[pull] master from ruby:master

.github/workflows/ubuntu.yml

@@ -49,9 +49,8 @@ jobs:
             os: ubuntu-24.04
             configure: 'CC=clang-18 cflags=-fsanitize=address cppflags=-DUSE_MN_THREADS=0'
             timeout: 60
-          # ubuntu-24.04-arm jobs don't start on ruby/ruby as of 2025-10-29
-          #- test_task: check
-          #  os: ubuntu-24.04-arm
+          - test_task: check
+            os: ubuntu-26.04-arm
       fail-fast: false
 
     env: &make-env

array.c

@@ -6750,6 +6750,8 @@ rb_ary_count(int argc, VALUE *argv, VALUE ary)
     return LONG2NUM(n);
 }
 
+VALUE rb_ident_set_new(void);
+
 static VALUE
 flatten(VALUE ary, int level)
 {
@@ -6777,9 +6779,9 @@ flatten(VALUE ary, int level)
     rb_ary_push(stack, LONG2NUM(i + 1));
 
     if (level < 0) {
-        memo = rb_obj_hide(rb_ident_hash_new());
-        rb_hash_aset(memo, ary, Qtrue);
-        rb_hash_aset(memo, tmp, Qtrue);
+        memo = rb_obj_hide(rb_ident_set_new());
+        rb_set_add(memo, ary);
+        rb_set_add(memo, tmp);
     }
 
     ary = tmp;
@@ -6795,7 +6797,7 @@ flatten(VALUE ary, int level)
             tmp = rb_check_array_type(elt);
             if (RBASIC(result)->klass) {
                 if (RTEST(memo)) {
-                    rb_hash_clear(memo);
+                    rb_set_clear(memo);
                 }
                 rb_raise(rb_eRuntimeError, "flatten reentered");
             }
@@ -6804,11 +6806,11 @@ flatten(VALUE ary, int level)
             }
             else {
                 if (memo) {
-                    if (rb_hash_aref(memo, tmp) == Qtrue) {
-                        rb_hash_clear(memo);
+                    if (rb_set_lookup(memo, tmp)) {
+                        rb_set_clear(memo);
                         rb_raise(rb_eArgError, "tried to flatten recursive array");
                     }
-                    rb_hash_aset(memo, tmp, Qtrue);
+                    rb_set_add(memo, tmp);
                 }
                 rb_ary_push(stack, ary);
                 rb_ary_push(stack, LONG2NUM(i));
@@ -6820,15 +6822,15 @@ flatten(VALUE ary, int level)
             break;
         }
         if (memo) {
-            rb_hash_delete(memo, ary);
+            rb_set_delete(memo, ary);
         }
         tmp = rb_ary_pop(stack);
         i = NUM2LONG(tmp);
         ary = rb_ary_pop(stack);
     }
 
     if (memo) {
-        rb_hash_clear(memo);
+        rb_set_clear(memo);
     }
 
     RBASIC_SET_CLASS(result, rb_cArray);

gc.c

@@ -3699,10 +3699,17 @@ rb_gc_obj_optimal_size(VALUE obj)
 
       case T_HASH:
         {
-            if (RB_OBJ_FROZEN(obj) && RHASH_AR_TABLE_P(obj)) {
-                return sizeof(struct RHash) + offsetof(ar_table, pairs) + RHASH_AR_TABLE_BOUND(obj) * sizeof(ar_table_pair);
+            const size_t st_size = sizeof(struct RHash) + sizeof(st_table);
+            if (RHASH_ST_TABLE_P(obj)) {
+                return st_size;
             }
-            return sizeof(struct RHash) + (RHASH_ST_TABLE_P(obj) ? sizeof(st_table) : sizeof(ar_table));
+
+            const size_t ar_size = sizeof(struct RHash) + offsetof(ar_table, pairs) + RHASH_AR_TABLE_BOUND(obj) * sizeof(ar_table_pair);
+            if (OBJ_FROZEN(obj) || ar_size > st_size) {
+                return ar_size;
+            }
+
+            return st_size;
         }
 
       default:

gc/default/default.c

@@ -5925,6 +5925,7 @@ static void pinned_roots_mark(rb_objspace_t *objspace, rb_heap_t *heap);
 static void
 mark_roots(rb_objspace_t *objspace, const char **categoryp)
 {
+    VALUE objspace_guard = (VALUE)objspace;
 #define MARK_CHECKPOINT(category) do { \
     if (categoryp) *categoryp = category; \
 } while (0)
@@ -5955,6 +5956,9 @@ mark_roots(rb_objspace_t *objspace, const char **categoryp)
 
     rb_gc_save_machine_context();
     rb_gc_mark_roots(objspace, categoryp);
+    /* Keep this frame, including its saved registers, until root marking has
+     * scanned the machine stack. */
+    RB_GC_GUARD(objspace_guard);
     gc_mark_set_parent_invalid(objspace);
 }