Increase tail dup threshold for -O3 from 3 to 4.

We see a modest performance improvement from this slightly higher tail dup threshold.

Differential Revision: https://reviews.llvm.org/D36775

llvm-svn: 311139
diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 374b307..6533030 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -143,7 +143,7 @@
     "tail-dup-placement-aggressive-threshold",
     cl::desc("Instruction cutoff for aggressive tail duplication during "
              "layout. Used at -O3. Tail merging during layout is forced to "
-             "have a threshold that won't conflict."), cl::init(3),
+             "have a threshold that won't conflict."), cl::init(4),
     cl::Hidden);
 
 // Heuristic for tail duplication.