MachineCopyPropagation: Do not consider undef operands as clobbers

This was originally introduced in r278321 to work around correctness
problems in the ExecutionDepsFix pass; Probably also to keep the
performance benefits of breaking the false dependencies which of course
also affect undef operands.

ExecutionDepsFix has been improved here recently (see for example
r278321) so we should not need this exception any longer.

Differential Revision: https://reviews.llvm.org/D29525

llvm-svn: 294087

1 files changed, 0 insertions, 8 deletions

diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index 92d043d..8662a30 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -280,14 +280,6 @@ void MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
           MaybeDeadCopies.remove(CI->second);
         }
       }
-      // Treat undef use like defs for copy propagation but not for
-      // dead copy. We would need to do a liveness check to be sure the copy
-      // is dead for undef uses.
-      // The backends are allowed to do whatever they want with undef value
-      // and we cannot be sure this register will not be rewritten to break
-      // some false dependencies for the hardware for instance.
-      if (MO.isUndef())
-        Defs.push_back(Reg);
     }
 
     // The instruction has a register mask operand which means that it clobbers