diff --git a/compiler-rt/test/sanitizer_common/ios_commands/iossim_prepare.py b/compiler-rt/test/sanitizer_common/ios_commands/iossim_prepare.py
--- a/compiler-rt/test/sanitizer_common/ios_commands/iossim_prepare.py
+++ b/compiler-rt/test/sanitizer_common/ios_commands/iossim_prepare.py
@@ -1,5 +1,17 @@
 #!/usr/bin/python
 
-import os, sys, subprocess, json
+import json
+import os
+import subprocess
+
+
+device_id = os.environ.get('SANITIZER_IOSSIM_TEST_DEVICE_IDENTIFIER')
+if not device_id:
+    raise EnvironmentError('Specify SANITIZER_IOSSIM_TEST_DEVICE_IDENTIFIER to select which simulator to use.')
+
+boot_cmd = ['xcrun', 'simctl', 'bootstatus', device_id, '-b']
+subprocess.check_call(boot_cmd)
+# TODO(rdar58118442): we start the simulator here, but we never tear it down
+
 
 print(json.dumps({"env": {}}))