Check if compiler optimizes fields by value

[bvssvni]

Investigate whether context types can set their fields by value and still get the same compiler optimization. This might solve the problems with current Rust.

Use http://play.rust-lang.org/

[bvssvni]

Here is the code based on the current assumption:

pub fn field(a: uint, b: uint) -> (uint, uint) {
    enum Field<'a, T> {
        Value(T),
        Borrowed(&'a T),
    }

    impl<'a, T> Field<'a, T> {
        /// Gets a read only value.
        #[inline(always)]
        fn get(&'a self) -> &'a T {
            match *self {
                Value(ref val) => val,
                Borrowed(rval) => rval,
            }
        }
    }

    struct Foo<'a> {
        foo: Field<'a, uint>,
    }

    impl<'a> Foo<'a> {
        #[inline(always)]
        fn bar(&'a self, val: uint) -> BarFoo<'a> {
            BarFoo {
                foo: Borrowed(self.foo.get()),
                bar: Value(val),
            }
        }
    }

    struct BarFoo<'a> {
        foo: Field<'a, uint>,
        bar: Field<'a, uint>,
    }

    let foo = Foo { foo: Value(a) };
    let bar = foo.bar(b);
    (*bar.foo.get(), *bar.bar.get())
}

fn main() {
    let (x, y) = field(2, 4);
    println!("{}, {}", x, y)
}

This emits the assembly with -O2:

.text
    .file   "-.rs"
    .section    .text._ZN4main20h5ac5e5cb32d0c663nca4v0.0E,"ax",@progbits
    .align  16, 0x90
    .type   _ZN4main20h5ac5e5cb32d0c663nca4v0.0E,@function
_ZN4main20h5ac5e5cb32d0c663nca4v0.0E:
    .cfi_startproc
    cmpq    %fs:112, %rsp
    ja  .LBB0_2
    movabsq $88, %r10
    movabsq $0, %r11
    callq   __morestack
    retq
.LBB0_2:
    subq    $88, %rsp
.Ltmp0:
    .cfi_def_cfa_offset 96
    movq    $2, 80(%rsp)
    movq    $4, 72(%rsp)
    leaq    _ZN3fmt11secret_show20h23973399929398009214v0.0E(%rip), %rax
    movq    %rax, 40(%rsp)
    leaq    80(%rsp), %rcx
    movq    %rcx, 48(%rsp)
    movq    %rax, 56(%rsp)
    leaq    72(%rsp), %rax
    movq    %rax, 64(%rsp)
    leaq    _ZN4main15__STATIC_FMTSTR20h0d54fec8ba6e9728Mca4v0.0E(%rip), %rax
    movq    %rax, 8(%rsp)
    movq    $3, 16(%rsp)
    leaq    40(%rsp), %rax
    movq    %rax, 24(%rsp)
    movq    $2, 32(%rsp)
    leaq    8(%rsp), %rdi
    callq   _ZN2io5stdio12println_args20h216979612568a473ggl11v0.11.0.preE@PLT
    addq    $88, %rsp
    retq
.Ltmp1:
    .size   _ZN4main20h5ac5e5cb32d0c663nca4v0.0E, .Ltmp1-_ZN4main20h5ac5e5cb32d0c663nca4v0.0E
    .cfi_endproc

    .section    .text.main,"ax",@progbits
    .globl  main
    .align  16, 0x90
    .type   main,@function
main:
    .cfi_startproc
    movq    %rsi, %rax
    movq    %rdi, %rcx
    leaq    _ZN4main20h5ac5e5cb32d0c663nca4v0.0E(%rip), %rdi
    movq    %rcx, %rsi
    movq    %rax, %rdx
    jmp _ZN10lang_start20h7779600900fd5515Rje11v0.11.0.preE@PLT
.Ltmp2:
    .size   main, .Ltmp2-main
    .cfi_endproc

    .section    .text._ZN3fmt11secret_show20h23973399929398009214v0.0E,"ax",@progbits
    .align  16, 0x90
    .type   _ZN3fmt11secret_show20h23973399929398009214v0.0E,@function
_ZN3fmt11secret_show20h23973399929398009214v0.0E:
    .cfi_startproc
    jmp _ZN3fmt3num14uint.fmt..Show3fmt20h0858b562b1543b44bOI11v0.11.0.preE@PLT
.Ltmp3:
    .size   _ZN3fmt11secret_show20h23973399929398009214v0.0E, .Ltmp3-_ZN3fmt11secret_show20h23973399929398009214v0.0E
    .cfi_endproc

    .type   str1337,@object
    .section    .rodata.str1337,"a",@progbits
str1337:
    .ascii  ", "
    .size   str1337, 2

    .type   _ZN4main15__STATIC_FMTSTR20h0d54fec8ba6e9728Mca4v0.0E,@object
    .section    .data.rel.ro.local._ZN4main15__STATIC_FMTSTR20h0d54fec8ba6e9728Mca4v0.0E,"aw",@progbits
    .align  16
_ZN4main15__STATIC_FMTSTR20h0d54fec8ba6e9728Mca4v0.0E:
    .byte   1
    .zero   7
    .byte   0
    .zero   15
    .long   32
    .byte   2
    .zero   3
    .quad   0
    .byte   3
    .zero   15
    .byte   3
    .zero   15
    .byte   0
    .zero   7
    .quad   str1337
    .quad   2
    .zero   48
    .byte   1
    .zero   7
    .byte   0
    .zero   15
    .long   32
    .byte   2
    .zero   3
    .quad   0
    .byte   3
    .zero   15
    .byte   3
    .zero   15
    .size   _ZN4main15__STATIC_FMTSTR20h0d54fec8ba6e9728Mca4v0.0E, 216


    .section    ".note.GNU-stack","",@progbits

[bvssvni]

Here is the code based on value:

pub fn field(a: uint, b: uint) -> (uint, uint) {
    enum Field<T> {
        Value(T),
    }

    impl<T: Copy> Field<T> {
        /// Gets a copy of the value.
        #[inline(always)]
        fn get(&self) -> T {
            match *self {
                Value(val) => val,
            }
        }
    }

    struct Foo {
        foo: Field<uint>,
    }

    impl Foo {
        #[inline(always)]
        fn bar(&self, val: uint) -> BarFoo {
            BarFoo {
                foo: Value(self.foo.get()),
                bar: Value(val),
            }
        }
    }

    struct BarFoo {
        foo: Field<uint>,
        bar: Field<uint>,
    }

    let foo = Foo { foo: Value(a) };
    let bar = foo.bar(b);
    (bar.foo.get(), bar.bar.get())
}

fn main() {
    let (x, y) = field(2, 4);
    println!("{}, {}", x, y)
}

This emits the assembly with -O2:

.text
    .file   "-.rs"
    .section    .text._ZN4main20h282b95531535ea8bYba4v0.0E,"ax",@progbits
    .align  16, 0x90
    .type   _ZN4main20h282b95531535ea8bYba4v0.0E,@function
_ZN4main20h282b95531535ea8bYba4v0.0E:
    .cfi_startproc
    cmpq    %fs:112, %rsp
    ja  .LBB0_2
    movabsq $88, %r10
    movabsq $0, %r11
    callq   __morestack
    retq
.LBB0_2:
    subq    $88, %rsp
.Ltmp0:
    .cfi_def_cfa_offset 96
    movq    $2, 80(%rsp)
    movq    $4, 72(%rsp)
    leaq    _ZN3fmt11secret_show21h114544777529901186294v0.0E(%rip), %rax
    movq    %rax, 40(%rsp)
    leaq    80(%rsp), %rcx
    movq    %rcx, 48(%rsp)
    movq    %rax, 56(%rsp)
    leaq    72(%rsp), %rax
    movq    %rax, 64(%rsp)
    leaq    _ZN4main15__STATIC_FMTSTR20h90e030ad4229642enca4v0.0E(%rip), %rax
    movq    %rax, 8(%rsp)
    movq    $3, 16(%rsp)
    leaq    40(%rsp), %rax
    movq    %rax, 24(%rsp)
    movq    $2, 32(%rsp)
    leaq    8(%rsp), %rdi
    callq   _ZN2io5stdio12println_args20h216979612568a473ggl11v0.11.0.preE@PLT
    addq    $88, %rsp
    retq
.Ltmp1:
    .size   _ZN4main20h282b95531535ea8bYba4v0.0E, .Ltmp1-_ZN4main20h282b95531535ea8bYba4v0.0E
    .cfi_endproc

    .section    .text.main,"ax",@progbits
    .globl  main
    .align  16, 0x90
    .type   main,@function
main:
    .cfi_startproc
    movq    %rsi, %rax
    movq    %rdi, %rcx
    leaq    _ZN4main20h282b95531535ea8bYba4v0.0E(%rip), %rdi
    movq    %rcx, %rsi
    movq    %rax, %rdx
    jmp _ZN10lang_start20h7779600900fd5515Rje11v0.11.0.preE@PLT
.Ltmp2:
    .size   main, .Ltmp2-main
    .cfi_endproc

    .section    .text._ZN3fmt11secret_show21h114544777529901186294v0.0E,"ax",@progbits
    .align  16, 0x90
    .type   _ZN3fmt11secret_show21h114544777529901186294v0.0E,@function
_ZN3fmt11secret_show21h114544777529901186294v0.0E:
    .cfi_startproc
    jmp _ZN3fmt3num14uint.fmt..Show3fmt20h0858b562b1543b44bOI11v0.11.0.preE@PLT
.Ltmp3:
    .size   _ZN3fmt11secret_show21h114544777529901186294v0.0E, .Ltmp3-_ZN3fmt11secret_show21h114544777529901186294v0.0E
    .cfi_endproc

    .type   str1334,@object
    .section    .rodata.str1334,"a",@progbits
str1334:
    .ascii  ", "
    .size   str1334, 2

    .type   _ZN4main15__STATIC_FMTSTR20h90e030ad4229642enca4v0.0E,@object
    .section    .data.rel.ro.local._ZN4main15__STATIC_FMTSTR20h90e030ad4229642enca4v0.0E,"aw",@progbits
    .align  16
_ZN4main15__STATIC_FMTSTR20h90e030ad4229642enca4v0.0E:
    .byte   1
    .zero   7
    .byte   0
    .zero   15
    .long   32
    .byte   2
    .zero   3
    .quad   0
    .byte   3
    .zero   15
    .byte   3
    .zero   15
    .byte   0
    .zero   7
    .quad   str1334
    .quad   2
    .zero   48
    .byte   1
    .zero   7
    .byte   0
    .zero   15
    .long   32
    .byte   2
    .zero   3
    .quad   0
    .byte   3
    .zero   15
    .byte   3
    .zero   15
    .size   _ZN4main15__STATIC_FMTSTR20h90e030ad4229642enca4v0.0E, 216


    .section    ".note.GNU-stack","",@progbits

[bvssvni]

The emitted assembly is almost identical.