CODEWITHSUNDEEP
webassembly
Himujjal
Himujjal

Reputation: 2127

How to represent operations in WebAssembly Text format?

Let's say I am writing the following C code in WebAssembly Text format:

if (a < 2) a = 5;
else a = 6;

WASM:

(if
  (i32.eq (get_local $x) (i32.const 10))
  (then (i32.local 5) (set_local $x))
  (else (i32.const 7) (local.set $a))
)

This also works:

(
    ;; ....
    get_local $a
    i32.const 2
    i32.lt_s ;; a < 2  
    (if
        (then
            i32.const 5
            local.set $a
        )
        (else 
            i32.const 7
            local.set $a
        )
    )
    ;; ...
)

Which one to follow? Why the difference over writing operations before and after the operands?

Upvotes: 0

Views: 306

Answers (2)

Anders Gustafsson
Anders Gustafsson

Reputation: 86

Webassembly text format is a (virtual) stack machine, meaning you can only add and remove things from the end of a stack (think list).

i32.const 10 ;;stack=[10]
i32.const 6 ;;stack=[10, 6]
i32.const 2 ;;stack=[10, 6, 2]
i31.add ;;consume two from stack and then put result on stack. stack=[10, 8]
i32.add ;;consume two from stack and then put result on stack. stack=[18]

This is how webassembly text format behaves, however you are allowed to use parenthesis (S-expressions) to choose the order they are put on the stack.

(i32.const 98 (i32.const 3)) ;;stack=[3, 98]

Allowing for much easier (for humans) to read syntax eg:

(i32.add (i32.const 10) (i32.add (i32.const 6) (i32.const 2)) ;;stack=[18]

So to answer your question: It doesnt matter which you use but if you are writing .wat by hand its clearly easier to use s-expressions.

Upvotes: 0

TonyK
TonyK

Reputation: 17114

It is purely a matter of personal preference -- the assembled code is the same. Which style do you prefer? (But I must say, your WASM code doesn't do what your C code does!)

Upvotes: 1

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