Index: lib/Transforms/Scalar/Scalarizer.cpp
===================================================================
--- lib/Transforms/Scalar/Scalarizer.cpp
+++ lib/Transforms/Scalar/Scalarizer.cpp
@@ -341,7 +341,8 @@
            MI != ME; ++MI)
         if (canTransferMetadata(MI->first))
           New->setMetadata(MI->first, MI->second);
-      New->setDebugLoc(Op->getDebugLoc());
+      if (Op->getDebugLoc() && !New->getDebugLoc())
+        New->setDebugLoc(Op->getDebugLoc());
     }
   }
 }
Index: test/Transforms/Scalarizer/dbgloc-bug.ll
===================================================================
--- /dev/null
+++ test/Transforms/Scalarizer/dbgloc-bug.ll
@@ -0,0 +1,86 @@
+; RUN: opt -S -march=x86 -scalarizer -o /dev/null %s
+
+; Reproducer for pr27938
+; https://llvm.org/bugs/show_bug.cgi?id=27938
+
+@d = internal unnamed_addr global i1 false
+@e = internal unnamed_addr global i16 0
+@a = global i16 0
+@b = global i16 0
+
+; Function Attrs: noinline norecurse nounwind readonly
+define i16 @f1() #0 !dbg !6 {
+  %_tmp1 = load i16, i16* @a, align 1, !dbg !10
+  ret i16 %_tmp1, !dbg !10
+}
+
+; Function Attrs: noinline norecurse nounwind
+define void @f2() #1 !dbg !11 {
+bb1:
+  %.old = load i16, i16* @b, align 1, !dbg !14
+  %_tmp3.old = icmp eq i16 %.old, 0, !dbg !14
+  br i1 %_tmp3.old, label %bb3.preheader, label %bb1.bb8_crit_edge, !dbg !14
+
+bb1.bb8_crit_edge:                                ; preds = %bb1
+  %_tmp19.b.pre = load i1, i1* @d, align 1
+  br label %bb8, !dbg !14
+
+bb3.preheader:                                    ; preds = %bb1
+  %e.promoted = load i16, i16* @e, align 1, !dbg !16
+  %_tmp7 = tail call i16 @f1(), !dbg !16
+  %0 = insertelement <4 x i16> <i16 undef, i16 0, i16 0, i16 0>, i16 %e.promoted, i32 0
+  %broadcast.splatinsert5 = insertelement <4 x i16> undef, i16 %_tmp7, i32 0
+  %broadcast.splat6 = shufflevector <4 x i16> %broadcast.splatinsert5, <4 x i16> undef, <4 x i32> zeroinitializer
+  br label %vector.body
+
+vector.body:                                      ; preds = %vector.body, %bb3.preheader
+  %index = phi i16 [ 0, %bb3.preheader ], [ %index.next, %vector.body ]
+  %vec.phi = phi <4 x i16> [ %0, %bb3.preheader ], [ %1, %vector.body ]
+  %1 = xor <4 x i16> %vec.phi, %broadcast.splat6, !dbg !16
+  %index.next = add i16 %index, 4
+  %2 = icmp eq i16 %index.next, -4
+  br i1 %2, label %middle.block, label %vector.body
+
+middle.block:                                     ; preds = %vector.body
+  %.lcssa = phi <4 x i16> [ %1, %vector.body ]
+  %rdx.shuf = shufflevector <4 x i16> %.lcssa, <4 x i16> undef, <4 x i32> <i32 2, i32 3, i32 undef, i32 undef>, !dbg !16
+  %bin.rdx = xor <4 x i16> %.lcssa, %rdx.shuf, !dbg !16
+  br label %bb3
+
+bb3:                                              ; preds = %middle.block
+  %rdx.shuf7 = shufflevector <4 x i16> %bin.rdx, <4 x i16> undef, <4 x i32> <i32 1, i32 undef, i32 undef, i32 undef>, !dbg !16
+  %bin.rdx8 = xor <4 x i16> %bin.rdx, %rdx.shuf7, !dbg !16
+  %3 = extractelement <4 x i16> %bin.rdx8, i32 0, !dbg !16
+  %_tmp10.2 = xor i16 %3, %_tmp7, !dbg !16
+  store i1 true, i1* @d, align 1
+  store i16 %_tmp10.2, i16* @e, align 1, !dbg !16
+  br label %bb8
+
+bb8:                                              ; preds = %bb1.bb8_crit_edge, %bb3
+  %_tmp19.b = phi i1 [ %_tmp19.b.pre, %bb1.bb8_crit_edge ], [ true, %bb3 ]
+  %_tmp19 = zext i1 %_tmp19.b to i16
+  store i16 %_tmp19, i16* @b, align 1, !dbg !22
+  ret void, !dbg !23
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2, retainedTypes: !2)
+!1 = !DIFile(filename: "dbgloc-bug.c", directory: ".")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "f1", scope: !1, file: !1, line: 9, type: !7, isLocal: false, isDefinition: true, scopeLine: 10, isOptimized: true, unit: !0, variables: !2)
+!7 = !DISubroutineType(types: !8)
+!8 = !{!9}
+!9 = !DIBasicType(name: "short", size: 16, align: 16, encoding: DW_ATE_signed)
+!10 = !DILocation(line: 11, column: 5, scope: !6)
+!11 = distinct !DISubprogram(name: "f2", scope: !1, file: !1, line: 14, type: !12, isLocal: false, isDefinition: true, scopeLine: 15, isOptimized: true, unit: !0, variables: !2)
+!12 = !DISubroutineType(types: !13)
+!13 = !{null}
+!14 = !DILocation(line: 19, column: 9, scope: !15)
+!15 = !DILexicalBlock(scope: !11, file: !1, line: 17, column: 5)
+!16 = !DILocation(line: 24, column: 9, scope: !15)
+!22 = !DILocation(line: 27, column: 5, scope: !11)
+!23 = !DILocation(line: 28, column: 1, scope: !11)