[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, 2 insertions, 2 deletions

diff --git a/llvm/unittests/CodeGen/GlobalISel/PatternMatchTest.cpp b/llvm/unittests/CodeGen/GlobalISel/PatternMatchTest.cpp
index b5f4e22..19d9cbe 100644
--- a/llvm/unittests/CodeGen/GlobalISel/PatternMatchTest.cpp
+++ b/llvm/unittests/CodeGen/GlobalISel/PatternMatchTest.cpp
@@ -499,8 +499,8 @@ TEST_F(AArch64GISelMITest, MatchMiscellaneous) {
   EXPECT_TRUE(mi_match(Reg, *MRI, m_OneNonDBGUse(m_GAdd(m_Reg(), m_Reg()))));
 
   // Add multiple debug uses of Reg.
-  B.buildInstr(TargetOpcode::DBG_VALUE, {}, {Reg})->getOperand(0).setIsDebug();
-  B.buildInstr(TargetOpcode::DBG_VALUE, {}, {Reg})->getOperand(0).setIsDebug();
+  B.buildInstr(TargetOpcode::DBG_VALUE, {}, {Reg});
+  B.buildInstr(TargetOpcode::DBG_VALUE, {}, {Reg});
 
   EXPECT_FALSE(mi_match(Reg, *MRI, m_OneUse(m_GAdd(m_Reg(), m_Reg()))));
   EXPECT_TRUE(mi_match(Reg, *MRI, m_OneNonDBGUse(m_GAdd(m_Reg(), m_Reg()))));