diff --git a/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 6925cb6bdde..e8055d7cf7c 100644
--- a/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -688,6 +688,11 @@ DwarfCompileUnit *DwarfDebug::constructDwarfCompileUnit(DICompileUnit DIUnit) {
   DwarfCompileUnit *NewCU = new DwarfCompileUnit(
       InfoHolder.getUnits().size(), Die, DIUnit, Asm, this, &InfoHolder);
   InfoHolder.addUnit(NewCU);
+
+  // LTO with assembly output shares a single line table amongst multiple CUs.
+  // To avoid the compilation directory being ambiguous, let the line table
+  // explicitly describe the directory of all files, never relying on the
+  // compilation directory.
   if (!Asm->OutStreamer.hasRawTextSupport() || SingleCU)
     Asm->OutStreamer.getContext().setMCLineTableCompilationDir(
         NewCU->getUniqueID(), CompilationDir);
diff --git a/test/DebugInfo/lto-comp-dir.ll b/test/DebugInfo/lto-comp-dir.ll
index eeaf4c0a658..d272dff6ea8 100644
--- a/test/DebugInfo/lto-comp-dir.ll
+++ b/test/DebugInfo/lto-comp-dir.ll
@@ -3,6 +3,10 @@
 ; RUN: %llc_dwarf < %s -filetype=obj | llvm-dwarfdump -debug-dump=line - | FileCheck %s
 ; RUN: %llc_dwarf < %s -filetype=asm | FileCheck --check-prefix=ASM %s
 
+; If multiple line tables are emitted, one per CU, those line tables can
+; unambiguously rely on the comp_dir of their owning CU and use directory '0'
+; to refer to it.
+
 ; CHECK: .debug_line contents:
 ; CHECK-NEXT: Line table prologue:
 ; CHECK-NOT: include_directories
@@ -14,6 +18,11 @@
 ; CHECK: file_names[   1]   0 {{.*}} b.cpp
 ; CHECK-NOT: file_names
 
+; However, if a single line table is emitted and shared between CUs, the
+; comp_dir is ambiguous and relying on it would lead to different path
+; interpretations depending on which CU lead to the table - so ensure that
+; full paths are always emitted in this case, never comp_dir relative.
+
 ; ASM: .file   1 "/tmp/dbginfo/a{{[/\\]+}}a.cpp"
 ; ASM: .file   2 "/tmp/dbginfo/b{{[/\\]+}}b.cpp"