Index: llvm/utils/lit/lit/llvm/subst.py
===================================================================
--- llvm/utils/lit/lit/llvm/subst.py
+++ llvm/utils/lit/lit/llvm/subst.py
@@ -1,5 +1,6 @@
 import os
 import re
+import sys
 
 import lit.util
 
@@ -118,8 +119,25 @@
             command_str = str(self.command)
 
         if command_str:
+            args = [command_str]
             if self.extra_args:
-                command_str = ' '.join([command_str] + self.extra_args)
+                args = args + self.extra_args
+
+            # This isn't intended to be perfect, as doing so is more trouble
+            # than it's worth.  We just want to catch strings with unquoted
+            # spaces, since that is the use case that comes up the most
+            # frequently.  However, as the user might explicitly quote them,
+            # we do try to avoid quoting already quoted strings.
+            def quote(x):
+                if ' ' not in x:
+                    return x
+                quote_ch = '"' if sys.platform == 'win32' else "'"
+                if x[0] != quote_ch:
+                    x = quote_ch + x
+                if x[-1] != quote_ch:
+                    x = x + quote_ch
+                return x
+            command_str = ' '.join(quote(x) for x in args)
         else:
             if self.unresolved == 'warn':
                 # Warn, but still provide a substitution.