wasmtime / issue #14452 GC: Copying and other disagree · git-wasmtime

Stream: git-wasmtime

Topic: wasmtime / issue #14452 GC: Copying and other disagree

Wasmtime GitHub notifications bot (Sep 30 2026 at 07:00):

Test Case

repro.wat

(module
  (type $obj (struct))
  (type $garbage (array (mut i32)))
  (tag $exn)
  (global $keep_garbage (mut (ref null $garbage)) (ref.null $garbage))
  (global $root (mut (ref null $obj)) (ref.null $obj))

  (func $collect_then_throw
    i32.const 100000
    array.new_default $garbage
    global.set $keep_garbage
    i32.const 100000
    array.new_default $garbage
    global.set $keep_garbage
    i32.const 100000
    array.new_default $garbage
    global.set $keep_garbage
    throw $exn)

  (func (export "_start") (local $local (ref null $obj))
    struct.new $obj
    local.set $local
    local.get $local
    global.set $root
    block $outer_handler
      try_table (catch $exn $outer_handler)
        block $skip_throw
          block $inner_handler
            try_table (catch $exn $inner_handler)
              br $skip_throw
            end
          end
          throw $exn
        end
        call $collect_then_throw
      end
      return
    end
    local.get $local
    global.get $root
    ref.eq
    i32.eqz
    if
      unreachable
    end)
)

Steps to Reproduce

$ wasmtime repro.wat

Expected Results

No output.
Exits with 0.

Actual Results

Error: failed to run main module `repro.wat`

Caused by:
    0: failed to invoke command default
    1: error while executing at wasm backtrace:
    0:     0x8d - <unknown>!<wasm function 1>
    2: wasm trap: wasm `unreachable` instruction executed

Versions and Environment

wasmtime 49.0.1 (46c23a87d 2026-09-24)

Operating system: macOS

Architecture: Arm64

Extra Info

Other collectors work fine:

$ wasmtime -C collector=drc repro.wat
$ wasmtime -C collector=null repro.wat

Node with V8 works fine:

$ wasm-tools --version
wasm-tools 1.212.0 (1cf71f9a9 2024-06-27)
$ node --version
v26.10.0
$ wasm-tools parse repro.wat | node --experimental-wasm-exnref -e 'WebAssembly.instantiate(require("fs").readFileSync(0)).then(r=>{r.instance.exports._start();console.log("ok")})'
ok

Wasmtime GitHub notifications bot (Sep 30 2026 at 07:00):

skuzmich added the bug label to Issue #14452.

Wasmtime GitHub notifications bot (Sep 30 2026 at 07:16):

skuzmich commented on issue #14452:

Also it is unclear from docs which collector should be ok to use:

https://docs.wasmtime.dev/api/wasmtime/enum.Collector.html#variant.Auto

Currently this always defaults to the copying collector, but the default value may change over time.

https://docs.wasmtime.dev/api/wasmtime/enum.Collector.html#variant.Copying

Note that this collector is still under construction and is not yet functional.

Wasmtime GitHub notifications bot (Oct 02 2026 at 19:50):

alexcrichton closed issue #14452: