8309956: Shenandoah: Strengthen the mark word check in string dedup

Reviewed-by: rkennke
Backport-of: 57b8251241e2044d5039ce162bf4637a9b2e5466

src/hotspot/share/gc/shenandoah/shenandoahStringDedup.inline.hpp

@@ -45,14 +45,15 @@ bool ShenandoahStringDedup::is_candidate(oop obj) {
     return false;
   }
 
-  if (StringDedup::is_below_threshold_age(obj->age())) {
-    const markWord mark = obj->mark();
-    // Having/had displaced header, too risk to deal with them, skip
-    if (mark == markWord::INFLATING() || mark.has_displaced_mark_helper()) {
-      return false;
-    }
+  const markWord mark = obj->mark();
+
+  // Having/had displaced header, too risky to deal with them, skip
+  if (mark == markWord::INFLATING() || mark.has_displaced_mark_helper()) {
+    return false;
+  }
 
-    // Increase string age and enqueue it when it rearches age threshold
+  if (StringDedup::is_below_threshold_age(mark.age())) {
+    // Increase string age and enqueue it when it reaches age threshold
     markWord new_mark = mark.incr_age();
     if (mark == obj->cas_set_mark(new_mark, mark)) {
       return StringDedup::is_threshold_age(new_mark.age()) &&