Should there be a constant i31ref initializer expression?

[dcodeIO]

Following up on observations in #124 (comment), I am wondering whether it'd make sense to introduce a constant initializer expression for i31ref globals, because currently there is no trivial way to declare an i31ref typed global, requiring workarounds like

(module
 (import "env" "trivial_i31ref" (global $trivial_i31ref i31ref))
 (global $global_i31ref (mut i31ref) (global.get $trivial_i31ref))
)

Suitable solutions from simplest to most general seem to be (1) the introduction of i31.zero

(module
 (global $global_i31ref (mut i31ref) (i31.zero))
)

or (2) a variant of i31.new taking an immediate instead of an expression

(module
 (global $global_i31ref (mut i31ref) (i31.new_with_value value=0))
)

or, radically, (3) allowing global initializers to be defaultable and making i31ref default to contain value 0

(module
 (global $global_i31ref (mut i31ref) (default))
)

The latter is more complicated but leads to potential code size savings, e.g. in the best case

(module
 (global $global_f64 (mut f64) (f64.const 0) ;; 9 bytes initializer
 (global $global_f64 (mut f64) (default) ;; 1 byte initializer
)

further leading to (4) allowing to omit global initializers entirely, like on locals

 (global $global_f64 (mut f64)) ;; 0 byte initializer

Given how fundamental the problem seems, I might as well be missing something?

[jakobkummerow]

As an alternative to (2): I think we could just mark i31.new as a "constant instruction", and then you could do:

(global $global_i31ref (mut? i31ref) (i31.new (i32.const 42)))

[dcodeIO]

Yeah, that would work as well, then just needs to evaluate that the argument to i31.new is constant.