Call Convention

Understanding WebAssembly's call convention is essential for interop between WASM and JavaScript (or other languages).

Stack-based Call Convention

WebAssembly uses a stack-based call convention:

flowchart LR
    subgraph Caller["Caller"]
        P1[Push arg 1] --> P2[Push arg 2]
        P2 --> CALL[Call function]
    end
    CALL --> CALLEE[Callee pops args]
    CALLEE --> COMPUTE[Compute result]
    COMPUTE --> PUSH[Push result]
    PUSH --> RET[Return]
    RET --> POP[Caller pops result]

Local Variables

(func $factorial (param $n i32) (result i32)
  (local $result i32)
  (local $temp i32)

  (local.set $result (i32.const 1))

  (block $done (result i32)
    (loop $loop
      (br_if $done (i32.le_s (local.get $n) (i32.const 1)))
      (local.set $temp (local.get $result))
      (local.set $result (i32.mul (local.get $result) (local.get $n)))
      (local.set $n (i32.sub (local.get $n) (i32.const 1)))
      (br $loop))
    (local.get $result)))

Branching and Control Flow

If-Else

(func $max (param $a i32) (param $b i32) (result i32)
  (if (i32.gt_s (local.get $a) (local.get $b))
    (then (local.get $a))
    (else (local.get $b))))

Loop

(func $sum_to (param $n i32) (result i32)
  (local $sum i32)
  (local.set $sum (i32.const 0))
  (block $done
    (loop $start
      (br_if $done (i32.eqz (local.get $n)))
      (local.set $sum (i32.add (local.get $sum) (local.get $n)))
      (local.set $n (i32.sub (local.get $n) (i32.const 1)))
      (br $start)))
  (local.get $sum))

Performance Tips

Technique	Note
Use smaller types	i32 faster than i64 on 32-bit architectures
Minimize parameters	Pass structs via pointer, not by value
Inline small functions	Reduce call overhead
Avoid excessive branching	Deep branching affects performance

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Continue learning Import and Export to understand module communication.