Index: llvm/trunk/lib/MC/MCContext.cpp
===================================================================
--- llvm/trunk/lib/MC/MCContext.cpp
+++ llvm/trunk/lib/MC/MCContext.cpp
@@ -582,10 +582,21 @@
   }
   // Canonicalize the root filename. It cannot be empty, and should not
   // repeat the compilation dir.
-  StringRef FileName =
-      !getMainFileName().empty() ? StringRef(getMainFileName()) : InputFileName;
-  if (FileName.empty() || FileName == "-")
-    FileName = "<stdin>";
+  // The MCContext ctor initializes MainFileName to the name associated with
+  // the SrcMgr's main file ID, which might be the same as InputFileName (and
+  // possibly include directory components).
+  // Or, MainFileName might have been overridden by a -main-file-name option,
+  // which is supposed to be just a base filename with no directory component.
+  // So, if the InputFileName and MainFileName are not equal, assume
+  // MainFileName is a substitute basename and replace the last component.
+  SmallString<1024> FileNameBuf = InputFileName;
+  if (FileNameBuf.empty() || FileNameBuf == "-")
+    FileNameBuf = "<stdin>";
+  if (!getMainFileName().empty() && FileNameBuf != getMainFileName()) {
+    llvm::sys::path::remove_filename(FileNameBuf);
+    llvm::sys::path::append(FileNameBuf, getMainFileName());
+  }
+  StringRef FileName = FileNameBuf;
   if (FileName.consume_front(getCompilationDir()))
     if (llvm::sys::path::is_separator(FileName.front()))
       FileName = FileName.drop_front();
Index: llvm/trunk/test/DebugInfo/X86/main-file-name.s
===================================================================
--- llvm/trunk/test/DebugInfo/X86/main-file-name.s
+++ llvm/trunk/test/DebugInfo/X86/main-file-name.s
@@ -1,17 +0,0 @@
-// RUN: llvm-mc -triple x86_64-unknown-linux-gnu -filetype obj -main-file-name foo.S -g -o %t %s
-// RUN: llvm-dwarfdump -v -debug-info %t | FileCheck %s
-
-// CHECK: DW_TAG_compile_unit [1]
-// CHECK-NOT: DW_TAG_
-// CHECK: DW_AT_name [DW_FORM_string]       ("foo.S")
-        
-
-# 1 "foo.S"
-# 1 "<built-in>" 1
-# 1 "foo.S" 2
-
-foo:
-  nop
-  nop
-  nop
-        
Index: llvm/trunk/test/MC/ELF/debug-main-file.s
===================================================================
--- llvm/trunk/test/MC/ELF/debug-main-file.s
+++ llvm/trunk/test/MC/ELF/debug-main-file.s
@@ -0,0 +1,41 @@
+// RUN: mkdir -p %t/Inputs
+// RUN: cp %s %t/base.s
+// RUN: cp %s %t/Inputs/subdir.s
+// RUN: cd %t
+
+// RUN: llvm-mc -triple=x86_64-linux-unknown -filetype=obj -dwarf-version=4 \
+// RUN:     -g base.s -o %t1.o
+// RUN: llvm-dwarfdump -debug-info %t1.o | \
+// RUN:     FileCheck %s --check-prefixes=CHECK,BASE
+// RUN: llvm-mc -triple=x86_64-linux-unknown -filetype=obj -dwarf-version=4 \
+// RUN:     -g base.s -o %t2.o -main-file-name rename.s
+// RUN: llvm-dwarfdump -debug-info %t2.o | \
+// RUN:     FileCheck %s --check-prefixes=CHECK,RENAME
+// RUN: llvm-mc -triple=x86_64-linux-unknown -filetype=obj -dwarf-version=4 \
+// RUN:     -g Inputs\subdir.s -o %t3.o
+// RUN: llvm-dwarfdump -debug-info %t3.o | \
+// RUN:     FileCheck %s --check-prefixes=CHECK,SUBDIR
+// RUN: llvm-mc -triple=x86_64-linux-unknown -filetype=obj -dwarf-version=4 \
+// RUN:     -g Inputs\subdir.s -main-file-name sub-rename.s -o %t4.o
+// RUN: llvm-dwarfdump -debug-info %t4.o | \
+// RUN:     FileCheck %s --check-prefixes=CHECK,SUB-RENAME
+
+// CHECK: DW_TAG_compile_unit
+// CHECK-NOT: DW_TAG
+// CHECK: DW_AT_name
+// BASE-SAME:       ("base.s")
+// RENAME-SAME:     ("rename.s")
+// SUBDIR-SAME:     ("Inputs{{(/|\\)+}}subdir.s")
+// SUB-RENAME-SAME: ("Inputs{{(/|\\)+}}sub-rename.s")
+
+// CHECK: DW_TAG_label
+// CHECK-NOT: DW_TAG
+// CHECK: DW_AT_decl_file
+// BASE-SAME:       ("{{.*(/|\\)}}base.s")
+// RENAME-SAME:     ("{{.*(/|\\)}}rename.s")
+// SUBDIR-SAME:     ("{{.*Inputs(/|\\)+}}subdir.s")
+// SUB-RENAME-SAME: ("{{.*Inputs(/|\\)+}}sub-rename.s")
+
+        .text
+start:
+        nop