Index: llvm/trunk/lib/Target/X86/X86DiscriminateMemOps.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/X86DiscriminateMemOps.cpp
+++ llvm/trunk/lib/Target/X86/X86DiscriminateMemOps.cpp
@@ -115,14 +115,15 @@
       if (X86II::getMemoryOperandNo(MI.getDesc().TSFlags) < 0)
         continue;
       const DILocation *DI = MI.getDebugLoc();
-      if (!DI) {
+      bool HasDebug = DI;
+      if (!HasDebug) {
         DI = ReferenceDI;
       }
       Location L = diToLocation(DI);
       DenseSet<unsigned> &Set = Seen[L];
       const std::pair<DenseSet<unsigned>::iterator, bool> TryInsert =
           Set.insert(DI->getBaseDiscriminator());
-      if (!TryInsert.second) {
+      if (!TryInsert.second || !HasDebug) {
         unsigned BF, DF, CI = 0;
         DILocation::decodeDiscriminator(DI->getDiscriminator(), BF, DF, CI);
         Optional<unsigned> EncodedDiscriminator = DILocation::encodeDiscriminator(
Index: llvm/trunk/test/CodeGen/X86/discriminate-mem-ops-missing-info.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/discriminate-mem-ops-missing-info.ll
+++ llvm/trunk/test/CodeGen/X86/discriminate-mem-ops-missing-info.ll
@@ -0,0 +1,55 @@
+; RUN: llc -x86-discriminate-memops  < %s | FileCheck %s
+;
+; original source, compiled with -O3 -gmlt -fdebug-info-for-profiling:
+; int sum(int* arr, int pos1, int pos2) {
+;   return arr[pos1] + arr[pos2];
+; }
+;
+; ModuleID = 'test.cc'
+source_filename = "test.cc"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @llvm.prefetch(i8 *, i32, i32, i32)
+; Function Attrs: norecurse nounwind readonly uwtable
+define i32 @sum(i32* %arr, i32 %pos1, i32 %pos2) !dbg !7 {
+entry:
+  %idxprom = sext i32 %pos1 to i64
+  %arrayidx = getelementptr inbounds i32, i32* %arr, i64 %idxprom
+  %0 = load i32, i32* %arrayidx, align 4
+  %idxprom1 = sext i32 %pos2 to i64
+  %arrayidx2 = getelementptr inbounds i32, i32* %arr, i64 %idxprom1
+  %1 = load i32, i32* %arrayidx2, align 4
+  %add = add nsw i32 %1, %0, !dbg !15
+  ret i32 %add
+}
+
+attributes #0 = { "target-cpu"="x86-64" }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2, debugInfoForProfiling: true)
+!1 = !DIFile(filename: "test.cc", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 7.0.0 (trunk 322155) (llvm/trunk 322159)"}
+!7 = distinct !DISubprogram(name: "sum", linkageName: "sum", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0)
+!8 = !DISubroutineType(types: !2)
+!9 = !DILocation(line: 2, column: 10, scope: !7)
+!10 = !{!11, !11, i64 0}
+!11 = !{!"int", !12, i64 0}
+!12 = !{!"omnipotent char", !13, i64 0}
+!13 = !{!"Simple C++ TBAA"}
+!15 = !DILocation(line: 2, column: 20, scope: !7)
+
+
+;CHECK-LABEL: sum:
+;CHECK:       # %bb.0:
+;CHECK:       .loc 1 1 0 {{.*}} discriminator 2
+;CHECK-NEXT:  movl (%rdi,%rax,4), %eax
+;CHECK-NEXT:  .loc 1 2 20
+;CHECK-NEXT:  addl (%rdi,%rcx,4), %eax