branching to invalid label?

[wanderer]

What is (br 0) branching too?

(module
  (func $if (result i32)
    (local $i i32)
    (if
      (i32.const 1)
      (br 0)
     )
     (get_local $i)
  )
  (export "if" $if)
)

There are no blocks or loops so there are no labels.

[ghost]

@wanderer To the end of the implicit if-then block, so to the end of the if operation.

[wanderer]

thanks @JSStats. There is no special if-then block in the binary spec, so it must be getting converted to this

(module
  (func $if (result i32)
    (local $i i32)
    (if
      (i32.const 1)
      (block (br 0) ) 
     )
     (get_local $i)
  )
  (export "if" $if)
)

But why is it inserting a block? Shouldn't the s-expressions be 1-1 with the binary?

[ghost]

@wanderer (if (cond) (exp1) (exp2)) is just an assembler short-hand for (if (cond) (then (exp1)) (else (exp2))), where the then and else are blocks.

[wanderer]

@JSStats I think i see that. I'm picking it apart with the sexpr-wasm-prototype. So something like

    (if
      (i32.const 1)
      (i32.const 1)
     )

Gets no block inserted. But

    (if
      (i32.const 1)
      (br 0)
     )

gets turned into

    (if
      (i32.const 1)
      (block (br 0))
     )

Which is annoying because there must be some rules to decide whether or not to insert a block. Also it means s-expressions are not 1-1 with the binary

[ghost]

@wanderer Oh, the sexpr-wasm-prototype de-facto standard is out of sync with the spec or design, and still uses a legacy if so translates. sexpr-wasm-prototype looks at block uses and does some simplifications.

[wanderer]

thanks @JSStats for taking time to explain

[binji]

@JSStats No, that's wrong. AIUI, the spec was updated before the demo to add blocks to the then and else branches, in preparation for the transition to the postorder format. But the binary format was not similarly updated. I modified sexpr-wasm-prototype so it could take this discrepancy into account (AFAIK the other binary translators don't bother).