diff --git a/llvm/test/tools/llvm-ml/basic.test b/llvm/test/tools/llvm-ml/basic.test
--- a/llvm/test/tools/llvm-ml/basic.test
+++ b/llvm/test/tools/llvm-ml/basic.test
@@ -1,4 +1,8 @@
 # RUN: not llvm-ml %t.blah.asm /nologo /Fo /dev/null 2>&1 | FileCheck -DMSG=%errc_ENOENT --check-prefix=ENOENT %s
+# RUN: not llvm-ml /Ta /nnlogo /Fo /dev/null 2>&1 | FileCheck -DMSG=%errc_ENOENT --check-prefix=ENOENT-TA %s
+# RUN: not llvm-ml - /Fo /dev/null /nnlogo 2>&1 | FileCheck --check-prefix=BADFLAG %s
 
 # ENOENT-NOT: nologo
 # ENOENT: {{.*}}.blah.asm: [[MSG]]
+# ENOENT-TA: /nnlogo: [[MSG]]
+# BADFLAG: llvm-ml: error: invalid option '/nnlogo', did you mean '/nologo'?
diff --git a/llvm/test/tools/llvm-ml/invalid_file_extension.blah b/llvm/test/tools/llvm-ml/invalid_file_extension.blah
deleted file mode 100644
--- a/llvm/test/tools/llvm-ml/invalid_file_extension.blah
+++ /dev/null
@@ -1,8 +0,0 @@
-; RUN: not llvm-ml %s /Fo /dev/null 2>&1 | FileCheck %s --check-prefixes=CHECK-INVALID
-; RUN: llvm-ml /Ta %s -m64 -filetype=s /Fo - | FileCheck %s --check-prefixes=CHECK-TA
-
-; CHECK-INVALID: error: invalid option '{{.*}}invalid_file_extension.blah'
-
-.code
-foo:
-; CHECK-TA: foo:
diff --git a/llvm/test/tools/llvm-ml/lit.local.cfg b/llvm/test/tools/llvm-ml/lit.local.cfg
--- a/llvm/test/tools/llvm-ml/lit.local.cfg
+++ b/llvm/test/tools/llvm-ml/lit.local.cfg
@@ -3,5 +3,4 @@
     config.unsupported = True
 
 config.suffixes.add('.asm')
-config.suffixes.add('.S')
 config.suffixes.add('.blah')
diff --git a/llvm/test/tools/llvm-ml/random_file_extension.blah b/llvm/test/tools/llvm-ml/random_file_extension.blah
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-ml/random_file_extension.blah
@@ -0,0 +1,6 @@
+; RUN: llvm-ml %s -m64 -filetype=s /Fo - | FileCheck %s
+; RUN: llvm-ml /Ta %s -m64 -filetype=s /Fo - | FileCheck %s
+
+.code
+foo:
+; CHECK: foo:
diff --git a/llvm/test/tools/llvm-ml/valid_file_extension.S b/llvm/test/tools/llvm-ml/valid_file_extension.S
deleted file mode 100644
--- a/llvm/test/tools/llvm-ml/valid_file_extension.S
+++ /dev/null
@@ -1,5 +0,0 @@
-; RUN: llvm-ml -m64 -filetype=s %s /Fo - | FileCheck %s
-
-.code
-foo:
-; CHECK: foo:
diff --git a/llvm/tools/llvm-ml/Opts.td b/llvm/tools/llvm-ml/Opts.td
--- a/llvm/tools/llvm-ml/Opts.td
+++ b/llvm/tools/llvm-ml/Opts.td
@@ -31,7 +31,7 @@
 def bitness : LLVMJoined<"m">, Values<"32,64">,
               HelpText<"Target platform (x86 or x86-64)">;
 def as_lex : LLVMFlag<"as-lex">,
-             HelpText<"Lex tokens from an .asm or .S file without assembling">;
+             HelpText<"Lex tokens from a file without assembling">;
 def debug : LLVMFlag<"debug">, Flags<[HelpHidden]>,
             HelpText<"Enable debug output">;
 def debug_only : LLVMCommaJoined<"debug-only=">, Flags<[HelpHidden]>,
@@ -81,8 +81,8 @@
                        "that are all declared with .SAFESEH. Only available in "
                        "32-bit.">;
 def assembly_file : MLJoinedOrSeparate<"Ta">,
-                    HelpText<"Assemble source file with name not ending with "
-                             "the .asm or the .S extension">;
+                    HelpText<"Assemble source file with the given name. Used "
+                    "if the filename begins with a forward slash.">;
 def error_on_warning : MLFlag<"WX">, Alias<fatal_warnings>;
 def parse_only : MLFlag<"Zs">, HelpText<"Run a syntax-check only">,
                  Alias<filetype>, AliasArgs<["null"]>;
diff --git a/llvm/tools/llvm-ml/llvm-ml.cpp b/llvm/tools/llvm-ml/llvm-ml.cpp
--- a/llvm/tools/llvm-ml/llvm-ml.cpp
+++ b/llvm/tools/llvm-ml/llvm-ml.cpp
@@ -39,6 +39,7 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Process.h"
+#include "llvm/Support/Regex.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/ToolOutputFile.h"
@@ -206,18 +207,20 @@
       T.ParseArgs(ArgsArr, MissingArgIndex, MissingArgCount);
 
   std::string InputFilename;
+  llvm::Regex SlashFlagRegex("^\\/[^\\/]+$");
   for (auto *Arg : InputArgs.filtered(OPT_INPUT)) {
     std::string ArgString = Arg->getAsString(InputArgs);
-    StringRef ArgStringRef(ArgString);
-    if (ArgString == "-" || ArgStringRef.endswith(".asm") ||
-        ArgStringRef.endswith(".S")) {
+    bool IsFile = false;
+    std::error_code IsFileEC =
+        llvm::sys::fs::is_regular_file(ArgString, IsFile);
+    if (ArgString == "-" || IsFile) {
       if (!InputFilename.empty()) {
         WithColor::warning(errs(), ProgName)
             << "does not support multiple assembly files in one command; "
             << "ignoring '" << InputFilename << "'\n";
       }
       InputFilename = ArgString;
-    } else {
+    } else if (SlashFlagRegex.match(ArgString)) {
       std::string Diag;
       raw_string_ostream OS(Diag);
       OS << "invalid option '" << ArgString << "'";
@@ -228,6 +231,9 @@
 
       WithColor::error(errs(), ProgName) << OS.str() << '\n';
       exit(1);
+    } else {
+      WithColor::error() << ArgString << ": " << IsFileEC.message() << '\n';
+      exit(1);
     }
   }
   for (auto *Arg : InputArgs.filtered(OPT_assembly_file)) {