Index: lib/CodeGen/IfConversion.cpp
===================================================================
--- lib/CodeGen/IfConversion.cpp
+++ lib/CodeGen/IfConversion.cpp
@@ -7,7 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This file implements the machine instruction level if-conversion pass.
+// This file implements the machine instruction level if-conversion pass, which
+// tries to convert conditional branches into predicated instructions.
 //
 //===----------------------------------------------------------------------===//
 
@@ -673,6 +674,12 @@
     if (I->isDebugValue())
       continue;
 
+    // We don't need to set CannotBeCopied to true if BBI contains a convergent
+    // op.  CannotBeCopied prevents us from moving a BB into a predecessor and
+    // predicating it, if the BB that has more than one predecessor.  But this
+    // transformation is safe on BBs that contain convergent ops, as we're
+    // moving the convergent op *up* the CFG, thus removing, rather than adding,
+    // control-flow dependencies.
     if (I->isNotDuplicable())
       BBI.CannotBeCopied = true;