> On Dec 6, 2015, at 5:16 PM, Karl Pickett via swift-users
> <swift-users@swift.org> wrote:
>
> I have a struct and this code:
>
> func test() {
> precondition(sizeof(Foo) == 128)
>
> let s = Foo()
> for _ in 0..<100_000_000 {
> doSomething(s)
> }
> }
>
> The asm (on LInux, with -O) is showing me that s is being re-initialized on
> every iteration of the loop. I was hoping that thanks to swift's strict
> constness rules on structs, it wouldn't have to do this - and just pass the
> same pointer to doSomething() each time.
>
> When I use an inout param, that is 2x as fast and doesn't re-initialize each
> time. However I don't see why passing something immutably wouldn't be as
> fast.
This definitely seems like a place where we ought to be able to peephole the
extra copies away. Mind filing a bug?
-Joe
>
> - Karl
>
> asm from perf:
>
> 2.71 │50:┌─→xorps %xmm0,%xmm0
> ▒
> 8.06 │ │ movaps %xmm0,-0x20(%rbp)
> ▒
> 2.71 │ │ movaps %xmm0,-0x30(%rbp)
> ▒
> 7.41 │ │ movaps %xmm0,-0x40(%rbp)
> ▒
> 10.59 │ │ movaps %xmm0,-0x50(%rbp)
> ▒
> 10.00 │ │ movaps %xmm0,-0x60(%rbp)
> ▒
> 9.53 │ │ movaps %xmm0,-0x70(%rbp)
> ▒
> 10.65 │ │ movaps %xmm0,-0x80(%rbp)
> ▒
> 11.24 │ │ movaps %xmm0,-0x90(%rbp)
> ▒
> 12.06 │ │ mov %r14,%rdi
> ▒
> 3.41 │ │→ callq _TF4main11doSomethingFVS_3FooT_
> ▒
> 2.82 │ │ dec %rbx
> ▒
> 8.82 │ └──jne 50
>
> main.swift:
>
> struct Vec4 {
> var a: Int64 = 0
> var b: Int64 = 0
> var c: Int64 = 0
> var d: Int64 = 0
> }
>
> struct Foo {
> var x: Vec4 = Vec4()
> var y: Vec4 = Vec4()
> var z: Vec4 = Vec4()
> var u: Vec4 = Vec4()
> }
>
> func test() {
> precondition(sizeof(Foo) == 128)
>
> let s = Foo()
> for _ in 0..<100_000_000 {
> doSomething(s)
> }
> }
>
> test()
>
>
> lib.swift:
>
> func doSomething(s: Foo) {
> precondition(s.x.a != 1)
> }
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