Index: lib/Transforms/Scalar/GVN.cpp
===================================================================
--- lib/Transforms/Scalar/GVN.cpp
+++ lib/Transforms/Scalar/GVN.cpp
@@ -67,6 +67,7 @@
 static cl::opt<bool> EnablePRE("enable-pre",
                                cl::init(true), cl::Hidden);
 static cl::opt<bool> EnableLoadPRE("enable-load-pre", cl::init(true));
+static cl::opt<bool> DisableZeroCostPHIPRE("disable-phi-pre", cl::init(false));
 
 // Maximum allowed recursion depth.
 static cl::opt<uint32_t>
@@ -2529,7 +2530,8 @@
 
   // Don't do PRE when it might increase code size, i.e. when
   // we would need to insert instructions in more than one pred.
-  if (NumWithout > 1 || NumWith == 0)
+  if ((DisableZeroCostPHIPRE && NumWithout != 1) || 
+      ((!DisableZeroCostPHIPRE) && NumWithout > 1) || NumWith == 0)
     return false;
 
   // We may have a case where all predecessors have the instruction,