Issue Description:

re: https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20151207/000172.html

I have a struct Foo 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.

• 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)
}