MDN / drop: Wasm data instruction
MDN / drop: Wasm data instruction
The data.drop data instruction discards the data contained by a passive data definition, freeing up its memory, after being used in a memory.init.
Active data segments are dropped automatically during module instantiation, and therefore are not available to drop via data.drop.
(module
(memory (export "memory") 1)
(data $greeting "Hello")
(func (export "init")
i32.const 0 ;; destination offset in memory
i32.const 0 ;; offset into the data segment
i32.const 5 ;; number of bytes to copy
memory.init $greeting
data.drop $greeting
)
)
WebAssembly.instantiateStreaming(fetch("{%wasm-url%}")).then((result) => {
result.instance.exports.init();
const memBuffer = result.instance.exports.memory.buffer;
const memArray = new Uint8Array(memBuffer, 0, 5);
console.log(new TextDecoder().decode(memArray));
});
In the above example, we define a memory and a data called $greeting that contains the string Hello. We then invoke memory.init to copy the string over to the memory. With this done, the data is no longer needed, so we call data.drop to free up the memory it was using.
Syntax
data.drop identifier
data.drop: The
data.dropinstruction type. Must always be included first.
identifier: The identifier for the
datayou want to drop. This can be one of the following:name: An identifying name set for the
datawhen it was first defined. This must begin with a$symbol, for example$my_data.
index: The
data’s index number, for example0for the firstdatain the wasm module,1for the second, etc.
Type
[] -> []
Binary encoding
InstructionBinary formatExample text => binarydata.drop0xfc 9:u32 x:dataidxdata.drop 0 => 0xfc 0x09 0x00