diff --git a/mypyc/build.py b/mypyc/build.py
index 566612ce9d33..1350576c6b47 100644
--- a/mypyc/build.py
+++ b/mypyc/build.py
@@ -680,10 +680,13 @@ def mypycify(
             cflags.append("-DMYPYC_LOG_TRACE")
         if experimental_features:
             cflags.append("-DMYPYC_EXPERIMENTAL")
+        if opt_level == "0":
+            cflags.append("-UNDEBUG")
     elif compiler.compiler_type == "msvc":
         # msvc doesn't have levels, '/O2' is full and '/Od' is disable
         if opt_level == "0":
             opt_level = "d"
+            cflags.append("/UNDEBUG")
         elif opt_level in ("1", "2", "3"):
             opt_level = "2"
         if debug_level == "0":
diff --git a/mypyc/lib-rt/CPy.h b/mypyc/lib-rt/CPy.h
index fc4906f41916..68c0be2bd748 100644
--- a/mypyc/lib-rt/CPy.h
+++ b/mypyc/lib-rt/CPy.h
@@ -331,6 +331,11 @@ static inline bool CPyTagged_IsLe(CPyTagged left, CPyTagged right) {
 static inline int64_t CPyLong_AsInt64(PyObject *o) {
     if (likely(PyLong_Check(o))) {
         PyLongObject *lobj = (PyLongObject *)o;
+    #if CPY_3_12_FEATURES
+        if (likely(PyUnstable_Long_IsCompact(lobj))) {
+            return PyUnstable_Long_CompactValue(lobj);
+        }
+    #else
         Py_ssize_t size = Py_SIZE(lobj);
         if (likely(size == 1)) {
             // Fast path
@@ -338,6 +343,7 @@ static inline int64_t CPyLong_AsInt64(PyObject *o) {
         } else if (likely(size == 0)) {
             return 0;
         }
+    #endif
     }
     // Slow path
     return CPyLong_AsInt64_(o);
diff --git a/mypyc/lib-rt/static_data.c b/mypyc/lib-rt/static_data.c
index 1fc93330bdc1..c119bec56467 100644
--- a/mypyc/lib-rt/static_data.c
+++ b/mypyc/lib-rt/static_data.c
@@ -16,6 +16,10 @@ mypyc_interned_str_struct mypyc_interned_str;
 
 int
 intern_strings(void) {
+    if (mypyc_interned_str.values != NULL) {
+        // Already interned.
+        return 0;
+    }
     INTERN_STRING(__init_subclass__, "__init_subclass__");
     INTERN_STRING(__mro_entries__, "__mro_entries__");
     INTERN_STRING(__name__, "__name__");