> On Dec 7, 2015, at 10:27 AM, Karl Pickett <karl.pick...@gmail.com> wrote: > > I created https://bugs.swift.org/browse/SR-110 > <https://bugs.swift.org/browse/SR-110> for the compiler. However, I also > think that the documentation needs an issue filed. (Where to do that at?)
Thanks! > The current docs say structs are always copied (the only exception being > inout to memory variable optimization). That would make programmers worried > about speed and stack usage run away screaming, and not give swift a try. If weren't already been chased away by 'vars are always allocated on the heap'. The docs generally discuss high-level semantic behavior rather than the real code emitted; in general, users can count on structs being copied whenever necessary to preserve value semantics between different names. How would you suggest rewording the documentation? -Joe > On Mon, Dec 7, 2015 at 11:53 AM, Joe Groff <jgr...@apple.com > <mailto:jgr...@apple.com>> wrote: > >> On Dec 6, 2015, at 5:16 PM, Karl Pickett via swift-users >> <swift-users@swift.org <mailto: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) >> } >> _______________________________________________ >> swift-users mailing list >> swift-users@swift.org <mailto:swift-users@swift.org> >> https://lists.swift.org/mailman/listinfo/swift-users >> <https://lists.swift.org/mailman/listinfo/swift-users>
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