block
The block statement creates a label that can later be branched out of with a br.
The loop statement is the opposite of the block statement, in the sense that while branching to a loop jumps to the beginning of the loop, branching to a block jumps to the end of the block; that is, out of the block.
Try it
(module
;; import the browser console object, you'll need to pass this in from JavaScript
(import "console" "log" (func $log (param i32)))
;; create a function that takes in a number as a param,
;; and logs that number if it's not equal to 100.
(func (export "log_if_not_100") (param $num i32)
(block $my_block
;; $num is equal to 100
local.get $num
i32.const 100
i32.eq
(if
(then
;; branch to the end of the block
br $my_block
)
)
;; not reachable when $num is 100
local.get $num
call $log
)
)
)
const url = "{%wasm-url%}";
await WebAssembly.instantiateStreaming(fetch(url), { console }).then(
(result) => {
const log_if_not_100 = result.instance.exports.log_if_not_100;
log_if_not_100(99);
// Expected output: 99
log_if_not_100(100);
// Should not log anything
log_if_not_100(101);
// Expected output: 101
},
);
Syntax
wat
;; label the block so that it can be branched to.
(block $my_block
;; branch to the block.
;; most of the time you'll want to put this in an if statement and only branch on condition,
;; otherwise the following control flow are unreachable.
br $my_block
;; this will never be reached, since the br jumped out of the block already.
unreachable
)
| Instruction | Binary opcode |
|---|---|
block |
0x02 |