diff --git a/llvm/include/llvm/MC/MCDwarf.h b/llvm/include/llvm/MC/MCDwarf.h
--- a/llvm/include/llvm/MC/MCDwarf.h
+++ b/llvm/include/llvm/MC/MCDwarf.h
@@ -261,6 +261,14 @@
     HasSource = Source.hasValue();
   }
 
+  void resetFileTable() {
+    MCDwarfDirs.clear();
+    MCDwarfFiles.clear();
+    RootFile.Name.clear();
+    resetMD5Usage();
+    HasSource = false;
+  }
+
 private:
   void emitV2FileDirTables(MCStreamer *MCOS) const;
   void emitV5FileDirTables(MCStreamer *MCOS, Optional<MCDwarfLineStr> &LineStr) const;
@@ -326,12 +334,7 @@
     Header.HasSource = Source.hasValue();
   }
 
-  void resetRootFile() {
-    assert(Header.MCDwarfFiles.empty());
-    Header.RootFile.Name.clear();
-    Header.resetMD5Usage();
-    Header.HasSource = false;
-  }
+  void resetFileTable() { Header.resetFileTable(); }
 
   bool hasRootFile() const { return !Header.RootFile.Name.empty(); }
 
diff --git a/llvm/lib/MC/MCParser/AsmParser.cpp b/llvm/lib/MC/MCParser/AsmParser.cpp
--- a/llvm/lib/MC/MCParser/AsmParser.cpp
+++ b/llvm/lib/MC/MCParser/AsmParser.cpp
@@ -3395,9 +3395,9 @@
   } else {
     // In case there is a -g option as well as debug info from directive .file,
     // we turn off the -g option, directly use the existing debug info instead.
-    // Also reset any implicit ".file 0" for the assembler source.
+    // Throw away any implicit file table for the assembler source.
     if (Ctx.getGenDwarfForAssembly()) {
-      Ctx.getMCDwarfLineTable(0).resetRootFile();
+      Ctx.getMCDwarfLineTable(0).resetFileTable();
       Ctx.setGenDwarfForAssembly(false);
     }
 
diff --git a/llvm/test/MC/AsmParser/directive_file-g.s b/llvm/test/MC/AsmParser/directive_file-g.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/AsmParser/directive_file-g.s
@@ -0,0 +1,24 @@
+## Make sure that using -g (or equivalent) on an asm file that already has
+## debug-info directives in it will correctly ignore the -g and produce
+## debug info corresponding to the directives in the source.
+## Note gcc accepts ".file 1" after a label, although not after an opcode.
+## If no other directives appear, gcc emits no debug info at all.
+
+# RUN: llvm-mc -g -triple i386-unknown-unknown -filetype=obj %s -o %t
+# RUN: llvm-dwarfdump -debug-info -debug-line %t | FileCheck %s
+
+foo:
+        .file 1 "a.c"
+        .loc 1 1 1
+        nop
+
+# CHECK: .debug_info
+## gcc does generate a DW_TAG_compile_unit in this case, with or without
+## -g on the command line, but we do not.
+# CHECK-EMPTY:
+# CHECK_NEXT: .debug_line
+# CHECK: file_names[ 1]:
+# CHECK-NEXT: name: "a.c"
+# CHECK-NEXT: dir_index: 0
+# CHECK: 0x{{0+}}0 1 1 1 0 0 is_stmt
+# CHECK: 0x{{0+}}1 1 1 1 0 0 is_stmt end_sequence