Are these 2 string allocations equivalent?

Question

I saw this guy, and he wrote this code:

func foo1() *string {
    var pointa *string
    pointa = new(string)
    *pointa = "stuff"
    return pointa
}

Very nice, very nice, however, can't help but notice that one could also write:

func foo2() *string {
    var pointa *string
    var str = "stuff"
    pointa = &str
    return pointa
}

The first code says:

• dynamically allocate a string container with empty underlying storage
• override the underlying storage with contents of this literal
• I assume, because of COW, underlying storage just points to the literal's data in the data segment of ELF

The second code says:

• allocate locally an instance of a string
• but because it will later escape the scope via reference &, it will be allocated dynamically
• and still, because of COW, no copy will take place

Am I right about these assumptions? Are these 2 dynamic string allocations equivalent? Is new(string); *pointer = "payload" exactly what is happening in the second code, but under the hood, implicitly?

Are these pieces of code gonna produce the same code that allocated and initializes the string?

Answer 1

The Go specification makes no guarantees, and whatever answer you discover today is subject to change without warning, but you can see for yourself using the version of gc available a godbolt.org that all three versions of your function produce identical output, differing only in the names and values of the string (stuff1/stuff2/stuff3):

https://godbolt.org/z/evP1M3fs5

func foo1() *string {
    var pointa *string
    pointa = new(string)
    *pointa = "stuff1"
    return pointa
}

func foo2() *string {
    var pointa *string
    var str = "stuff2"
    pointa = &str
    return pointa
}

func foo3() *string {
    x := "stuff3"
    return &x;
}

Output:

main_foo1_pc0:
        TEXT    main.foo1(SB), ABIInternal, $24-0
        CMPQ    SP, 16(R14)
        PCDATA  $0, $-2
        JLS     main_foo1_pc50
        PCDATA  $0, $-1
        PUSHQ   BP
        MOVQ    SP, BP
        SUBQ    $16, SP
        FUNCDATA        $0, gclocals·g2BeySu+wFnoycgXfElmcg==(SB)
        FUNCDATA        $1, gclocals·g2BeySu+wFnoycgXfElmcg==(SB)
        LEAQ    type:string(SB), AX
        PCDATA  $1, $0
        CALL    runtime.newobject(SB)
        MOVQ    $6, 8(AX)
        LEAQ    go:string."stuff1"(SB), CX
        MOVQ    CX, (AX)
        ADDQ    $16, SP
        POPQ    BP
        RET
main_foo1_pc50:
        NOP
        PCDATA  $1, $-1
        PCDATA  $0, $-2
        CALL    runtime.morestack_noctxt(SB)
        PCDATA  $0, $-1
        JMP     main_foo1_pc0
main_foo2_pc0:
        TEXT    main.foo2(SB), ABIInternal, $24-0
        CMPQ    SP, 16(R14)
        PCDATA  $0, $-2
        JLS     main_foo2_pc50
        PCDATA  $0, $-1
        PUSHQ   BP
        MOVQ    SP, BP
        SUBQ    $16, SP
        FUNCDATA        $0, gclocals·g2BeySu+wFnoycgXfElmcg==(SB)
        FUNCDATA        $1, gclocals·g2BeySu+wFnoycgXfElmcg==(SB)
        LEAQ    type:string(SB), AX
        PCDATA  $1, $0
        CALL    runtime.newobject(SB)
        MOVQ    $6, 8(AX)
        LEAQ    go:string."stuff2"(SB), CX
        MOVQ    CX, (AX)
        ADDQ    $16, SP
        POPQ    BP
        RET
main_foo2_pc50:
        NOP
        PCDATA  $1, $-1
        PCDATA  $0, $-2
        CALL    runtime.morestack_noctxt(SB)
        PCDATA  $0, $-1
        JMP     main_foo2_pc0
main_foo3_pc0:
        TEXT    main.foo3(SB), ABIInternal, $24-0
        CMPQ    SP, 16(R14)
        PCDATA  $0, $-2
        JLS     main_foo3_pc50
        PCDATA  $0, $-1
        PUSHQ   BP
        MOVQ    SP, BP
        SUBQ    $16, SP
        FUNCDATA        $0, gclocals·g2BeySu+wFnoycgXfElmcg==(SB)
        FUNCDATA        $1, gclocals·g2BeySu+wFnoycgXfElmcg==(SB)
        LEAQ    type:string(SB), AX
        PCDATA  $1, $0
        CALL    runtime.newobject(SB)
        MOVQ    $6, 8(AX)
        LEAQ    go:string."stuff3"(SB), CX
        MOVQ    CX, (AX)
        ADDQ    $16, SP
        POPQ    BP
        RET
main_foo3_pc50:
        NOP
        PCDATA  $1, $-1
        PCDATA  $0, $-2
        CALL    runtime.morestack_noctxt(SB)
        PCDATA  $0, $-1
        JMP     main_foo3_pc0