Index: lib/CodeGen/LiveDebugVariables.cpp
===================================================================
--- lib/CodeGen/LiveDebugVariables.cpp
+++ lib/CodeGen/LiveDebugVariables.cpp
@@ -662,12 +662,12 @@
       continue;
     }
 
-    // For physregs, use the live range of the first regunit as a guide.
-    unsigned Unit = *MCRegUnitIterator(Loc.getReg(), &TRI);
-    LiveRange *LR = &LIS.getRegUnit(Unit);
-    const VNInfo *VNI = LR->getVNInfoAt(Idx);
-    // Don't track copies from physregs, it is too expensive.
-    extendDef(Idx, LocNo, LR, VNI, nullptr, LIS);
+    // For physregs, we only mark the start slot idx. DwarfDebug will see it
+    // as if the DBG_VALUE is valid up until the end of the basic block, or
+    // the next def of the physical register. So we do not need to extend the
+    // range. It might actually happen that the DBG_VALUE is the last use of
+    // the physical register (e.g. if this is an unused input argument to a
+    // function).
   }
 
   // Erase all the undefs.
Index: test/DebugInfo/MIR/X86/live-debug-vars-unused-arg.mir
===================================================================
--- /dev/null
+++ test/DebugInfo/MIR/X86/live-debug-vars-unused-arg.mir
@@ -0,0 +1,175 @@
+# RUN: llc -O1 -start-before=greedy -stop-after=virtregrewriter -o /dev/null %s -debug-only=livedebugvars 2>&1 | FileCheck -check-prefix=CHECKDBG %s
+# RUN: llc -O1 -start-before=greedy -stop-after=virtregrewriter -o - %s | FileCheck -check-prefix=CHECKMIR %s
+
+# REQUIRES: asserts
+
+# This test case was generated by using the following c program:
+#   extern void foo(int, int);
+#
+#   int bar[2] = {1, 2};
+#
+#   int main(int argc, char** argv)
+#   {
+#     int a0 = bar[0];
+#     int a1 = bar[1];
+#     foo(a0, a1);
+#     return 0;
+#   }
+#
+# It was compiled with -g and -O1, and the mir was dumped before ra greedy.
+
+--- |
+  ; ModuleID = 'live-debug-vars-unused-arg.ll'
+  source_filename = "live-debug-vars-unused-arg.c"
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+  target triple = "x86_64-unknown-linux-gnu"
+
+  @bar = local_unnamed_addr global [2 x i32] [i32 1, i32 2], align 4, !dbg !0
+
+  ; Function Attrs: nounwind uwtable
+  define i32 @main(i32 %argc, i8** nocapture readnone %argv) local_unnamed_addr #0 !dbg !14 {
+  entry:
+    tail call void @llvm.dbg.value(metadata i32 %argc, metadata !21, metadata !DIExpression()), !dbg !25
+    tail call void @llvm.dbg.value(metadata i8** %argv, metadata !22, metadata !DIExpression()), !dbg !26
+    %0 = load i32, i32* getelementptr inbounds ([2 x i32], [2 x i32]* @bar, i64 0, i64 0), align 4, !dbg !27, !tbaa !28
+    tail call void @llvm.dbg.value(metadata i32 %0, metadata !23, metadata !DIExpression()), !dbg !32
+    %1 = load i32, i32* getelementptr inbounds ([2 x i32], [2 x i32]* @bar, i64 0, i64 1), align 4, !dbg !33, !tbaa !28
+    tail call void @llvm.dbg.value(metadata i32 %1, metadata !24, metadata !DIExpression()), !dbg !34
+    tail call void @foo(i32 %0, i32 %1) #2, !dbg !35
+    ret i32 0, !dbg !36
+  }
+
+  declare void @foo(i32, i32) local_unnamed_addr
+
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+  ; Function Attrs: nounwind
+  declare void @llvm.stackprotector(i8*, i8**) #2
+
+  attributes #0 = { nounwind uwtable }
+  attributes #1 = { nounwind readnone speculatable }
+  attributes #2 = { nounwind }
+
+  !llvm.dbg.cu = !{!2}
+  !llvm.module.flags = !{!10, !11, !12}
+  !llvm.ident = !{!13}
+
+  !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+  !1 = distinct !DIGlobalVariable(name: "bar", scope: !2, file: !3, line: 3, type: !6, isLocal: false, isDefinition: true)
+  !2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 6.0.0 (trunk 313866) (llvm/trunk 313875)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5)
+  !3 = !DIFile(filename: "live-debug-vars-unused-arg.c", directory: "/repo/uabbpet/master")
+  !4 = !{}
+  !5 = !{!0}
+  !6 = !DICompositeType(tag: DW_TAG_array_type, baseType: !7, size: 64, elements: !8)
+  !7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !8 = !{!9}
+  !9 = !DISubrange(count: 2)
+  !10 = !{i32 2, !"Dwarf Version", i32 4}
+  !11 = !{i32 2, !"Debug Info Version", i32 3}
+  !12 = !{i32 1, !"wchar_size", i32 4}
+  !13 = !{!"clang version 6.0.0 (trunk 313866) (llvm/trunk 313875)"}
+  !14 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 5, type: !15, isLocal: false, isDefinition: true, scopeLine: 6, flags: DIFlagPrototyped, isOptimized: true, unit: !2, variables: !20)
+  !15 = !DISubroutineType(types: !16)
+  !16 = !{!7, !7, !17}
+  !17 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !18, size: 64)
+  !18 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !19, size: 64)
+  !19 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+  !20 = !{!21, !22, !23, !24}
+  !21 = !DILocalVariable(name: "argc", arg: 1, scope: !14, file: !3, line: 5, type: !7)
+  !22 = !DILocalVariable(name: "argv", arg: 2, scope: !14, file: !3, line: 5, type: !17)
+  !23 = !DILocalVariable(name: "a0", scope: !14, file: !3, line: 7, type: !7)
+  !24 = !DILocalVariable(name: "a1", scope: !14, file: !3, line: 8, type: !7)
+  !25 = !DILocation(line: 5, column: 14, scope: !14)
+  !26 = !DILocation(line: 5, column: 27, scope: !14)
+  !27 = !DILocation(line: 7, column: 12, scope: !14)
+  !28 = !{!29, !29, i64 0}
+  !29 = !{!"int", !30, i64 0}
+  !30 = !{!"omnipotent char", !31, i64 0}
+  !31 = !{!"Simple C/C++ TBAA"}
+  !32 = !DILocation(line: 7, column: 7, scope: !14)
+  !33 = !DILocation(line: 8, column: 12, scope: !14)
+  !34 = !DILocation(line: 8, column: 7, scope: !14)
+  !35 = !DILocation(line: 9, column: 3, scope: !14)
+  !36 = !DILocation(line: 10, column: 3, scope: !14)
+
+...
+---
+name:            main
+alignment:       4
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: gr32, preferred-register: '' }
+  - { id: 1, class: gr64, preferred-register: '' }
+  - { id: 2, class: gr32, preferred-register: '' }
+  - { id: 3, class: gr32, preferred-register: '' }
+  - { id: 4, class: gr32, preferred-register: '' }
+liveins:
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    0
+  adjustsStack:    false
+  hasCalls:        true
+  stackProtector:  ''
+  maxCallFrameSize: 4294967295
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:
+stack:
+constants:
+body:             |
+  bb.0.entry:
+    DBG_VALUE debug-use %edi, debug-use _, !21, !DIExpression(), debug-location !25
+    DBG_VALUE debug-use %rsi, debug-use _, !22, !DIExpression(), debug-location !26
+    %2 = MOV32rm %rip, 1, _, @bar, _, debug-location !27 :: (dereferenceable load 4 from `i32* getelementptr inbounds ([2 x i32], [2 x i32]* @bar, i64 0, i64 0)`, !tbaa !28)
+    DBG_VALUE debug-use %2, debug-use _, !23, !DIExpression(), debug-location !32
+    %3 = MOV32rm %rip, 1, _, @bar + 4, _, debug-location !33 :: (dereferenceable load 4 from `i32* getelementptr inbounds ([2 x i32], [2 x i32]* @bar, i64 0, i64 1)`, !tbaa !28)
+    DBG_VALUE debug-use %3, debug-use _, !24, !DIExpression(), debug-location !34
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead %rsp, implicit-def dead %eflags, implicit %rsp, debug-location !35
+    %edi = COPY %2, debug-location !35
+    %esi = COPY %3, debug-location !35
+    CALL64pcrel32 @foo, csr_64, implicit %rsp, implicit killed %edi, implicit killed %esi, implicit-def %rsp, debug-location !35
+    ADJCALLSTACKUP64 0, 0, implicit-def dead %rsp, implicit-def dead %eflags, implicit %rsp, debug-location !35
+    %eax = MOV32r0 implicit-def dead %eflags, debug-location !36
+    RET 0, killed %eax, debug-location !36
+
+...
+
+# Let's verify that the slot index ranges for the unused variables argc/argv,
+# connected to physical regs %EDI and %RSI, does not overlap with the ranges
+# for %vreg2 and %vreg3. The register allocator is actually allocating the
+# virtual registers # to %EDI and %ESI, so the ranges for argc/argv should
+# not cover the whole BB.
+#
+# CHECKDBG-LABEL: ********** EMITTING LIVE DEBUG VARIABLES **********
+# CHECKDBG-NEXT: !"argc,5"        [0B;0e):0 Loc0=%EDI
+# CHECKDBG-NEXT:         [0B;0e):0 BB#0-160B
+# CHECKDBG-NEXT: !"argv,5"        [0B;0e):0 Loc0=%RSI
+# CHECKDBG-NEXT:         [0B;0e):0 BB#0-160B
+# CHECKDBG-NEXT: !"a0,7"  [16r;64r):0 Loc0=%vreg2
+# CHECKDBG-NEXT:         [16r;64r):0 BB#0-160B
+# CHECKDBG-NEXT: !"a1,8"  [32r;80r):0 Loc0=%vreg3
+# CHECKDBG-NEXT:         [32r;80r):0 BB#0-160B
+
+# Verify that we only get one DBG_VALUE for argc and one for argv.
+# CHECKMIR: ![[ARGC:[0-9]+]] = !DILocalVariable(name: "argc", arg: 1
+# CHECKMIR: ![[ARGV:[0-9]+]] = !DILocalVariable(name: "argv", arg: 2
+# CHECKMIR: name:            main
+# CHECKMIR: body:
+# CHECKMIR: DBG_VALUE debug-use %edi, debug-use _, ![[ARGC]]
+# CHECKMIR: DBG_VALUE debug-use %rsi, debug-use _, ![[ARGV]]
+# CHECKMIR-NOT: DBG_VALUE debug-use %{{.*}}, debug-use _, ![[ARGC]]
+# CHECKMIR-NOT: DBG_VALUE debug-use %{{.*}}, debug-use _, ![[ARGV]]
+