br_if question: order of evaluation #200
Description
As always, it seems I come back to ensure that I'm reading things right. With the new testsuite, I have come up with an issue with the labels.wast test.
Before I go into the problem I am facing, let me first mention that it seems that the ml-proto is supporting:
(br_if (i32.const 1) $outer (set_local $i (i32.or (get_local $i) (i32.const 0x10))))
But the https://github.com/WebAssembly/spec/tree/master/ml-proto file says only:
( br_if <expr> <var> ) ;; = (if_else <expr> (br <var>) (nop))
When it should really be:
( br_if <expr> <var> <expr>?) ;; = (if_else <expr> (br <var> <expr>?) (nop))
no?
Second, this last part seems not to be entirely true because the order of evaluation seems then off. When I consider the test below:
(func $br_if (result i32)
(local $i i32)
(set_local $i (i32.const 0))
(block $outer
(block $inner
(br_if (i32.const 0) $inner)
(set_local $i (i32.or (get_local $i) (i32.const 0x1)))
(br_if (i32.const 1) $inner)
(set_local $i (i32.or (get_local $i) (i32.const 0x2)))
)
(br_if (i32.const 0) $outer (set_local $i (i32.or (get_local $i) (i32.const 0x4))))
(set_local $i (i32.or (get_local $i) (i32.const 0x8)))
(br_if (i32.const 1) $outer (set_local $i (i32.or (get_local $i) (i32.const 0x10))))
(set_local $i (i32.or (get_local $i) (i32.const 0x20)))
)
)
The assertion requires this method to return 0x1d. This means, we evaluate the sets with the 0x1, 0x4, 0x8, and 0x10.
It is the 0x4 that I have problems with:
(br_if (i32.const 0) $outer (set_local $i (i32.or (get_local $i) (i32.const 0x4))))
In this case, we thus evaluate first the child before branching. Therefore, this is not equivalent to putting a if around the (i32.const 0) and doing a br with the label and the set_local expression.
So, which is it?
- Is br_if a short-hand for an if with a branch, ie:
( br_if <expr> <var> <expr>?) ;; = (if_else <expr> (br <var> <expr>?) (nop)) - Is br_if a short-hand to:
( br_if <cond_expr> <var> <expr>?) ;; = do the expr if there and remember the result, do the if <cond_expr> and, if it is true, jump to <var> while return the result
Not sure I was entirely clear here but bringing up little issues I see.
Note about what the design repo says:
- "All nodes other than control flow constructs" , so this does not work here, right?
- The Branches and nesting section does not express when the br_if should evaluate the expression if it is there
Thanks for your input and I hope you all don' t mind me spamming about these questions :)