Index: lib/CodeGen/RegisterCoalescer.cpp
===================================================================
--- lib/CodeGen/RegisterCoalescer.cpp
+++ lib/CodeGen/RegisterCoalescer.cpp
@@ -244,6 +244,14 @@
     /// Return true if a copy involving a physreg should be joined.
     bool canJoinPhys(const CoalescerPair &CP);
 
+    /// When merging SrcReg and DstReg together, and the operand of the
+    /// specified DBG_VALUE refers to one of them, would the def that a
+    /// DBG_VALUE refers to change? This can happen when the DBG_VALUEs
+    /// operand is dead and it's merged into a different live value,
+    /// meaning the DBG_VALUE operands must be updated.
+    bool mergingChangesDbgValue(MachineInstr *DbgV, unsigned SrcReg,
+                                unsigned DstReg) 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 +1625,58 @@
   }
 }
 
+bool RegisterCoalescer::mergingChangesDbgValue(MachineInstr *DbgV,
+                                              unsigned SrcReg,
+                                              unsigned DstReg) const {
+  assert(DbgV->isDebugValue());
+  assert(DbgV->getParent() && "DbgValue with no parent");
+  assert(DbgV->getOperand(0).isReg());
+  unsigned DbgReg = DbgV->getOperand(0).getReg();
+
+  SlotIndex MIIdx = LIS->getSlotIndexes()->getIndexAfter(*DbgV);
+  const LiveInterval &SrcLI = LIS->getInterval(SrcReg);
+
+  // Is the src reg live across the DBG_VALUE?
+  bool SrcLive = false;
+  auto LII = SrcLI.find(MIIdx);
+  if (LII != SrcLI.end() && LII->contains(MIIdx))
+    SrcLive = true;
+
+  bool DstLive = false;
+  // Destination register can be physical or virtual.
+  if (TargetRegisterInfo::isVirtualRegister(DstReg)) {
+    // Is DstReg live across the DBG_VALUE?
+    const LiveInterval &DstLI = LIS->getInterval(DstReg);
+    LII = DstLI.find(MIIdx);
+    DstLive = (LII != DstLI.end() && LII->contains(MIIdx));
+  } else if (MRI->isConstantPhysReg(DstReg)) {
+    // Constant phys regs are always live.
+    DstLive = true;
+  } else {
+    // For physical registers, see if any register unit containing DstReg
+    // is live across the DBG_VALUE.
+    for (MCRegUnitIterator UI(DstReg, TRI); UI.isValid(); ++UI) {
+      const LiveRange &DstLI = LIS->getRegUnit(*UI);
+      auto DstLII = DstLI.find(MIIdx);
+      if (DstLII != DstLI.end() && DstLII->contains(MIIdx)) {
+        DstLive = true;
+        break;
+      }
+    }
+  }
+
+  // We now have whether src and dst are live. Best case: we have a DBG_VALUE
+  // of a live register, coalesing won't change its value.
+  if ((DstLive && DbgReg == DstReg) || (SrcLive && DbgReg == SrcReg))
+    return false;
+  // If neither register are live, no damage done.
+  if (!DstLive && !SrcLive)
+    return false;
+  // Otherwise, we will end up resurrecting the DBG_VALUE with a different
+  // register, which is unsafe.
+  return true;
+}
+
 void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
                                           unsigned DstReg,
                                           unsigned SubIdx) {
@@ -1830,6 +1890,20 @@
   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 to succeed.
+  SmallVector<MachineInstr *, 4> DbgValuesToChange;
+  auto CheckForDbgUser = [this, &CP, &DbgValuesToChange](MachineInstr &MI) {
+    if (MI.isDebugValue() && MI.getOperand(0).isReg() &&
+        mergingChangesDbgValue(&MI, CP.getSrcReg(), CP.getDstReg()))
+      DbgValuesToChange.push_back(&MI);
+  };
+  for (auto &RegIt : MRI->reg_instructions(CP.getSrcReg()))
+    CheckForDbgUser(RegIt);
+  for (auto &RegIt : MRI->reg_instructions(CP.getDstReg()))
+    CheckForDbgUser(RegIt);
+
   // 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 +1972,16 @@
     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 +1,243 @@
+# 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
+  }
+
+  define i32 @test4(i32* %pin) {
+  entry:
+    ret i32 0
+  start.test4:
+    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
+tracksRegLiveness: true
+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:
+    successors: %bb.2(0x04000000), %bb.1(0x7c000000)
+
+    ; CHECK-LABEL: name: test
+    ;
+    ; We currently expect %1 and %0 to merge into %7
+    ;
+    ; CHECK: %[[REG1:[0-9]+]]:gr32 = MOV32rm
+    ; CHECK-NEXT: %[[REG2:[0-9]+]]:gr32 = XOR32rr %[[REG1]]
+    ; CHECK-NEXT: %[[REG3:[0-9]+]]:gr32 = ADD32rr %[[REG3]], %[[REG2]]
+    ; CHECK-NEXT: 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
+    DBG_VALUE %0, $noreg, !3, !DIExpression(), debug-location !5
+    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
+tracksRegLiveness: true
+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:
+    successors: %bb.2(0x04000000), %bb.1(0x7c000000)
+
+    ; CHECK-LABEL: name: test2
+    ;
+    ; %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.
+    ;
+    ; CHECK: %[[REG11:[0-9]+]]:gr32 = MOV32rm
+    ; CHECK-NEXT: %[[REG12:[0-9]+]]:gr32 = XOR32rr %[[REG11]]
+    ; CHECK-NEXT: DBG_VALUE %[[REG13:[0-9]+]]
+    ; CHECK-NEXT: %[[REG13]]:gr32 = ADD32rr %[[REG13]], %[[REG12]]
+
+    %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
+    DBG_VALUE %0, $noreg, !3, !DIExpression(), debug-location !5
+    %1:gr32 = COPY killed %0
+    %1:gr32 = ADD32rr %1, killed %5, implicit-def dead $eflags
+    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
+tracksRegLiveness: true
+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:
+    successors: %bb.2(0x04000000), %bb.1(0x7c000000)
+
+    ; CHECK-LABEL: name: test3
+    ;
+    ; This is a use-before-def, merging new registers into %0 could unsoundly
+    ; make it live again, on merge mark it undef.
+    ;
+    ; CHECK: DBG_VALUE $noreg
+
+    DBG_VALUE %0, $noreg, !3, !DIExpression(), debug-location !5
+    %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
+    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:            test4
+tracksRegLiveness: true
+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.test4:
+    successors: %bb.2(0x04000000), %bb.1(0x7c000000)
+
+    ; CHECK-LABEL: name: test4
+    ;
+    ; Using a dead register, even if we coalesce it to the right value, should
+    ; be marked undef. The coalescer can't prove it's correct without
+    ; considering control flow in the general case.
+    ;
+    ; CHECK: DBG_VALUE $noreg
+
+    %0:gr32 = COPY killed %7
+    DBG_VALUE %7, $noreg, !3, !DIExpression(), debug-location !5
+    %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
+    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
+
+...