loop and br in WebAssembly

On a loop, br jumps to the top: that is the whole while story.

(module
  ;; A loop label points at the top, so br means "go around again".
  ;; The surrounding block gives br_if somewhere to exit to.
  (func (export "sum_to") (param $n i32) (result i32)
    (local $i i32)
    (local $acc i32)
    (block $done
      (loop $top
        (br_if $done (i32.gt_s (local.get $i) (local.get $n)))
        (local.set $acc (i32.add (local.get $acc) (local.get $i)))
        (local.set $i (i32.add (local.get $i) (i32.const 1)))
        (br $top)))
    (local.get $acc)))

How it works

1. (loop $top ...) labels its own start; (br $top) goes around.
2. The enclosing (block $done ...) gives br_if an exit.
3. The accumulator and counter are plain locals moved by set and add.

Keywords and builtins used here

• block
• br
• br_if
• export
• func
• i32
• local
• loop
• module
• param
• result

The run, in numbers

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13

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439

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Three-star pace

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Type this snippet

Step 3 of 3 in Control flow, step 12 of 28 in Language basics.

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