br_table
The br_table
statement branches to different loop
, block
, or if
statements, based on an argument.
In some ways, br_table
is similar to the switch
statement, branching to different code blocks depending on the argument.
Try it
(module
;; Import the browser console object, which you'll need to pass in from JavaScript
(import "console" "log" (func $log (param i32)))
(func
;; Label each block for easy reference
;; (they can also be referenced by their index)
(block $outer_block
(block $middle_block
(block $inner_block
;; Choose which block to break out of based on their order in the br_table
;; 0 is `$inner_block`, 1 is `$outer_block`, 2 is `$middle_block`
i32.const 0
;; Create a br_table with three targets
(br_table $inner_block $outer_block $middle_block)
;; The code will never reach this point since we broke out of the block
unreachable
)
;; If you jump out of `$inner_block` but stay in `$middle_block`,
;; 42 will be logged
;; If you jump out of `$middle_block` also,
;; by jumping out of either `$middle_block` or `$outer_block`,
;; this will be skipped
i32.const 42
call $log
)
)
)
(start 1) ;; Run the first function automatically
)
const url = "{%wasm-url%}";
await WebAssembly.instantiateStreaming(fetch(url), { console });
Syntax
wat
;; Add true to the top of the stack so that the `if` statement is executed.
i32.const 1
(if ;; 2
(then
(block ;; 1
(loop ;; 0
;; Add a variable to the top of the stack
i32.const 2
;; 0 = jump to `block`; since item 0 in br_table is 1, it jumps up one level
;; 1 = jump to `if`; since item 1 in br_table is 2, it jumps up two levels
;; 2 = jump to `loop`; since item 2 in br_table is 0, it doesn't jump any levels (causing an infinite loop)
;; Create a br_table with the targets
(br_table 1 2 0)
)
)
)
)
Instruction | Binary opcode |
---|---|
br_table |
0x0e |