[swift-dev] question about performance of dispatches on existentials

Johannes WeiƟ johannesweiss at apple.com
Fri Jul 7 13:27:02 CDT 2017


Hi swift-dev,

If I have basically this program (full program see at the tail end of this mail)

public class A { func bar() { ... }}
public protocol B {
    func foo(_ a: A)
}
extension B {
    func foo(_ a: A) { a.bar() }
}
public class ActualB: B {
}
public class OtherB: B {
}
func abc() {
    let b: B = makeB()
    b.foo(a)
}

I get the following call frames when running it (compiled with `swiftc -O -g -o test test.swift`):

    frame #1: 0x0000000100001dbf test`specialized A.bar() at test.swift:6 [opt]
    frame #2: 0x0000000100001e6f test`specialized B.foo(_:) [inlined] test.SubA.bar() -> () at test.swift:0 [opt]
    frame #3: 0x0000000100001e6a test`specialized B.foo(a=<unavailable>) at test.swift:23 [opt]
    frame #4: 0x0000000100001a6e test`B.foo(_:) at test.swift:0 [opt]
    frame #5: 0x0000000100001b3e test`protocol witness for B.foo(_:) in conformance OtherB at test.swift:0 [opt]
    frame #6: 0x0000000100001ccd test`abc() at test.swift:45 [opt]
    frame #7: 0x0000000100001969 test`main at test.swift:48 [opt]

1, 6, and 7 are obviously totally fine and expected.

In 6 we are also building and destroying an existential box, also understandable and fine.

But there's two things I don't quite understand:

I) Why (in 5) will the existential container be retained and released?

--- SNIP ---
                     __T04test6OtherBCAA1BA2aDP3fooyAA1ACFTW:        // protocol witness for test.B.foo(test.A) -> () in conformance test.OtherB : test.B in test
0000000100001b20         push       rbp                                         ; CODE XREF=__T04test7ActualBCAA1BA2aDP3fooyAA1ACFTW+4
0000000100001b21         mov        rbp, rsp
0000000100001b24         push       r14
0000000100001b26         push       rbx
0000000100001b27         mov        r14, rdi
0000000100001b2a         mov        rbx, qword [r13]
0000000100001b2e         mov        rdi, rbx
0000000100001b31         call       _swift_rt_swift_retain
0000000100001b36         mov        rdi, r14                                    ; argument #1 for method __T04test1BPAAE3fooyAA1ACF
0000000100001b39         call       __T04test1BPAAE3fooyAA1ACF                  ; (extension in test):test.B.foo(test.A) -> ()
0000000100001b3e         mov        rdi, rbx
0000000100001b41         pop        rbx
0000000100001b42         pop        r14
0000000100001b44         pop        rbp
0000000100001b45         jmp        _swift_rt_swift_release
                        ; endp
--- SNAP ---

II) Why are 2, 3, 4 and 5 not one stack frame? Seems like we could just JMP from one to the next. Sure in 5 the call is surrounded by a release/retain but in the others we could just JMP.


We see quite a measurable performance issue in a project we're working on (email me directly for details/code) and so I thought I'd ask because I'd like to understand why this is all needed (if it is).


Many thanks,
  Johannes

--- SNIP ---
import Darwin

public class A {
    @inline(never)
    public func bar() {
        print("bar")
    }
}
public class SubA: A {
    @inline(never)
    public override func bar() {
        print("bar")
    }
}

public protocol B {
    func foo(_ a: A)
}

public extension B {
    @inline(never)
    func foo(_ a: A) {
        a.bar()
    }
}

public class ActualB: B {
}

public class OtherB: B {
}

public func makeB() -> B {
    if arc4random() == 1231231 {
        return ActualB()
    } else {
        return OtherB()
    }
}

@inline(never)
func abc() {
    let a = SubA()
    let b: B = makeB()
    b.foo(a)
}

abc()
--- SNAP ---



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