[ScheduleDAG] Move DBG_VALUEs after first term forward.

MBBs are not allowed to have non-terminator instructions after the first
terminator. Currently in some cases (see the modified test),
EmitSchedule can add DBG_VALUEs after the last terminator, for example
when referring a debug value that gets folded into a TCRETURN
instruction on ARM.

This patch updates EmitSchedule to move inserted DBG_VALUEs just before
the first terminator. I am not sure if there are terminators produce
values that can in turn be used by a DBG_VALUE. In that case, moving the
DBG_VALUE might result in referencing an undefined register. But in any
case, it seems like currently there is no way to insert a proper DBG_VALUEs
for such registers anyways.

Alternatively it might make sense to just remove those extra DBG_VALUES.

I am not too familiar with the details of debug info in the backend and
would appreciate any suggestions on how to address the issue in the best
possible way.

Reviewers: vsk, aprantl, jpaquette, efriedma, paquette

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D83561

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineInstr.cpp b/llvm/lib/CodeGen/MachineInstr.cpp
index d418159..5fbf91e 100644
--- a/llvm/lib/CodeGen/MachineInstr.cpp
+++ b/llvm/lib/CodeGen/MachineInstr.cpp
@@ -147,6 +147,10 @@ MachineInstr::MachineInstr(MachineFunction &MF, const MachineInstr &MI)
   setFlags(MI.Flags);
 }
 
+void MachineInstr::moveBefore(MachineInstr *MovePos) {
+  MovePos->getParent()->splice(MovePos, getParent(), getIterator());
+}
+
 /// getRegInfo - If this instruction is embedded into a MachineFunction,
 /// return the MachineRegisterInfo object for the current function, otherwise
 /// return null.