[MachineInstr] Move MIParser's DBG_VALUE RegState::Debug invariant into MachineInstr::addOperand

Based on the reasoning of D53903, register operands of DBG_VALUE are
invariably treated as RegState::Debug operands. This change enforces
this invariant as part of MachineInstr::addOperand so that all passes
emit this flag consistently.

RegState::Debug is inconsistently set on DBG_VALUE registers throughout
LLVM. This runs the risk of a filtering iterator like
MachineRegisterInfo::reg_nodbg_iterator to process these operands
erroneously when not parsed from MIR sources.

This issue was observed in the development of the llvm-mos fork which
adds a backend that relies on physical register operands much more than
existing targets. Physical RegUnit 0 has the same numeric encoding as
$noreg (indicating an undef for DBG_VALUE). Allowing debug operands into
the machine scheduler correlates $noreg with RegUnit 0 (i.e. a collision
of register numbers with different zero semantics). Eventually, this
causes an assert where DBG_VALUE instructions are prohibited from
participating in live register ranges.

Reviewed By: MatzeB, StephenTozer

Differential Revision: https://reviews.llvm.org/D110105

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineOperand.cpp b/llvm/lib/CodeGen/MachineOperand.cpp
index b8ba045..4d080e1 100644
--- a/llvm/lib/CodeGen/MachineOperand.cpp
+++ b/llvm/lib/CodeGen/MachineOperand.cpp
@@ -250,6 +250,11 @@ void MachineOperand::ChangeToRegister(Register Reg, bool isDef, bool isImp,
   if (RegInfo && WasReg)
     RegInfo->removeRegOperandFromUseList(this);
 
+  // Ensure debug instructions set debug flag on register uses.
+  const MachineInstr *MI = getParent();
+  if (!isDef && MI && MI->isDebugInstr())
+    isDebug = true;
+
   // Change this to a register and set the reg#.
   assert(!(isDead && !isDef) && "Dead flag on non-def");
   assert(!(isKill && isDef) && "Kill flag on def");