diff --git a/py/asmbase.c b/py/asmbase.c
index c941e917b..83a699cd4 100644
--- a/py/asmbase.c
+++ b/py/asmbase.c
@@ -47,8 +47,10 @@ void mp_asm_base_deinit(mp_asm_base_t *as, bool free_code) {
 
 void mp_asm_base_start_pass(mp_asm_base_t *as, int pass) {
     if (pass == MP_ASM_PASS_COMPUTE) {
-        // reset all labels
+        #ifndef NDEBUG
+        // With debugging enabled labels are checked for unique assignment
         memset(as->label_offsets, -1, as->max_num_labels * sizeof(size_t));
+        #endif
     } else if (pass == MP_ASM_PASS_EMIT) {
         // allocating executable RAM is platform specific
         MP_PLAT_ALLOC_EXEC(as->code_offset, (void**)&as->code_base, &as->code_size);
diff --git a/py/emitbc.c b/py/emitbc.c
index 3f4dfc178..5e7fa623a 100644
--- a/py/emitbc.c
+++ b/py/emitbc.c
@@ -313,9 +313,12 @@ void mp_emit_bc_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scope) {
     emit->scope = scope;
     emit->last_source_line_offset = 0;
     emit->last_source_line = 1;
+    #ifndef NDEBUG
+    // With debugging enabled labels are checked for unique assignment
     if (pass < MP_PASS_EMIT) {
         memset(emit->label_offsets, -1, emit->max_num_labels * sizeof(mp_uint_t));
     }
+    #endif
     emit->bytecode_offset = 0;
     emit->code_info_offset = 0;
 
@@ -495,7 +498,6 @@ void mp_emit_bc_label_assign(emit_t *emit, mp_uint_t l) {
         emit->label_offsets[l] = emit->bytecode_offset;
     } else {
         // ensure label offset has not changed from MP_PASS_CODE_SIZE to MP_PASS_EMIT
-        //printf("l%d: (at %d vs %d)\n", l, emit->bytecode_offset, emit->label_offsets[l]);
         assert(emit->label_offsets[l] == emit->bytecode_offset);
     }
 }