diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3355,6 +3355,10 @@
   I->moveBefore(&*InsertPos);
   ++NumSunkInst;
 
+  // Drop the debug loc. This prevents single-stepping from going backwards.
+  // See HowToUpdateDebugInfo.rst for the full rationale.
+  I->setDebugLoc(DebugLoc());
+
   // Also sink all related debug uses from the source basic block. Otherwise we
   // get debug use before the def. Attempt to salvage debug uses first, to
   // maximise the range variables have location for. If we cannot salvage, then
diff --git a/llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll b/llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll
@@ -0,0 +1,42 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+; CHECK-LABEL: @test(
+; CHECK: [[phi:%.*]] = phi i32
+; CHECK-NEXT: [[add:%.*]] = add i32 %x, 1{{$}}
+; CHECK-NEXT: add i32 [[phi]], [[add]], !dbg
+define i32 @test(i32 %x, i1 %c) !dbg !6 {
+bb0:
+  %a = add i32 %x, 1, !dbg !8
+  br i1 %c, label %bb1, label %bb2, !dbg !9
+
+bb1:                                              ; preds = %bb0
+  br label %bb3, !dbg !10
+
+bb2:                                              ; preds = %bb0
+  br label %bb3, !dbg !11
+
+bb3:                                              ; preds = %bb2, %bb1
+  %p = phi i32 [ 0, %bb1 ], [ 1, %bb2 ], !dbg !12
+  %r = add i32 %p, %a, !dbg !13
+  ret i32 %r, !dbg !14
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!3, !4}
+!llvm.module.flags = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "sink_to_unreachable_dbg.ll", directory: "/")
+!2 = !{}
+!3 = !{i32 7}
+!4 = !{i32 0}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "test", linkageName: "test", scope: null, file: !1, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!7 = !DISubroutineType(types: !2)
+!8 = !DILocation(line: 1, column: 1, scope: !6)
+!9 = !DILocation(line: 2, column: 1, scope: !6)
+!10 = !DILocation(line: 3, column: 2, scope: !6)
+!11 = !DILocation(line: 4, column: 3, scope: !6)
+!12 = !DILocation(line: 5, column: 4, scope: !6)
+!13 = !DILocation(line: 6, column: 4, scope: !6)
+!14 = !DILocation(line: 7, column: 4, scope: !6)