immediate ascii for string path

source/quickjs-internal.h

@@ -78,7 +78,7 @@
  */
 // #define DUMP_BYTECODE  (1)
 /* dump GC summary: old/new heap, recovery %, heap growth */
-#define DUMP_GC
+// #define DUMP_GC
 /* dump detailed GC: roots, scanning, object traversal (implies DUMP_GC) */
 // #define DUMP_GC_DETAIL
 #ifdef DUMP_GC_DETAIL

source/runtime.c

@@ -1732,6 +1732,21 @@ static JSValue js_sub_string (JSContext *ctx, JSText *p, int start, int end) {
     return JS_MKPTR (p);
   }
 
+  /* Try immediate ASCII for short substrings (avoids heap allocation) */
+  if (len <= MIST_ASCII_MAX_LEN && len > 0) {
+    char buf[MIST_ASCII_MAX_LEN];
+    int all_ascii = 1;
+    for (i = 0; i < len; i++) {
+      uint32_t c = string_get (p, start + i);
+      if (c >= 128) { all_ascii = 0; break; }
+      buf[i] = (char)c;
+    }
+    if (all_ascii) {
+      JSValue imm = MIST_TryNewImmediateASCII (buf, len);
+      if (!JS_IsNull (imm)) return imm;
+    }
+  }
+
   /* Root the source string as a JSValue so it survives js_alloc_string GC */
   JSGCRef src_ref;
   JS_PushGCRef (ctx, &src_ref);