diff --git a/compiler-rt/test/crt/lit.cfg.py b/compiler-rt/test/crt/lit.cfg.py
--- a/compiler-rt/test/crt/lit.cfg.py
+++ b/compiler-rt/test/crt/lit.cfg.py
@@ -10,6 +10,17 @@
 config.test_source_root = os.path.dirname(__file__)
 
 
+# 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'])
+
 def get_library_path(file):
     cmd = subprocess.Popen([config.clang.strip(),
                             config.target_cflags.strip(),