Index: llvm/trunk/utils/lit/lit/TestRunner.py
===================================================================
--- llvm/trunk/utils/lit/lit/TestRunner.py
+++ llvm/trunk/utils/lit/lit/TestRunner.py
@@ -217,7 +217,20 @@
 # cmd is export or env
 def updateEnv(env, cmd):
     arg_idx = 1
+    unset_next_env_var = False
     for arg_idx, arg in enumerate(cmd.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 env.env:
+                del env.env[arg]
+            continue
+
         # Partition the string into KEY=VALUE.
         key, eq, val = arg.partition('=')
         # Stop if there was no equals.
Index: llvm/trunk/utils/lit/tests/Inputs/shtest-env/env-u.txt
===================================================================
--- llvm/trunk/utils/lit/tests/Inputs/shtest-env/env-u.txt
+++ llvm/trunk/utils/lit/tests/Inputs/shtest-env/env-u.txt
@@ -0,0 +1,23 @@
+# Check and make sure preset environment variable were set in lit.cfg
+#
+# RUN: %{python} print_environment.py \
+# RUN:   | FileCheck --check-prefix=CHECK-ENV-PRESET %s
+#
+# Check single unset of environment variable
+#
+# RUN: env -u FOO %{python} print_environment.py \
+# RUN:  | FileCheck --check-prefix=CHECK-ENV-UNSET-1 %s
+#
+# Check multiple unsets of environment variables
+#
+# RUN: env -u FOO -u BAR %{python} print_environment.py \
+# RUN:  | FileCheck --check-prefix=CHECK-ENV-UNSET-MULTIPLE %s
+
+# CHECK-ENV-PRESET: BAR = 2
+# CHECK-ENV-PRESET: FOO = 1
+
+# CHECK-ENV-UNSET-1: BAR = 2
+# CHECK-ENV-UNSET-1-NOT: FOO
+
+# CHECK-ENV-UNSET-MULTIPLE-NOT: BAR
+# CHECK-ENV-UNSET-MULTIPLE-NOT: FOO
Index: llvm/trunk/utils/lit/tests/Inputs/shtest-env/env.txt
===================================================================
--- llvm/trunk/utils/lit/tests/Inputs/shtest-env/env.txt
+++ llvm/trunk/utils/lit/tests/Inputs/shtest-env/env.txt
@@ -0,0 +1,15 @@
+# Check for simple one environment variable setting
+#
+# RUN: env A_FOO=999 %{python} print_environment.py \
+# RUN:   | FileCheck --check-prefix=CHECK-ENV-1 %s
+#
+# Check for multiple environment variable settings
+#
+# RUN: env A_FOO=1 B_BAR=2 C_OOF=3 %{python} print_environment.py \
+# RUN:   | FileCheck --check-prefix=CHECK-ENV-MULTIPLE %s
+
+# CHECK-ENV-1: A_FOO = 999
+
+# CHECK-ENV-MULTIPLE: A_FOO = 1
+# CHECK-ENV-MULTIPLE: B_BAR = 2
+# CHECK-ENV-MULTIPLE: C_OOF = 3
Index: llvm/trunk/utils/lit/tests/Inputs/shtest-env/lit.cfg
===================================================================
--- llvm/trunk/utils/lit/tests/Inputs/shtest-env/lit.cfg
+++ llvm/trunk/utils/lit/tests/Inputs/shtest-env/lit.cfg
@@ -0,0 +1,9 @@
+import lit.formats
+config.name = 'shtest-env'
+config.suffixes = ['.txt']
+config.test_format = lit.formats.ShTest()
+config.test_source_root = None
+config.test_exec_root = None
+config.environment['FOO'] = '1'
+config.environment['BAR'] = '2'
+config.substitutions.append(('%{python}', sys.executable))
Index: llvm/trunk/utils/lit/tests/Inputs/shtest-env/mixed.txt
===================================================================
--- llvm/trunk/utils/lit/tests/Inputs/shtest-env/mixed.txt
+++ llvm/trunk/utils/lit/tests/Inputs/shtest-env/mixed.txt
@@ -0,0 +1,18 @@
+# Check for setting and removing one environment variable
+#
+# RUN: env A_FOO=999 -u FOO %{python} print_environment.py \
+# RUN:   | FileCheck --check-prefix=CHECK-ENV-1 %s
+#
+# Check for setting/unsetting multiple environment variables
+#
+# RUN: env A_FOO=1 -u FOO B_BAR=2 -u BAR C_OOF=3 %{python} print_environment.py \
+# RUN:   | FileCheck --check-prefix=CHECK-ENV-MULTIPLE %s
+
+# CHECK-ENV-1: A_FOO = 999
+# CHECK-ENV-1-NOT: FOO
+
+# CHECK-ENV-MULTIPLE: A_FOO = 1
+# CHECK-ENV-MULTIPLE-NOT: BAR
+# CHECK-ENV-MULTIPLE: B_BAR = 2
+# CHECK-ENV-MULTIPLE: C_OOF = 3
+# CHECK-ENV-MULTIPLE-NOT: FOO
Index: llvm/trunk/utils/lit/tests/Inputs/shtest-env/print_environment.py
===================================================================
--- llvm/trunk/utils/lit/tests/Inputs/shtest-env/print_environment.py
+++ llvm/trunk/utils/lit/tests/Inputs/shtest-env/print_environment.py
@@ -0,0 +1,8 @@
+#!/usr/bin/env python
+
+import os
+
+sorted_environment = sorted(os.environ.items())
+
+for name,value in sorted_environment:
+    print name,'=',value
Index: llvm/trunk/utils/lit/tests/Inputs/shtest-env/shtest-env.py
===================================================================
--- llvm/trunk/utils/lit/tests/Inputs/shtest-env/shtest-env.py
+++ llvm/trunk/utils/lit/tests/Inputs/shtest-env/shtest-env.py
@@ -0,0 +1,3 @@
+# Check the env command
+#
+# RUN: %{lit} -a -v %{inputs}/shtest-env