[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

llvm/include/llvm/CodeGen/MachineInstr.h

@@ -280,6 +280,9 @@ public:
   const MachineBasicBlock* getParent() const { return Parent; }
   MachineBasicBlock* getParent() { return Parent; }
 
+  /// Move the instruction before \p MovePos.
+  void moveBefore(MachineInstr *MovePos);
+
   /// Return the function that contains the basic block that this instruction
   /// belongs to.
   ///

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.

llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp

@@ -1034,7 +1034,29 @@ EmitSchedule(MachineBasicBlock::iterator &InsertPos) {
   }
 
   InsertPos = Emitter.getInsertPos();
-  return Emitter.getBlock();
+  // In some cases, DBG_VALUEs might be inserted after the first terminator,
+  // which results in an invalid MBB. If that happens, move the DBG_VALUEs
+  // before the first terminator.
+  MachineBasicBlock *InsertBB = Emitter.getBlock();
+  auto FirstTerm = InsertBB->getFirstTerminator();
+  if (FirstTerm != InsertBB->end()) {
+    assert(!FirstTerm->isDebugValue() &&
+           "first terminator cannot be a debug value");
+    for (MachineInstr &MI : make_early_inc_range(
+             make_range(std::next(FirstTerm), InsertBB->end()))) {
+      if (!MI.isDebugValue())
+        continue;
+
+      if (&MI == InsertPos)
+        InsertPos = std::prev(InsertPos->getIterator());
+
+      // The DBG_VALUE was referencing a value produced by a terminator. By
+      // moving the DBG_VALUE, the referenced value also needs invalidating.
+      MI.getOperand(0).ChangeToRegister(0, false);
+      MI.moveBefore(&*FirstTerm);
+    }
+  }
+  return InsertBB;
 }
 
 /// Return the basic block label.

llvm/test/CodeGen/ARM/dbg-tcreturn.ll

@@ -1,5 +1,4 @@
-; XFAIL: *
-; RUN: llc %s -o - -stop-after=finalize-isel -verify-machineinstr | FileCheck %s
+; RUN: llc %s -o - -stop-after=finalize-isel -verify-machineinstrs | FileCheck %s
 
 target datalayout = "e-m:o-p:32:32-Fi8-f64:32:64-v64:32:64-v128:32:128-a:0:32-n32-S32"
 target triple = "thumbv7-apple-ios7.0.0"
@@ -12,8 +11,8 @@ target triple = "thumbv7-apple-ios7.0.0"
 ; CHECK-NEXT:     %0:gpr = COPY $r0
 ; CHECK-NEXT:     $r0 = COPY %0
 ; CHECK-NEXT:     $r1 = COPY %1
-; CHECK-NEXT:     TCRETURNdi &__divsi3, implicit $sp, implicit $r0, implicit $r1
 ; CHECK-NEXT:     DBG_VALUE $noreg, $noreg, !13, !DIExpression(), debug-location !16
+; CHECK-NEXT:     TCRETURNdi &__divsi3, implicit $sp, implicit $r0, implicit $r1
 
 define i32 @test(i32 %a1, i32 %a2) !dbg !5 {
 entry: