YJIT: call alloc func directly in new #13219

jhawthorn · 2025-04-30T23:21:03Z

This removes an indirect branch by calling the allocator function from the class directly.

Benchmark 1: ./miniruby_master --yjit -e '20_000_000.times { Object.new }'
  Time (mean ± σ):      1.203 s ±  0.020 s    [User: 1.187 s, System: 0.011 s]
  Range (min … max):    1.168 s …  1.230 s    10 runs

Benchmark 2: ./miniruby_allocator --yjit -e '20_000_000.times { Object.new }'
  Time (mean ± σ):     967.5 ms ±  22.4 ms    [User: 951.6 ms, System: 11.5 ms]
  Range (min … max):   949.9 ms … 1026.0 ms    10 runs

Summary
  ./miniruby_allocator --yjit -e '20_000_000.times { Object.new }' ran
    1.24 ± 0.04 times faster than ./miniruby_master --yjit -e '20_000_000.times { Object.new }'

cc @tenderlove

This removes an indirect branch by calling the allocator function from the class directly.

k0kubun · 2025-04-30T23:26:52Z

yjit/src/codegen.rs

+
+        let allocator = 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()]);


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?

Wouldn't this only be a problem for T_DATA objects? I don't think they use rb_class_allocate_instance as their allocator.

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).

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.

YJIT: call alloc func directly in new

3d7525a

This removes an indirect branch by calling the allocator function from the class directly.

matzbot requested a review from a team April 30, 2025 23:21

k0kubun reviewed Apr 30, 2025

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Don't allow allocating singleton class

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YJIT: call alloc func directly in new #13219

YJIT: call alloc func directly in new #13219

jhawthorn commented Apr 30, 2025 •

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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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jhawthorn commented Apr 30, 2025 •

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