Index: compiler-rt/test/lit.common.cfg
===================================================================
--- compiler-rt/test/lit.common.cfg
+++ compiler-rt/test/lit.common.cfg
@@ -11,8 +11,19 @@
 import lit.formats
 import lit.util
 
-# Setup test format. Use bash on Unix and the lit shell on Windows.
-execute_external = (not sys.platform in ['win32'])
+
+# Choose between lit's internal shell pipeline runner and a real shell.  If
+# LIT_USE_INTERNAL_SHELL is in the environment, we use that as an override.
+use_lit_shell = os.environ.get("LIT_USE_INTERNAL_SHELL")
+if use_lit_shell:
+    # 0 is external, "" is default, and everything else is internal.
+    execute_external = (use_lit_shell == "0")
+else:
+    # Otherwise we default to internal on Windows and external elsewhere, as
+    # bash on Windows is usually very slow.
+    execute_external = (not sys.platform in ['win32'])
+
+# Setup test format.
 config.test_format = lit.formats.ShTest(execute_external)
 if execute_external:
   config.available_features.add('shell')