test/wasm2js/sign_ext.wast

(module
  (func "test8" (param $x i32) (result i32)
    (i32.extend8_s (local.get $x))
  )
  (func "test16" (param $x i32) (result i32)
    (i32.extend16_s (local.get $x))
  )
  (func "test8_i64" (param $x i64) (result i64)
    (i64.extend8_s (local.get $x))
  )
  (func "test16_i64" (param $x i64) (result i64)
    (i64.extend16_s (local.get $x))
  )
  (func "test32_i64" (param $x i64) (result i64)
    (i64.extend32_s (local.get $x))
  )
)