ZJIT: Specialize Struct class .new (ruby#18835)

We can use `ObjectAllocClass` for Structs because they are basically
a `__members__` ivar lookup for sizing plus the default allocator.

Requires ruby#18888 to make sure the constructor does not raise.

Author: tekknolagi

struct.c

@@ -859,6 +859,13 @@ struct_alloc(VALUE klass)
     }
 }
 
+// Whether `klass` allocates its instances with struct_alloc.
+bool
+rb_zjit_class_has_struct_allocator(VALUE klass)
+{
+    return rb_get_alloc_func(klass) == struct_alloc;
+}
+
 VALUE
 rb_struct_alloc(VALUE klass, VALUE values)
 {

zjit.c

@@ -356,6 +356,9 @@ rb_zjit_can_load_superclass_p(VALUE klass)
 
 rb_alloc_func_t rb_zjit_class_get_alloc_func(VALUE klass);
 
+// Defined in struct.c, where struct_alloc is visible.
+bool rb_zjit_class_has_struct_allocator(VALUE klass);
+
 VALUE rb_class_allocate_instance(VALUE klass);
 
 bool

zjit/bindgen/src/main.rs

@@ -407,6 +407,7 @@ fn main() {
         .allowlist_function("rb_zjit_class_initialized_p")
         .allowlist_function("rb_zjit_can_load_superclass_p")
         .allowlist_function("rb_zjit_class_has_default_allocator")
+        .allowlist_function("rb_zjit_class_has_struct_allocator")
         .allowlist_function("rb_zjit_class_get_alloc_func")
         .allowlist_function("rb_get_ec_cfp")
         .allowlist_function("rb_get_cfp_iseq")

zjit/src/cruby.rs

@@ -1648,6 +1648,10 @@ pub fn class_has_leaf_allocator(class: VALUE) -> bool {
     if class == unsafe { rb_cString } { return true; }
     // rb_reg_s_alloc
     if class == unsafe { rb_cRegexp } { return true; }
+    // struct_alloc, used by every Struct subclass, is leaf: it reads the hidden __members__ ivar
+    // and allocates, without calling into Ruby. It does modify the class's __members__ ivar once
+    // to cache the members, but without a Ractor check.
+    if unsafe { rb_zjit_class_has_struct_allocator(class) } { return true; }
     // rb_class_allocate_instance
     unsafe { rb_zjit_class_has_default_allocator(class) }
 }

zjit/src/cruby_bindings.inc.rs

@@ -12365,6 +12365,37 @@ mod hir_opt_tests {
         ");
     }
 
+    #[test]
+    fn test_specialize_struct_new_generates_object_alloc_class() {
+        eval(r#"
+            C = Struct.new
+            def test = C.new
+            test
+        "#);
+        assert_snapshot!(hir_string("test"), @"
+        fn test@<compiled>:3:
+        bb1():
+          EntryPoint interpreter
+          v1:BasicObject = LoadSelf
+          Jump bb3(v1)
+        bb2():
+          EntryPoint JIT(0)
+          v4:BasicObject = LoadArg :self@0
+          Jump bb3(v4)
+        bb3(v6:BasicObject):
+          v10:NilClass = Const Value(nil)
+          PatchPoint StableConstantNames(0x1000, C)
+          v13:ClassSubclass[C@0x1008] = Const Value(VALUE(0x1008))
+          PatchPoint MethodRedefined(C@0x1008, new@0x1009, cme:0x1010)
+          v42:ObjectSubclass[class_exact:C] = ObjectAllocClass C:VALUE(0x1008)
+          PatchPoint NoSingletonClass(C@0x1008)
+          PatchPoint MethodRedefined(C@0x1008, initialize@0x1038, cme:0x1040)
+          v47:BasicObject = CCallVariadic v42, :Struct#initialize@0x1068
+          CheckInterrupts
+          Return v42
+        ");
+    }
+
     #[test]
     fn test_inline_struct_aref_embedded() {
         eval(r#"