From f70c081b8ffe9d9e0f189cf5fd38ed6e0426d21b Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Fri, 10 Apr 2026 09:34:36 +0200
Subject: Always allocate CDHASH in 80B slots

Up to size 8, `rb_hash_new_with_size()` will allocate a 160B
slot to fit a full `ar_table`.

But in the case of CDHASH, we immediately assign a custom hash
type, which trigger the conversion into an `st_table`, potentially
wasting 80B.
---
 compile.c       | 33 +++++++++++++++++++++------------
 hash.c          |  8 ++++++++
 internal/hash.h |  1 +
 3 files changed, 30 insertions(+), 12 deletions(-)

diff --git a/compile.c b/compile.c
index ac1bdfe0f9..68cf715c4e 100644
--- a/compile.c
+++ b/compile.c
@@ -7067,7 +7067,7 @@ compile_case(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const orig_nod
     DECL_ANCHOR(body_seq);
     DECL_ANCHOR(cond_seq);
     int only_special_literals = 1;
-    VALUE literals = rb_hash_new();
+    VALUE literals = rb_hash_new_with_size_and_type(0, 0, &cdhash_type);
     int line;
     enum node_type type;
     const NODE *line_node;
@@ -7078,8 +7078,6 @@ compile_case(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const orig_nod
     INIT_ANCHOR(body_seq);
     INIT_ANCHOR(cond_seq);
 
-    RHASH_TBL_RAW(literals)->type = &cdhash_type;
-
     CHECK(COMPILE(head, "case base", RNODE_CASE(node)->nd_head));
 
     branches = decl_branch_base(iseq, PTR2NUM(node), nd_code_loc(node), "case");
@@ -7165,7 +7163,6 @@ compile_case(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const orig_nod
 
     if (only_special_literals && ISEQ_COMPILE_DATA(iseq)->option->specialized_instruction) {
         ADD_INSN(ret, orig_node, dup);
-        rb_obj_hide(literals);
         ADD_INSN2(ret, orig_node, opt_case_dispatch, literals, elselabel);
         RB_OBJ_WRITTEN(iseq, Qundef, literals);
         LABEL_REF(elselabel);
@@ -12167,9 +12164,8 @@ iseq_build_from_ary_body(rb_iseq_t *iseq, LINK_ANCHOR *const anchor,
                       case TS_CDHASH:
                         {
                             int i;
-                            VALUE map = rb_hash_new_with_size(RARRAY_LEN(op)/2);
+                            VALUE map = rb_hash_new_with_size_and_type(0, RARRAY_LEN(op)/2, &cdhash_type);
 
-                            RHASH_TBL_RAW(map)->type = &cdhash_type;
                             op = rb_to_array_type(op);
                             for (i=0; i<RARRAY_LEN(op); i+=2) {
                                 VALUE key = RARRAY_AREF(op, i);
@@ -12179,7 +12175,7 @@ iseq_build_from_ary_body(rb_iseq_t *iseq, LINK_ANCHOR *const anchor,
                                 rb_hash_aset(map, key, (VALUE)label | 1);
                             }
                             RB_GC_GUARD(op);
-                            RB_OBJ_SET_SHAREABLE(rb_obj_hide(map)); // allow mutation while compiling
+                            RB_OBJ_SET_SHAREABLE(map); // allow mutation while compiling
                             argv[j] = map;
                             RB_OBJ_WRITTEN(iseq, Qundef, map);
                         }
@@ -13004,6 +13000,20 @@ ibf_dump_code(struct ibf_dump *dump, const rb_iseq_t *iseq)
     return offset;
 }
 
+static int
+cdhash_copy_i(st_data_t key, st_data_t val, st_data_t arg)
+{
+    rb_hash_aset((VALUE)arg, (VALUE)key, (VALUE)val);
+    return ST_CONTINUE;
+}
+
+static VALUE
+cdhash_copy(VALUE dest, VALUE src)
+{
+    rb_hash_stlike_foreach(src, cdhash_copy_i, dest);
+    return dest;
+}
+
 static VALUE *
 ibf_load_code(const struct ibf_load *load, rb_iseq_t *iseq, ibf_offset_t bytecode_offset, ibf_offset_t bytecode_size, unsigned int iseq_size)
 {
@@ -13058,11 +13068,10 @@ ibf_load_code(const struct ibf_load *load, rb_iseq_t *iseq, ibf_offset_t bytecod
               case TS_CDHASH:
                 {
                     VALUE op = ibf_load_small_value(load, &reading_pos);
-                    VALUE v = ibf_load_object(load, op);
-                    v = rb_hash_dup(v); // hash dumped as frozen
-                    RHASH_TBL_RAW(v)->type = &cdhash_type;
-                    rb_hash_rehash(v); // hash function changed
-                    RB_OBJ_SET_SHAREABLE(freeze_hide_obj(v));
+                    VALUE src = ibf_load_object(load, op);
+                    VALUE v = rb_hash_new_with_size_and_type(0, RHASH_SIZE(src), &cdhash_type);
+                    rb_hash_rehash(cdhash_copy(v, src));
+                    RB_OBJ_SET_SHAREABLE(v);
 
                     // Overwrite the existing hash in the object list.  This
                     // is to keep the object alive during load time.
diff --git a/hash.c b/hash.c
index 8507c9c8f5..751d4564d5 100644
--- a/hash.c
+++ b/hash.c
@@ -1490,6 +1490,14 @@ rb_hash_new_with_size(st_index_t size)
     return ret;
 }
 
+VALUE
+rb_hash_new_with_size_and_type(VALUE klass, st_index_t size, const struct st_hash_type *type)
+{
+    VALUE ret = hash_alloc_flags(klass, 0, Qnil, true);
+    hash_st_table_init(ret, type, size);
+    return ret;
+}
+
 VALUE
 rb_hash_new_capa(long capa)
 {
diff --git a/internal/hash.h b/internal/hash.h
index 9688478d1e..3a5be14ad7 100644
--- a/internal/hash.h
+++ b/internal/hash.h
@@ -112,6 +112,7 @@ int rb_hash_stlike_foreach(VALUE hash, st_foreach_callback_func *func, st_data_t
 RUBY_SYMBOL_EXPORT_END
 
 VALUE rb_hash_new_with_size(st_index_t size);
+VALUE rb_hash_new_with_size_and_type(VALUE klass, st_index_t size, const struct st_hash_type *type);
 VALUE rb_hash_resurrect(VALUE hash);
 int rb_hash_stlike_lookup(VALUE hash, st_data_t key, st_data_t *pval);
 VALUE rb_hash_keys(VALUE hash);
-- 
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