Index: lib/CodeGen/RegisterCoalescer.cpp
===================================================================
--- lib/CodeGen/RegisterCoalescer.cpp
+++ lib/CodeGen/RegisterCoalescer.cpp
@@ -244,6 +244,13 @@
     /// Return true if a copy involving a physreg should be joined.
     bool canJoinPhys(const CoalescerPair &CP);
 
+    /// When merging DstLI into the vreg operand of the specified DBG_VALUE,
+    /// would it change the valuation of the DBG_VALUE? This can happen when
+    /// the DBG_VALUE is in a non-live range and DstLI is merged into that
+    /// range.
+    bool mergingChangesDbgValue(MachineInstr *DbgV,
+                                const LiveInterval &DstLI) const;
+
     /// Replace all defs and uses of SrcReg to DstReg and update the subregister
     /// number if it is not zero. If DstReg is a physical register and the
     /// existing subregister number of the def / use being updated is not zero,
@@ -1617,6 +1624,57 @@
   }
 }
 
+bool RegisterCoalescer::mergingChangesDbgValue(MachineInstr *DbgV,
+                                              const LiveInterval &DstLI) const {
+  assert(DbgV->isDebugValue());
+  assert(DbgV->getParent() && "DbgValue with no parent");
+  const MachineOperand &MO = DbgV->getOperand(0);
+  if (!MO.isReg())
+    return false;
+
+  SlotIndex MIIdx = LIS->getSlotIndexes()->getIndexAfter(*DbgV);
+  const LiveInterval &SrcLI = LIS->getInterval(MO.getReg());
+
+  // Now then -- is the src reg live at the point of the DBG_VALUE?
+  auto LII = SrcLI.find(MIIdx);
+  if (LII != SrcLI.end() && LII->contains(MIIdx))
+    // Reg is live across the DBG_VALUE, so coalescing will protect against
+    // its valuation changing.
+    return false;
+
+  // If the src is a use-before-def, it's already unclear which def it was
+  // supposed to refer to, and it'll be even more unclear after we merge with
+  // the destination register. Flag this to be undef. This accounts for
+  // end==begin too.
+  if (LII == SrcLI.begin())
+    return true;
+
+  // Get the value number for whatever was live before this slot. Move to
+  // previous interval segment, and get live VN at its end.
+  auto LIIPrev = std::prev(LII);
+  VNInfo *SrcVN = SrcLI.getVNInfoBefore(LIIPrev->end);
+
+  // If it's not live, and we can't tell what it _was_, it's likely impossible
+  // to work out what it _should_ be.
+  if (!SrcVN)
+    return true;
+
+  // And how about the dest reg?
+  LII = DstLI.find(MIIdx);
+  if (LII == DstLI.end() || !LII->contains(MIIdx))
+    return false;
+
+  VNInfo *DstVN = DstLI.getVNInfoAt(MIIdx);
+
+  // We know src was dead, dst is live, and thus the DBG_VALUE will be
+  // resurrected. If src and dst refer to a different def, then there will
+  // be an unsound alteration.
+  if (DstVN)
+    return DstVN->def != SrcVN->def;
+  else
+    return false;
+}
+
 void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
                                           unsigned DstReg,
                                           unsigned SubIdx) {
@@ -1830,6 +1888,21 @@
   ShrinkMask = LaneBitmask::getNone();
   ShrinkMainRange = false;
 
+  // Although we can update the DBG_VALUEs to the merged register, as debug uses
+  // do not contribute to liveness it might not be a sound update. Collect
+  // DBG_VALUEs that would change value were this interval merging succeed.
+  SmallVector<MachineInstr *, 4> DbgValuesToChange;
+  for (MachineRegisterInfo::reg_instr_iterator
+           I = MRI->reg_instr_begin(CP.getSrcReg()),
+           E = MRI->reg_instr_end();
+       I != E;) {
+    MachineInstr *UseMI = &*(I++);
+    if (UseMI->isDebugValue() && UseMI->getOperand(0).isReg() &&
+        TargetRegisterInfo::isVirtualRegister(CP.getDstReg()) &&
+        mergingChangesDbgValue(UseMI, LIS->getInterval(CP.getDstReg())))
+      DbgValuesToChange.push_back(UseMI);
+  }
+
   // Okay, attempt to join these two intervals.  On failure, this returns false.
   // Otherwise, if one of the intervals being joined is a physreg, this method
   // always canonicalizes DstInt to be it.  The output "SrcInt" will not have
@@ -1898,6 +1971,16 @@
     updateRegDefsUses(CP.getDstReg(), CP.getDstReg(), CP.getDstIdx());
   updateRegDefsUses(CP.getSrcReg(), CP.getDstReg(), CP.getSrcIdx());
 
+  // The updates to these DBG_VALUEs are not sound -- mark them undef.
+  // FIXME: further analysis might recover them, this is the minimal sound
+  // solution.
+  for (MachineInstr *MI : DbgValuesToChange) {
+    assert(MI->getOperand(0).isReg());
+    LLVM_DEBUG(dbgs() << "Update of " << MI->getOperand(0) << " would be "
+                      << "unsound, setting undef\n");
+    MI->getOperand(0).setReg(0);
+  }
+
   // Shrink subregister ranges if necessary.
   if (ShrinkMask.any()) {
     LiveInterval &LI = LIS->getInterval(CP.getDstReg());
Index: test/CodeGen/X86/pr40010.mir
===================================================================
--- /dev/null
+++ test/CodeGen/X86/pr40010.mir
@@ -0,0 +1,248 @@
+# RUN: llc %s -o - -run-pass=simple-register-coalescing | FileCheck %s
+# PR40010: DBG_VALUEs do not contribute to the liveness of virtual registers,
+# and the register coalescer would merge new live values on top of DBG_VALUEs,
+# leading to them presenting new (wrong) values to the debugger. Test that
+# when out of liveness, coalescing will mark DBG_VALUEs in non-live locations
+# as undef.
+--- |
+  ; ModuleID = './test.ll'
+  source_filename = "./test.ll"
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+  ; Original IR source here:
+  define i32 @test(i32* %pin) {
+  entry:
+    br label %start.test1
+
+  start.test1:                                       ; preds = %start, %entry
+    %foo = phi i32 [ 0, %entry ], [ %bar, %start.test1 ]
+    %baz = load i32, i32* %pin, align 1
+    %qux = xor i32 %baz, 1234
+    %bar = add i32 %qux, %foo
+    call void @llvm.dbg.value(metadata i32 %foo, metadata !3, metadata !DIExpression()), !dbg !5
+    %cmp = icmp ugt i32 %bar, 1000000
+    br i1 %cmp, label %leave, label %start.test1
+
+  leave:                                            ; preds = %start
+    ret i32 %bar
+  }
+
+  ; Stubs to appease the MIR parser
+  define i32 @test2(i32* %pin) {
+  entry:
+    ret i32 0
+  start.test2:
+    ret i32 0
+  leave:
+    ret i32 0
+  }
+
+  define i32 @test3(i32* %pin) {
+  entry:
+    ret i32 0
+  start.test3:
+    ret i32 0
+  leave:
+    ret i32 0
+  }
+
+  ; Function Attrs: nounwind
+  declare void @llvm.stackprotector(i8*, i8**) #1
+
+  attributes #0 = { nounwind readnone speculatable }
+  attributes #1 = { nounwind }
+
+  !llvm.module.flags = !{!0}
+  !llvm.dbg.cu = !{!1}
+
+  !0 = !{i32 2, !"Debug Info Version", i32 3}
+  !1 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !2, producer: "beards", isOptimized: true, runtimeVersion: 4, emissionKind: FullDebug)
+  !2 = !DIFile(filename: "bees.cpp", directory: "")
+  !3 = !DILocalVariable(name: "bees", scope: !4)
+  !4 = distinct !DISubprogram(name: "nope", scope: !1, file: !2, line: 1, spFlags: DISPFlagDefinition, unit: !1)
+  !5 = !DILocation(line: 0, scope: !4)
+
+...
+---
+name:            test
+alignment:       4
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+registers:
+  - { id: 0, class: gr32, preferred-register: '' }
+  - { id: 1, class: gr32, preferred-register: '' }
+  - { id: 2, class: gr64, preferred-register: '' }
+  - { id: 3, class: gr32, preferred-register: '' }
+  - { id: 4, class: gr32, preferred-register: '' }
+  - { id: 5, class: gr32, preferred-register: '' }
+  - { id: 6, class: gr32, preferred-register: '' }
+  - { id: 7, class: gr32, preferred-register: '' }
+  - { id: 8, class: gr32, preferred-register: '' }
+liveins:
+  - { reg: '$rdi', virtual-reg: '%2' }
+fixedStack:      []
+stack:           []
+constants:       []
+body:             |
+  bb.0.entry:
+    successors: %bb.1(0x80000000)
+    liveins: $rdi
+
+    %2:gr64 = COPY killed $rdi
+    %3:gr32 = MOV32r0 implicit-def dead $eflags
+    %4:gr32 = MOV32ri 1234
+    %7:gr32 = COPY killed %3
+
+  bb.1.start.test1:
+    ; CHECK-LABEL: bb.1.start.test1
+    successors: %bb.2(0x04000000), %bb.1(0x7c000000)
+
+    %0:gr32 = COPY killed %7
+    %8:gr32 = MOV32rm %2, 1, $noreg, 0, $noreg :: (load 4 from %ir.pin, align 1)
+    %5:gr32 = COPY killed %8
+    %5:gr32 = XOR32rr %5, %4, implicit-def dead $eflags
+    %1:gr32 = COPY killed %0
+    ; We currently expect %1 and %0 to merge into %7
+    %1:gr32 = ADD32rr %1, killed %5, implicit-def dead $eflags
+    ; CHECK: %7:gr32 = ADD32rr %7
+    DBG_VALUE %0, $noreg, !3, !DIExpression(), debug-location !5
+    ; CHECK-NEXT: DBG_VALUE $noreg
+    CMP32ri %1, 1000001, implicit-def $eflags
+    %7:gr32 = COPY %1
+    JB_1 %bb.1, implicit killed $eflags
+    JMP_1 %bb.2
+
+  bb.2.leave:
+    $eax = COPY killed %1
+    RET 0, killed $eax
+
+...
+---
+name:            test2
+alignment:       4
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+registers:
+  - { id: 0, class: gr32, preferred-register: '' }
+  - { id: 1, class: gr32, preferred-register: '' }
+  - { id: 2, class: gr64, preferred-register: '' }
+  - { id: 3, class: gr32, preferred-register: '' }
+  - { id: 4, class: gr32, preferred-register: '' }
+  - { id: 5, class: gr32, preferred-register: '' }
+  - { id: 6, class: gr32, preferred-register: '' }
+  - { id: 7, class: gr32, preferred-register: '' }
+  - { id: 8, class: gr32, preferred-register: '' }
+liveins:
+  - { reg: '$rdi', virtual-reg: '%2' }
+fixedStack:      []
+stack:           []
+constants:       []
+body:             |
+  bb.0.entry:
+    successors: %bb.1(0x80000000)
+    liveins: $rdi
+
+    %2:gr64 = COPY killed $rdi
+    %3:gr32 = MOV32r0 implicit-def dead $eflags
+    %4:gr32 = MOV32ri 1234
+    %7:gr32 = COPY killed %3
+
+  bb.1.start.test2:
+    ; CHECK-LABEL: bb.1.start.test2
+    successors: %bb.2(0x04000000), %bb.1(0x7c000000)
+
+    %0:gr32 = COPY killed %7
+    %8:gr32 = MOV32rm %2, 1, $noreg, 0, $noreg :: (load 4 from %ir.pin, align 1)
+    %5:gr32 = COPY killed %8
+    %5:gr32 = XOR32rr %5, %4, implicit-def dead $eflags
+    ; %0 should be merged into %7, but as %0 is live at this location the
+    ; DBG_VALUE should be preserved and point at the operand of ADD32rr.
+    DBG_VALUE %0, $noreg, !3, !DIExpression(), debug-location !5
+    ; CHECK: DBG_VALUE %7, $noreg
+    %1:gr32 = COPY killed %0
+    %1:gr32 = ADD32rr %1, killed %5, implicit-def dead $eflags
+    ; CHECK: %7:gr32 = ADD32rr %7
+    CMP32ri %1, 1000001, implicit-def $eflags
+    %7:gr32 = COPY %1
+    JB_1 %bb.1, implicit killed $eflags
+    JMP_1 %bb.2
+
+  bb.2.leave:
+    $eax = COPY killed %1
+    RET 0, killed $eax
+
+...
+---
+name:            test3
+alignment:       4
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+registers:
+  - { id: 0, class: gr32, preferred-register: '' }
+  - { id: 1, class: gr32, preferred-register: '' }
+  - { id: 2, class: gr64, preferred-register: '' }
+  - { id: 3, class: gr32, preferred-register: '' }
+  - { id: 4, class: gr32, preferred-register: '' }
+  - { id: 5, class: gr32, preferred-register: '' }
+  - { id: 6, class: gr32, preferred-register: '' }
+  - { id: 7, class: gr32, preferred-register: '' }
+  - { id: 8, class: gr32, preferred-register: '' }
+liveins:
+  - { reg: '$rdi', virtual-reg: '%2' }
+fixedStack:      []
+stack:           []
+constants:       []
+body:             |
+  bb.0.entry:
+    successors: %bb.1(0x80000000)
+    liveins: $rdi
+
+    %2:gr64 = COPY killed $rdi
+    %3:gr32 = MOV32r0 implicit-def dead $eflags
+    %4:gr32 = MOV32ri 1234
+    %7:gr32 = COPY killed %3
+
+  bb.1.start.test3:
+    ; CHECK-LABEL: bb.1.start.test3
+    successors: %bb.2(0x04000000), %bb.1(0x7c000000)
+
+    ; This is a use-before-def, merging new registers into %0 could unsoundly
+    ; make it live again, on merge mark it undef.
+    DBG_VALUE %0, $noreg, !3, !DIExpression(), debug-location !5
+    ; CHECK: DBG_VALUE $noreg
+    %0:gr32 = COPY killed %7
+    %8:gr32 = MOV32rm %2, 1, $noreg, 0, $noreg :: (load 4 from %ir.pin, align 1)
+    %5:gr32 = COPY killed %8
+    %5:gr32 = XOR32rr %5, %4, implicit-def dead $eflags
+    %1:gr32 = COPY killed %0
+    %1:gr32 = ADD32rr %1, killed %5, implicit-def dead $eflags
+    ; Check merging happened
+    ; CHECK: %7:gr32 = ADD32rr %7
+    CMP32ri %1, 1000001, implicit-def $eflags
+    %7:gr32 = COPY %1
+    JB_1 %bb.1, implicit killed $eflags
+    JMP_1 %bb.2
+
+  bb.2.leave:
+    $eax = COPY killed %1
+    RET 0, killed $eax
+
+...