Unnecessary memcpy during struct initialization

[jrmuizel]

pub struct Bar {
    l: u8,
    f: [u8; 200],
}

pub fn roo(f: &mut Bar) {
    f.f = noo();
}

#[inline(never)]
pub fn noo() -> [u8; 200] {
    [0; 200]
}

compiles to

example::roo:
        push    rbp
        mov     rbp, rsp
        push    r14
        push    rbx
        sub     rsp, 208
        mov     rbx, rdi
        lea     r14, [rbp - 216]
        mov     rdi, r14
        call    example::noo@PLT
        inc     rbx
        mov     edx, 200
        mov     rdi, rbx
        mov     rsi, r14
        call    memcpy@PLT
        add     rsp, 208
        pop     rbx
        pop     r14
        pop     rbp
        ret

example::noo:
        push    rbp
        mov     rbp, rsp
        xorps   xmm0, xmm0
        movups  xmmword ptr [rdi + 176], xmm0
        movups  xmmword ptr [rdi + 160], xmm0
        movups  xmmword ptr [rdi + 144], xmm0
        movups  xmmword ptr [rdi + 128], xmm0
        movups  xmmword ptr [rdi + 112], xmm0
        movups  xmmword ptr [rdi + 96], xmm0
        movups  xmmword ptr [rdi + 80], xmm0
        movups  xmmword ptr [rdi + 64], xmm0
        movups  xmmword ptr [rdi + 48], xmm0
        movups  xmmword ptr [rdi + 32], xmm0
        movups  xmmword ptr [rdi + 16], xmm0
        movups  xmmword ptr [rdi], xmm0
        mov     qword ptr [rdi + 192], 0
        mov     rax, rdi
        pop     rbp
        ret

If I drop 'l' field the copy goes away and I get:

example::roo:
        push    rbp
        mov     rbp, rsp
        call    example::noo@PLT
        pop     rbp
        ret

example::noo:
        push    rbp
        mov     rbp, rsp
        xorps   xmm0, xmm0
        movups  xmmword ptr [rdi + 176], xmm0
        movups  xmmword ptr [rdi + 160], xmm0
        movups  xmmword ptr [rdi + 144], xmm0
        movups  xmmword ptr [rdi + 128], xmm0
        movups  xmmword ptr [rdi + 112], xmm0
        movups  xmmword ptr [rdi + 96], xmm0
        movups  xmmword ptr [rdi + 80], xmm0
        movups  xmmword ptr [rdi + 64], xmm0
        movups  xmmword ptr [rdi + 48], xmm0
        movups  xmmword ptr [rdi + 32], xmm0
        movups  xmmword ptr [rdi + 16], xmm0
        movups  xmmword ptr [rdi], xmm0
        mov     qword ptr [rdi + 192], 0
        mov     rax, rdi
        pop     rbp
        ret

See also https://bugs.llvm.org/show_bug.cgi?id=35134

[dotdash]

I have a rough patch for the example in the LLVM bug report, but that is not equal to the rust code above. The above example would then still be blocked on #31681

[dotdash]

https://reviews.llvm.org/D40723

[dotdash]

That patch handles the original test case if -Zmutable-noalias is used. Turning roo into

fn roo() -> Bar {
    Bar {
        l: 4,
        f: noo(),
    }
}

is also handled without -Zmutable-noalias and gets rid of an extra memcpy that is currently still in there.

[nikic]

This optimizes well on nightly with -Zmutable-noalias, as a result of the LLVM 12 upgrade (call slot optimization with GEP dest now supported).

[jrmuizel]

We have -Zmutable-noalias by default now so closing.