Fix re-embedding of strings during compaction

The reference updating code for strings is not re-embedding strings
because the code is incorrectly wrapped inside of a
`if (STR_SHARED_P(obj))` clause. Shared strings can't be re-embedded
so this ends up being a no-op. This means that strings can be moved to a
large size pool during compaction, but won't be re-embedded, which would
waste the space.

gc.c

@@ -10609,16 +10609,18 @@ gc_update_object_references(rb_objspace_t *objspace, VALUE obj)
 #if USE_RVARGC
                 VALUE new_root = any->as.string.as.heap.aux.shared;
                 rb_str_update_shared_ary(obj, old_root, new_root);
+#endif
+            }
 
-                // if, after move the string is not embedded, and can fit in the
-                // slot it's been placed in, then re-embed it
-                if (rb_gc_obj_slot_size(obj) >= rb_str_size_as_embedded(obj)) {
-                    if (!STR_EMBED_P(obj) && rb_str_reembeddable_p(obj)) {
-                        rb_str_make_embedded(obj);
-                    }
+#if USE_RVARGC
+            /* If, after move the string is not embedded, and can fit in the
+             * slot it's been placed in, then re-embed it. */
+            if (rb_gc_obj_slot_size(obj) >= rb_str_size_as_embedded(obj)) {
+                if (!STR_EMBED_P(obj) && rb_str_reembeddable_p(obj)) {
+                    rb_str_make_embedded(obj);
                 }
-#endif
             }
+#endif
 
             break;
         }

string.c

@@ -312,14 +312,18 @@ rb_str_make_embedded(VALUE str)
     RUBY_ASSERT(rb_str_reembeddable_p(str));
     RUBY_ASSERT(!STR_EMBED_P(str));
 
-    char *buf = RSTRING_PTR(str);
-    long len = RSTRING_LEN(str);
+    char *buf = RSTRING(str)->as.heap.ptr;
+    long len = RSTRING(str)->as.heap.len;
 
     STR_SET_EMBED(str);
     STR_SET_EMBED_LEN(str, len);
 
-    memmove(RSTRING_PTR(str), buf, len);
-    ruby_xfree(buf);
+    if (len > 0) {
+        memcpy(RSTRING_PTR(str), buf, len);
+        ruby_xfree(buf);
+    }
+
+    TERM_FILL(RSTRING(str)->as.embed.ary + len, TERM_LEN(str));
 }
 
 void

test/ruby/test_gc_compact.rb

@@ -382,7 +382,7 @@ def add_ivars
   def test_moving_strings_up_size_pools
     omit if GC::INTERNAL_CONSTANTS[:SIZE_POOL_COUNT] == 1
 
-    assert_separately([], "#{<<~"begin;"}\n#{<<~"end;"}", timeout: 10, signal: :SEGV)
+    assert_separately(%w[-robjspace], "#{<<~"begin;"}\n#{<<~"end;"}", timeout: 10, signal: :SEGV)
     begin;
       STR_COUNT = 500
 
@@ -394,14 +394,14 @@ def test_moving_strings_up_size_pools
       stats = GC.verify_compaction_references(expand_heap: true, toward: :empty)
 
       assert_operator(stats[:moved_up][:T_STRING], :>=, STR_COUNT)
-      assert(ary) # warning: assigned but unused variable - ary
+      assert_include(ObjectSpace.dump(ary[0]), '"embedded":true')
     end;
   end
 
   def test_moving_strings_down_size_pools
     omit if GC::INTERNAL_CONSTANTS[:SIZE_POOL_COUNT] == 1
 
-    assert_separately([], "#{<<~"begin;"}\n#{<<~"end;"}", timeout: 10, signal: :SEGV)
+    assert_separately(%w[-robjspace], "#{<<~"begin;"}\n#{<<~"end;"}", timeout: 10, signal: :SEGV)
     begin;
       STR_COUNT = 500
 
@@ -412,7 +412,7 @@ def test_moving_strings_down_size_pools
       stats = GC.verify_compaction_references(expand_heap: true, toward: :empty)
 
       assert_operator(stats[:moved_down][:T_STRING], :>=, STR_COUNT)
-      assert(ary) # warning: assigned but unused variable - ary
+      assert_include(ObjectSpace.dump(ary[0]), '"embedded":true')
     end;
   end
 end