Select
select 指令根據其第三個運算元是否為零來選擇其前兩個運算元中的一個。它與許多其他語言中的三元運算子（例如 false ? 10 : 20）有一些相似之處，但它不會 短路。該指令後面可以跟一個即時值型別：select (result T)。select (result T) 使用不同的二進位制操作碼，並允許除了 WebAssembly MVP 引入的型別（i32、i64、f32、f64）之外的型別，例如，它允許在兩個 externref 值之間進行選擇。
試一試
(module
(func (export "select_simple") (result i32)
;; load two values onto the stack
i32.const 10
i32.const 20
;; change to `1` (true) to get the first value (`10`)
i32.const 0
select
)
(func (export "select_externref") (param $value externref) (param $condition i32) (result externref)
;; this is "select t", the explicitly typed variant
ref.null extern
local.get $value
local.get $condition
select (result externref)
)
)
const url = "{%wasm-url%}";
await WebAssembly.instantiateStreaming(fetch(url)).then((result) => {
const { select_simple, select_externref } = result.instance.exports;
console.log(select_simple());
// Expected output: 20
// If the second parameter is zero, returns the first parameter (which may be an arbitrary JS value)
const map = new Map();
console.log(select_externref(map, 0));
// Expected output: [object Map]
console.log(select_externref(map, -1));
// Expected output: null
});
語法
wat
;; push two values onto the stack
i32.const 10
i32.const 20
;; change to `1` (true) to get the first value (`10`)
i32.const 0
select
f32.const nan f32.const -54.1 ;; change to `1` (true) to get the first value (`nan`) i32.const 0 select (result f32)
| 指令 | 二進位制操作碼 |
|---|---|
select |
0x1b |
select t |
0x1c |