local.tee in WebAssembly

Store a value and keep using it, in one instruction.

(module
  ;; local.tee stores like local.set but leaves the value on the
  ;; stack, so one computation can feed a local and the next step.
  (func (export "square_plus_orig") (param $n i32) (result i32)
    (local $doubled i32)
    (i32.add
      (local.tee $doubled (i32.mul (local.get $n) (i32.const 2)))
      (i32.mul (local.get $doubled) (local.get $doubled)))))

How it works

local.tee $doubled (...) writes the local and leaves the value.
The kept copy feeds i32.add while the local feeds the square.
Without tee this takes a set and an extra get.

Keywords and builtins used here

export
func
i32
local
module
param
result

The run, in numbers

Lines: 8
Characters to type: 342
Tokens: 57
Three-star pace: 65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 53 seconds.

Type this snippet

Step 2 of 3 in Locals & the stack, step 8 of 28 in Language basics.

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