Index: lit/TestRunner.py
===================================================================
--- lit/TestRunner.py
+++ lit/TestRunner.py
@@ -205,7 +205,20 @@
             #   env FOO=1 llc < %s | env BAR=2 llvm-mc | FileCheck %s
             cmd_shenv = ShellEnvironment(shenv.cwd, shenv.env)
             arg_idx = 1
+            unset_next_env_var = False
             for arg_idx, arg in enumerate(j.args[1:]):
+                # Support for the -u flag (unsetting) for env command
+                # e.g., env -u FOO -u BAR will remove both FOO and BAR
+                # from the environment.
+                if arg == '-u':
+                    unset_next_env_var = True
+                    continue
+                if unset_next_env_var:
+                    unset_next_env_var = False
+                    if arg in cmd_shenv.env:
+                        del cmd_shenv.env[arg]
+                    continue
+
                 # Partition the string into KEY=VALUE.
                 key, eq, val = arg.partition('=')
                 # Stop if there was no equals.