Stream: git-wasmtime

Topic: wasmtime / issue #10171 Understanding `ref.test` failing

Wasmtime GitHub notifications bot (Feb 03 2025 at 11:46):

apoorvaanand1998 opened issue #10171:

(module
    (type $appNode
        (struct (field $left anyref)
                (field $right anyref)))

    (type $comb (struct (field $asciiTag i32)))

    (func $prob1 (export "prob1") (param $p (ref null $appNode)) (result i32)
        (local.get $p)
        (struct.get $appNode $left)
        (ref.cast (ref null $appNode))
        (struct.get $appNode $right)
        (ref.test (ref null $comb))
    )

    (func $prob2 (export "prob2") (param $p (ref null $appNode)) (result i32)
        (local.get $p)
        (struct.get $appNode $left)
        (ref.cast (ref null $appNode))
        (struct.get $appNode $left)
        (ref.test (ref null $comb))
    )

    (func $main (export "main") (result i32)
        (struct.new $appNode (struct.new $appNode (struct.new $comb (i32.const 67))
                                                  (struct.new $comb (i32.const 73)))
                             (ref.i31 (i32.const 2)))
        (call $prob2)
    )
)

Hey Wasmtime team! calling $prob1 returns 1 but $prob2 returns 2. Why is that? Shouldn't it be reading the struct.new $comb (i32.const 67) which indeed is a comb? I've tried reading the GC spec but haven't been able to figure out why the runtime behaves this way.

Wasmtime GitHub notifications bot (Feb 03 2025 at 11:47):

apoorvaanand1998 edited issue #10171:

(module
    (type $appNode
        (struct (field $left anyref)
                (field $right anyref)))

    (type $comb (struct (field $asciiTag i32)))

    (func $prob1 (export "prob1") (param $p (ref null $appNode)) (result i32)
        (local.get $p)
        (struct.get $appNode $left)
        (ref.cast (ref null $appNode))
        (struct.get $appNode $right)
        (ref.test (ref null $comb))
    )

    (func $prob2 (export "prob2") (param $p (ref null $appNode)) (result i32)
        (local.get $p)
        (struct.get $appNode $left)
        (ref.cast (ref null $appNode))
        (struct.get $appNode $left)
        (ref.test (ref null $comb))
    )

    (func $main (export "main") (result i32)
        (struct.new $appNode (struct.new $appNode (struct.new $comb (i32.const 67))
                                                  (struct.new $comb (i32.const 73)))
                             (ref.i31 (i32.const 2)))
        (call $prob2)
    )
)

Hey Wasmtime team! calling $prob1 returns 1 but $prob2 returns 0. Why is that? Shouldn't it be reading the (struct.new $comb (i32.const 67)) which indeed is a comb? I've tried reading the GC spec but haven't been able to figure out why the runtime behaves this way.

Wasmtime GitHub notifications bot (Feb 03 2025 at 15:42):

alexcrichton assigned fitzgen to issue #10171.

Wasmtime GitHub notifications bot (Feb 03 2025 at 18:22):

fitzgen commented on issue #10171:

This looks like a bug in Wasmtime, I think prob2 should indeed return 1.

FWIW, it also returns 1 in Firefox.

Wasmtime GitHub notifications bot (Feb 03 2025 at 18:27):

fitzgen commented on issue #10171:

Interestingly enough, removing $right makes the test pass.

Wasmtime GitHub notifications bot (Feb 03 2025 at 18:27):

fitzgen added the bug label to Issue #10171.

Wasmtime GitHub notifications bot (Feb 03 2025 at 18:27):

fitzgen added the wasm-proposal:gc label to Issue #10171.

Last updated: Dec 13 2025 at 19:03 UTC