNInfo*> &Copies);
 
+  /// getShallowDominator - Returns the least busy dominator of MBB that is
+  /// also dominated by DefMBB.  Busy is measured by loop depth.
+  MachineBasicBlock *findShallowDominator(MachineBasicBlock *MBB,
+                                          MachineBasicBlock *DefMBB);
+
   /// hoistCopiesForSize - Hoist back-copies to the complement interval in a
   /// way that minimizes code size. This implements the SM_Size spill mode.
   void hoistCopiesForSize();