golang: how to simulate union type efficiently

You can use arrays to represent a single int32 as two int16s and then assemble them with shifts as Rob Pike recommends:

func test3() (total int64) {
    type A struct {
        t int32
        u [2]int16
    }
    a := [...]A{
        {1, [2]int16{100, 0}},
        {2, [2]int16{3, 0}},
    }

    for i := 0; i < N; i++ {
        p := &a[i%2]
        switch p.t {
        case 1:
            total += int64(p.u[0]<<0 | p.u[1]<<8)
        case 2:
            total += int64(p.u[0])
        }
    }
    return
}

With the original Go compiler it runs about 2 times slower than the C version, and with gccgo (-O3) it runs about as fast as C.

Be warned though that this approach assumes little-endian ints. You’ll need to switch the order of shifts for big-endian architecture.

Also, if you need to decode structures from a byte slice, you should really be using encoding/binary. This library is created to translate between byte sequences and other types.