Index: llvm/trunk/lib/CodeGen/LiveDebugVariables.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/LiveDebugVariables.cpp
+++ llvm/trunk/lib/CodeGen/LiveDebugVariables.cpp
@@ -752,7 +752,15 @@
       }
       SmallVector<SlotIndex, 16> Kills;
       extendDef(Idx, Loc, LI, VNI, &Kills, LIS);
-      if (LI)
+      // FIXME: Handle sub-registers in addDefsFromCopies. The problem is that
+      // if the original location for example is %vreg0:sub_hi, and we find a
+      // full register copy in addDefsFromCopies (at the moment it only handles
+      // full register copies), then we must add the sub1 sub-register index to
+      // the new location. However, that is only possible if the new virtual
+      // register is of the same regclass (or if there is an equivalent
+      // sub-register in that regclass). For now, simply skip handling copies if
+      // a sub-register is involved.
+      if (LI && !LocMO.getSubReg())
         addDefsFromCopies(LI, Loc.locNo(), Loc.wasIndirect(), Kills, Defs, MRI,
                           LIS);
       continue;
Index: llvm/trunk/test/CodeGen/X86/dbg-value-superreg-copy.mir
===================================================================
--- llvm/trunk/test/CodeGen/X86/dbg-value-superreg-copy.mir
+++ llvm/trunk/test/CodeGen/X86/dbg-value-superreg-copy.mir
@@ -0,0 +1,67 @@
+# RUN: llc -O1 -start-after simple-register-coalescing -o - %s | FileCheck %s
+
+--- |
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+  target triple = "x86_64-pc-linux-gnu"
+
+  define i16 @foo(i8 %zzz) !dbg !4 {
+  entry:
+    ret i16 1
+  }
+
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2)
+  !1 = !DIFile(filename: "test.c", directory: "")
+  !2 = !{}
+  !3 = !{i32 1, !"Debug Info Version", i32 3}
+  !4 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 3, type: !5, isLocal: false, isDefinition: true, scopeLine: 3, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: false, unit: !0)
+  !5 = !DISubroutineType(types: !6)
+  !6 = !{null}
+  !7 = !DILocalVariable(name: "zzz", arg: 1, scope: !4, file: !1, line: 3, type: !8)
+  !8 = !DIBasicType(name: "char", size: 8, align: 8, encoding: DW_ATE_signed)
+  !9 = !DILocation(line: 0, scope: !4)
+  !10 = !DILocation(line: 4, column: 22, scope: !11)
+  !11 = distinct !DILexicalBlock(scope: !4, file: !1, line: 3, column: 19)
+
+...
+---
+name:            foo
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    %0:gr16_abcd = MOV16ri 1
+
+  bb.1:
+    DBG_VALUE debug-use %0.sub_8bit_hi, debug-use $noreg, !7, !DIExpression(), debug-location !9
+    %1:gr16 = COPY %0
+    %2:gr16 = COPY %0
+
+  bb.2:
+    $ax = COPY %1
+    $dx = COPY %2
+    RETQ killed $ax, killed $dx
+...
+
+# This test case was created as a reproducer for a bug when we got incorrect
+# DBG_VALUE instructions after regalloc like this:
+#
+#        movw    $1, %cx
+#        #DEBUG_VALUE: foo:zzz <- $ch
+#        movl    %ecx, %eax
+#        #DEBUG_VALUE: foo:zzz <- $ax
+#
+# The above is incorrect since the DBG_VALUE in the input is refering to the
+# hi subreg, so after the COPY/movl the value is in $ah and not $ax (nor $al).
+#
+# We currently only get one DEBUG_VALUE here. In the future we could allow a
+# second DEBUG_VALUE, as long as it is mapped to the hi subreg of the movl
+# dst.
+#
+# CHECK-NOT:    #DEBUG_VALUE:
+# CHECK:        #DEBUG_VALUE: foo:zzz <- ${{[abcd]+}}h
+# CHECK-NOT:    #DEBUG_VALUE: