diff --git a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
--- a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
@@ -3716,7 +3716,12 @@
 
     unsigned BlockNo = Num.getBlock();
     LocIdx LocNo = Num.getLoc();
-    Num = MInLocs[BlockNo][LocNo.asU64()];
+    ValueIDNum ResolvedValue = MInLocs[BlockNo][LocNo.asU64()];
+    // If there is no resolved value for this live-in then it is not directly
+    // reachable from the entry block -- model it as a PHI on entry to this
+    // block, which means we leave the ValueIDNum unchanged.
+    if (ResolvedValue != ValueIDNum::EmptyValue)
+      Num = ResolvedValue;
   }
   // Later, we'll be looking up ranges of instruction numbers.
   llvm::sort(DebugPHINumToValue);
diff --git a/llvm/test/DebugInfo/X86/instr-ref-unreachable.mir b/llvm/test/DebugInfo/X86/instr-ref-unreachable.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/DebugInfo/X86/instr-ref-unreachable.mir
@@ -0,0 +1,110 @@
+# RUN: llc --run-pass=livedebugvalues %s -o - -experimental-debug-variable-locations | FileCheck %s
+
+## Tests that when a block that is not directly reachable from the entry block
+## contains a DBG_PHI, we treat those DBG_PHIs as referring to live-in values,
+## and resolve them for users as normal.
+## FIXME: "def" currently does not have a value due to blocks that are not
+## directly reachable from the entry block not being fully covered by the
+## LiveDebugValues analysis.
+
+# CHECK-DAG: ![[UNDEFVAR:[0-9]+]] = !DILocalVariable(name: "undef"
+# CHECK-DAG: ![[DEFVAR:[0-9]+]] = !DILocalVariable(name: "def"
+
+# CHECK-LABEL: bb.1.bb1
+# CHECK: DBG_VALUE_LIST ![[UNDEFVAR]], {{.+}}, $noreg
+# CHECK-LABEL: bb.4.bb3
+# CHECK: DBG_VALUE_LIST ![[DEFVAR]], {{.+}}, $noreg
+
+--- |
+  ; ModuleID = 'llvm/test/DebugInfo/X86/instr-ref-unreachable.mir'
+  source_filename = "/tmp/b.ll"
+  target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+  target triple = "x86_64-unknown-linux-gnu"
+
+  @global = private constant [2 x ptr] [ptr blockaddress(@foo, %bb3), ptr blockaddress(@foo, %bb1)]
+
+  define ptr @foo() !dbg !4 {
+  bb:
+    br label %bb3
+
+  bb1:                                              ; preds = %bb3
+    call void @llvm.dbg.value(metadata i64 %phi, metadata !6, metadata !DIExpression()), !dbg !8
+    call void @llvm.dbg.value(metadata i64 %phi, metadata !9, metadata !DIExpression()), !dbg !8
+    store i32 0, ptr null, align 4, !dbg !10
+    %getelementptr = getelementptr i32, ptr null, i64 %phi
+    store i32 0, ptr %getelementptr, align 4
+    br label %bb2
+
+  bb2:                                              ; preds = %bb2, %bb1
+    %select = select i1 false, i1 false, i1 false
+    br i1 %select, label %bb3, label %bb2
+
+  bb3:                                              ; preds = %bb2, %bb
+    %phi = phi i64 [ 1, %bb ], [ 0, %bb2 ]
+    br label %bb1
+  }
+
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2, globals: !2, imports: !2, splitDebugInlining: false, nameTableKind: GNU)
+  !1 = !DIFile(filename: "test.cpp", directory: ".")
+  !2 = !{}
+  !3 = !{i32 2, !"Debug Info Version", i32 3}
+  !4 = distinct !DISubprogram(name: "GetNumericFormat", linkageName: "f", scope: null, file: !1, line: 980, type: !5, scopeLine: 984, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+  !5 = distinct !DISubroutineType(types: !2)
+  !6 = !DILocalVariable(name: "undef", scope: !4, file: !1, line: 263, type: !7)
+  !7 = !DIBasicType(name: "unsigned long", size: 64, encoding: DW_ATE_unsigned)
+  !8 = !DILocation(line: 0, scope: !4)
+  !9 = !DILocalVariable(name: "def", scope: !4, file: !1, line: 263, type: !7)
+  !10 = !DILocation(line: 151, column: 41, scope: !4)
+
+...
+---
+name:            foo
+alignment:       16
+tracksRegLiveness: true
+debugInstrRef:   true
+tracksDebugUserValues: true
+frameInfo:
+  maxAlignment:    1
+body:             |
+  bb.0.bb:
+    successors: %bb.1(0x80000000)
+
+    $ecx = MOV32ri 1, implicit-def $rcx
+    renamable $al = MOV8ri 1
+
+  bb.1.bb1 (ir-block-address-taken %ir-block.bb1, align 16):
+    successors: %bb.2(0x80000000)
+    liveins: $al, $rcx
+
+    DBG_INSTR_REF !6, !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(1, 0), debug-location !8
+    MOV32mi $noreg, 1, $noreg, 0, $noreg, 0, debug-location !10 :: (store (s32) into `ptr null`)
+    MOV32mi $noreg, 4, killed renamable $rcx, 0, $noreg, 0 :: (store (s32) into %ir.getelementptr)
+
+  bb.2.bb2 (align 16):
+    successors: %bb.3(0x04000000), %bb.2(0x7c000000)
+    liveins: $al
+
+    TEST8rr renamable $al, renamable $al, implicit-def $eflags
+    JCC_1 %bb.2, 5, implicit killed $eflags
+
+  bb.3:
+    successors: %bb.1(0x80000000)
+    liveins: $al
+
+    renamable $ecx = XOR32rr undef $ecx, undef $ecx, implicit-def dead $eflags, implicit-def $rcx
+    JMP_1 %bb.1
+
+  bb.4.bb3 (ir-block-address-taken %ir-block.bb3):
+    successors: %bb.1(0x80000000)
+    liveins: $al, $rcx
+
+    DBG_PHI $rcx, 1
+    DBG_INSTR_REF !9, !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(1, 0), debug-location !8
+    JMP_1 %bb.1
+
+...