diff -r e47e00723e5d Lib/test/test_dev.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/test/test_dev.py	Wed Mar 30 10:19:44 2016 +0200
@@ -0,0 +1,36 @@
+import subprocess
+import sys
+import unittest
+
+
+class DevModeTests(unittest.TestCase):
+    def check(self, strict, code, expected):
+        dev = "dev=strict" if strict else "dev"
+        args = [sys.executable, "-X", dev, "-c", code]
+        cmd = subprocess.run(args,
+                             stdout=subprocess.PIPE,
+                             stderr=subprocess.STDOUT,
+                             check=True)
+        self.assertEqual(cmd.stdout.rstrip(), expected)
+
+    def test_args(self):
+        code = ";".join(("import subprocess",
+                         "args = subprocess._args_from_interpreter_flags()",
+                         "print(' '.join(args))"))
+        self.check(False, code, b"-b -Wd")
+        self.check(True, code, b"-bb -Werror")
+
+    def test_faulthandler(self):
+        code = "import faulthandler; print(faulthandler.is_enabled())"
+        cmd = self.check(False, code, b'True')
+        cmd = self.check(True, code, b'True')
+
+    def test_malloc(self):
+        code = "import os; print(os.environ.get('PYTHONMALLOC'))"
+        cmd = self.check(False, code, b'debug')
+        cmd = self.check(True, code, b'debug')
+
+
+if __name__ == "__main__":
+    unittest.main()
+
diff -r e47e00723e5d Modules/main.c
--- a/Modules/main.c	Wed Mar 30 08:51:15 2016 +0200
+++ b/Modules/main.c	Wed Mar 30 10:19:44 2016 +0200
@@ -321,6 +321,92 @@ run_file(FILE *fp, const wchar_t *filena
     return run != 0;
 }
 
+static const char*
+exec_dev_mode(int strict, int argc, wchar_t **wargv)
+{
+    /* FIXME: copy code from posixmodule.c */
+    extern char **environ;
+
+    const char* errmsg;
+    char **argv, **parg;
+    int i;
+
+    argv = PyMem_RawMalloc((4 + argc + 1) * sizeof(argv[0]));
+    if (argv == NULL) {
+        errmsg = "building argv failed (malloc error)";
+        goto error;
+    }
+
+    parg = argv;
+    *parg = Py_EncodeLocale(wargv[0], NULL);
+    if (*parg == NULL) {
+        errmsg = "building argv failed (encode error)";
+        goto error;
+    }
+    parg++;
+
+    *parg++ = strict ? "-bb" : "-b";
+    *parg++ = strict ? "-Werror" : "-Wd";
+    *parg++ = "-Xfaulthandler";
+    *parg++ = "-X_devset";
+
+    for (i=1; i < argc; i++) {
+        *parg = Py_EncodeLocale(wargv[i], NULL);
+        if (*parg == NULL) {
+            errmsg = "building argv failed (encode error)";
+            goto error;
+        }
+        parg++;
+    }
+    *parg++ = NULL;
+
+    /* FIXME: use Py_GetProgramFullPath() */
+    execve(argv[0], argv, environ);
+
+    errmsg = "exec() failed";
+
+error:
+    PyMem_RawFree(argv);
+    return errmsg;
+}
+
+static void
+setup_dev_mode(int argc, wchar_t **argv)
+{
+    PyObject *xoptions, *value;
+    const char* errmsg;
+    int strict = 0;
+
+    xoptions = PySys_GetXOptions();
+    if (xoptions == NULL) {
+        Py_FatalError("failed to get sys._xoptions");
+    }
+
+    /* FIXME: catch errors */
+    if (PyDict_GetItemString(xoptions, "_devset") != NULL) {
+        return;
+    }
+
+    /* FIXME: catch errors */
+    value = PyDict_GetItemString(xoptions, "dev");
+    if (value == NULL) {
+        return;
+    }
+
+    if (PyUnicode_Check(value) && PyUnicode_CompareWithASCIIString(value, "strict") == 0) {
+        strict = 1;
+    }
+
+    /* FIXME: don't override but add _malloc suffix for malloc/pymalloc */
+    if (setenv("PYTHONMALLOC", "debug", 1)) {
+        Py_FatalError("failed to set PYTHONMALLOC env variable");
+    }
+
+    errmsg = exec_dev_mode(strict, argc, argv);
+    printf("setup_dev_mode() failed: %s\n", errmsg);
+    exit(1);
+}
+
 
 /* Main program */
 
@@ -590,6 +676,8 @@ Py_Main(int argc, wchar_t **argv)
         filename = argv[_PyOS_optind];
     }
 
+    setup_dev_mode(orig_argc, orig_argv);
+
     stdin_is_interactive = Py_FdIsInteractive(stdin, (char *)0);
 
 #if defined(MS_WINDOWS) || defined(__CYGWIN__)