drop: Wasm elem instruction

The elem.drop elem instruction discards the data contained by a passive elem segment, freeing up its memory, after being used in a table.init.

Note: Active elem segments are dropped automatically during module instantiation, and therefore are not available to drop via elem.drop.

Try it

(module
  (table $return_values 2 funcref)

  (func $f1 (result i32)
    i32.const 42
  )
  (func $f2 (result i32)
    i32.const 100
  )

  (elem $funcs funcref (ref.func $f1) (ref.func $f2))

  (func (export "init")
    i32.const 0    ;; destination table index
    i32.const 0    ;; offset into elem segment
    i32.const 2    ;; number of elements to copy
    table.init $funcs
    ;; segment data no longer needed
    elem.drop $funcs
  )

  (func (export "accessTable") (param $index i32) (result i32)
    local.get $index
    call_indirect (result i32)
  )
)

WebAssembly.instantiateStreaming(fetch("{%wasm-url%}")).then((result) => {
  result.instance.exports.init();
  const value = result.instance.exports.accessTable(1);
  console.log(value);
});

In the above example, we define a table, two functions, and an elem called $funcs that references the two functions. We then invoke table.init to copy the references from the $funcs elem over to the table. With this done, the elem is no longer needed, so we call elem.drop to free up the memory it was using.

Syntax

elem.drop identifier

elem.drop

The elem.drop instruction type. Must always be included first.

identifier

The identifier for the elem you want to drop. This can be one of the following:

name: An identifying name set for the elem when it was first defined. This must begin with a $ symbol, for example $my_elem.
index: The elem's index number, for example 0 for the first elem in the wasm module, 1 for the second, etc.

Type

[] -> []

Binary encoding

Instruction	Binary format	Example text => binary
`elem.drop`	`0xfc 13:u32 x:elemidx`	`elem.drop 0` => `0xfc 0x0d 0x00`

Specifications

WebAssembly Core Specification