Wasm: try..finally with non-nullable reference type produces invalid code #5165

sjrd · 2025-05-10T09:10:52Z

Reproduction:

  def test(): Unit = {
    var a = 1
    val some = try {
      Some(a)
    } finally {
      a = 2
    }
    assert(some.value == 1)
  }

produces invalid Wasm code. Trying to run the program yields

[CompileError: WebAssembly.instantiate():
Compiling function #165:"f.helloworld.Main$.test;V" failed:
uninitialized non-defaultable local: 2 @+28176]

The pretty-printed code for the function is

  (func $f.helloworld.Main$.test_V (type $128)
     (param $this (ref $c.helloworld.Main$))
     (local $a i32) (local $1 (ref $c.scala.Some)) (local $2 (ref $c.scala.Some)) (local $some (ref $c.scala.Some))
     i32.const 1
     local.set $a
     block $1
       block $2 (result exnref)
         try_table (catch_all_ref $2)
           call $new.scala.Some
           local.tee $2
           local.get $a
           call $bI
           call $ct.scala.Some.<init>_Ljava.lang.Object_V
           local.get $2
           local.set $1
         end
         ref.null exn
       end
       i32.const 2
       local.set $a
       br_on_null $1
       throw_ref
     end
     local.get $1
     local.set $some
     call $m.scala.Predef$
     ref.as_non_null
     local.get $some
     struct.get $c.scala.Some $f.scala.Some.value
     call $as.int
     i32.const 1
     i32.eq
     call $f.scala.Predef$.assert_Z_V)

Wasm cannot prove that, in all the code paths that lead to reading local.get $1 after the big block $1, the variable was initialized. That's fair, since it wouldn't be initialized if we caught an exception; we know that we cannot reach there in that case (because of the re-throw) but Wasm validation cannot prove it. Since $1 has a non-nullable reference type (ref $c.Some), that is not valid code.

We have to always use a defaultable type for the partial results involved in try..finallys (which means we have to cast away nullability when we read them).

The text was updated successfully, but these errors were encountered:

…y`s. As well as for the locals of `Labeled` blocks whose `Return`s cross a `try..finally` boundary. If their original type was not defaultable, we cast away nullability when reading them back.

sjrd self-assigned this May 10, 2025

sjrd added bug Confirmed bug. Needs to be fixed. wasm Applies to the WebAssembly backend only labels May 10, 2025

sjrd added this to the v1.19.1 milestone May 10, 2025

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Wasm: try..finally with non-nullable reference type produces invalid code #5165

Wasm: try..finally with non-nullable reference type produces invalid code #5165

sjrd commented May 10, 2025

Wasm: try..finally with non-nullable reference type produces invalid code #5165

Wasm: try..finally with non-nullable reference type produces invalid code #5165

Comments

sjrd commented May 10, 2025