cmd/compile: unnecessary instructions generated

[mariecurried]

What version of Go are you using (go version)?

$ go version
go version go1.11.4 windows/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

I compiled the following code to assembly:

package test

import "encoding/binary"

func test(x uint64) [8]byte {
    var out [8]byte
    binary.LittleEndian.PutUint64(out[:], x)
    return out
}

Assembly code generated:

movq    $0, "".~r1+32(SP)
movq    $0, "".out(SP)
movq    "".x+24(SP), AX
movq    AX, "".out(SP)
movq    "".out(SP), AX
movq    AX, "".~r1+32(SP)
movq    8(SP), BP
addq    $16, SP
ret

What did you expect to see?

I was expecting the generated code to be much simpler

What did you see instead?

The assembly code shown above has many unnecessary instructions.
A solution I found was to use a named return. When I do that, the assembly code becomes:

movq    $0, "".out+16(SP)
movq    "".x+8(SP), AX
movq    AX, "".out+16(SP)
ret

In spite of being much better, it also has an extra instruction, the first one, where "out" is being zeroed.

[randall77]

The extra copy with unnamed return values is #14762.

The unneeded zeroing might be #25132 or #24926.

[FiloSottile]

@randall77 close as duplicate then?