diff --git a/llvm/lib/CodeGen/MachineFunction.cpp b/llvm/lib/CodeGen/MachineFunction.cpp
--- a/llvm/lib/CodeGen/MachineFunction.cpp
+++ b/llvm/lib/CodeGen/MachineFunction.cpp
@@ -1131,15 +1131,24 @@
     }
   }
 
+  MachineBasicBlock &InsertBB = *CurInst->getParent();
+
   // We reached the start of the block before finding a defining instruction.
-  // It must be an argument: assert that this is the entry block, and produce
-  // a DBG_PHI.
-  assert(!State.first.isVirtual());
-  MachineBasicBlock &TargetBB = *CurInst->getParent();
-  assert(&*TargetBB.getParent()->begin() == &TargetBB);
+  // It could be from a constant register, otherwise it must be an argument.
+  if (TRI.isConstantPhysReg(State.first)) {
+    // We can produce a DBG_PHI that identifies the constant physreg. Doesn't
+    // matter where we put it, as it's constant valued.
+    assert(CurInst->isCopy());
+  } else {
+    // Assert that this is the entry block. If it isn't, then there is some
+    // code construct we don't recognise that deals with physregs across
+    // blocks.
+    assert(!State.first.isVirtual());
+    assert(&*InsertBB.getParent()->begin() == &InsertBB);
+  }
 
   // Create DBG_PHI for specified physreg.
-  auto Builder = BuildMI(TargetBB, TargetBB.getFirstNonPHI(), DebugLoc(),
+  auto Builder = BuildMI(InsertBB, InsertBB.getFirstNonPHI(), DebugLoc(),
                          TII.get(TargetOpcode::DBG_PHI));
   Builder.addReg(State.first, RegState::Debug);
   unsigned NewNum = getNewDebugInstrNum();
diff --git a/llvm/test/DebugInfo/AArch64/instr-ref-const-physreg.ll b/llvm/test/DebugInfo/AArch64/instr-ref-const-physreg.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/DebugInfo/AArch64/instr-ref-const-physreg.ll
@@ -0,0 +1,38 @@
+; RUN: llc %s -stop-before=finalize-isel -march=aarch64 -o - \
+; RUN:     -experimental-debug-variable-locations | FileCheck %s
+
+; Test that when an SSA Value becomes a constant-physreg copy, under the
+; instruction referencing model, the COPY is labelled. In the general case
+; labelling a copy is undesirable -- this test really checks that we don't
+; crash, and we don't just drop the information.
+
+; CHECK: DBG_PHI $xzr, 1
+; CHECK: DBG_INSTR_REF 1, 0
+
+define i64 @test() !dbg !7 {
+  %foo = add i64 0, 0
+  call void @llvm.dbg.value(metadata i64 %foo, metadata !12, metadata !DIExpression()), !dbg !13
+  ret i64 %foo, !dbg !13
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/tmp/out.c")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!""}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 3, type: !8, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !11, !11}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+!12 = !DILocalVariable(name: "bar", arg: 1, scope: !7, file: !1, line: 3, type: !11)
+!13 = !DILocation(line: 0, scope: !7)
+!14 = !DILocalVariable(name: "baz", arg: 2, scope: !7, file: !1, line: 3, type: !11)