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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