Index: llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp
+++ llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp
@@ -2137,7 +2137,8 @@
     DEBUG(dbgs() << "Reassoc to scalar: " << *V << '\n');
     I->replaceAllUsesWith(V);
     if (Instruction *VI = dyn_cast<Instruction>(V))
-      VI->setDebugLoc(I->getDebugLoc());
+      if (I->getDebugLoc())
+        VI->setDebugLoc(I->getDebugLoc());
     RedoInsts.insert(I);
     ++NumAnnihil;
     return;
Index: llvm/trunk/test/Transforms/Reassociate/keep-debug-loc.ll
===================================================================
--- llvm/trunk/test/Transforms/Reassociate/keep-debug-loc.ll
+++ llvm/trunk/test/Transforms/Reassociate/keep-debug-loc.ll
@@ -0,0 +1,48 @@
+; RUN: opt -S -reassociate < %s | FileCheck %s
+
+; PR34231
+;
+; Verify that the original debug location is kept if the
+; replacement debug location is missing when
+; reassociating expressions.
+
+define i16 @fn1() !dbg !3 {
+  ret i16 undef
+}
+
+define void @fn2() !dbg !6 {
+; CHECK-LABEL: @fn2
+; CHECK: call i16 @fn1(), !dbg ![[LOC1:[0-9]+]]
+; CHECK-NOT: or i16
+  %inlinable_call = call i16 @fn1(), !dbg !7
+  %dbgless_instruction = or i16 %inlinable_call, 0
+  store i16 %dbgless_instruction, i16* undef, align 1
+  unreachable
+}
+
+define void @fn3() !dbg !8 {
+; CHECK-LABEL: @fn3
+; CHECK: load i16, i16* undef, !dbg ![[LOC2:[0-9]+]]
+; CHECK-NOT: or i16
+  %instruction = load i16, i16* undef, !dbg !9
+  %dbgless_instruction = or i16 %instruction, 0
+  store i16 %dbgless_instruction, i16* undef, align 1
+  unreachable
+}
+
+; CHECK: ![[LOC1]] = !DILocation(line: 7
+; CHECK: ![[LOC2]] = !DILocation(line: 9
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 6.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly)
+!1 = !DIFile(filename: "foo.c", directory: "/")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = distinct !DISubprogram(name: "fn1", scope: !1, file: !1, line: 2, type: !4, isLocal: false, isDefinition: true, scopeLine: 2, isOptimized: true, unit: !0)
+!4 = !DISubroutineType(types: !5)
+!5 = !{}
+!6 = distinct !DISubprogram(name: "fn2", scope: !1, file: !1, line: 3, type: !4, isLocal: false, isDefinition: true, scopeLine: 3, isOptimized: true, unit: !0)
+!7 = !DILocation(line: 7, column: 10, scope: !6)
+!8 = distinct !DISubprogram(name: "fn3", scope: !1, file: !1, line: 8, type: !4, isLocal: false, isDefinition: true, scopeLine: 3, isOptimized: true, unit: !0)
+!9 = !DILocation(line: 9, column: 10, scope: !8)