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YJIT: call alloc func directly in new #13219

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YJIT: call alloc func directly in new #13219

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YJIT: call alloc func directly in new
jhawthorn Apr 30, 2025
7548512
Don't allow allocating singleton class
jhawthorn Apr 30, 2025
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11 changes: 10 additions & 1 deletion yjit/src/codegen.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4912,7 +4912,16 @@ fn gen_opt_new(
// Fast path
// call rb_class_alloc to actually allocate
jit_prepare_non_leaf_call(jit, asm);
let obj = asm.ccall(rb_obj_alloc as _, vec![comptime_recv.into()]);

let allocator =
if unsafe { FL_TEST(comptime_recv, VALUE(RUBY_FL_SINGLETON as usize)) } != VALUE(0) {
None
} else {
unsafe { rb_get_alloc_func(comptime_recv) }
};

let allocator = allocator.unwrap_or(rb_obj_alloc);
let obj = asm.ccall(allocator as _, vec![comptime_recv.into()]);
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@k0kubun k0kubun Apr 30, 2025 •
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When an allocator function is found, it seems to skip the if (rb_obj_class(obj) != rb_class_real(klass)) { rb_raise(rb_eTypeError, "wrong instance allocation"); } check in class_call_alloc_func. Is the error unreachable?

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@tenderlove tenderlove Apr 30, 2025 •
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Wouldn't this only be a problem for T_DATA objects? I don't think they use rb_class_allocate_instance as their allocator.

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@jhawthorn jhawthorn Apr 30, 2025 •
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I think it's only reachable from a custom allocator which is implemented incorrectly (there's no reason to ever return an object with the wrong class), so I'm not sure if it's necessary to keep.

If it is I suppose we could allowlist just the most common builtin allocators (or even just the T_OBJECT allocator).

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I honestly didn't know when it happens (it'd be nice to have a comment there) and thus asked a question. If the check is useless (on opt_new's rb_obj_alloc call) on the interpreter too, it's fine.


// Get a reference to the stack location where we need to save the
// return instance.
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