This is a failure in the optimizer of identifying two loads to return the same
value and so it can’t remove a retain/release pair.
/ protocol witness for B.foo(_:) in conformance OtherB
sil shared [transparent] [serialized] [thunk]
@_T04test6OtherBCAA1BA2aDP3fooyAA1ACFTW : $@convention(witness_method) (@owned
A, @in_guaranteed OtherB) -> () {
// %0 // user: %7
// %1 // user: %3
bb0(%0 : $A, %1 : $*OtherB):
%2 = alloc_stack $OtherB // users: %9, %4, %11, %7
%3 = load %1 : $*OtherB // users: %6, %4
store %3 to %2 : $*OtherB // id: %4
// function_ref B.foo(_:)
%5 = function_ref @_T04test1BPAAE3fooyAA1ACF : $@convention(method) <τ_0_0
where τ_0_0 : B> (@owned A, @in_guaranteed τ_0_0) -> () // user: %7
strong_retain %3 : $OtherB // id: %6
%7 = apply %5<OtherB>(%0, %2) : $@convention(method) <τ_0_0 where τ_0_0 : B>
(@owned A, @in_guaranteed τ_0_0) -> ()
%8 = tuple () // user: %12
%9 = load %2 : $*OtherB // user: %10
strong_release %9 : $OtherB // id: %10
dealloc_stack %2 : $*OtherB // id: %11
return %8 : $() // id: %12
} // end sil function ‘_T04test6OtherBCAA1BA2aDP3fooyAA1ACFTW’
If load store forwarding could just tell that the apply does not write to the
alloc_stack (It could because @in_guaranteed guarantees no write) … i would
expect it to mem promote this … ARC could then remove the retain/release pair
(AFAICT).
https://bugs.swift.org/browse/SR-5403 <https://bugs.swift.org/browse/SR-5403>
> On Jul 7, 2017, at 11:27 AM, Johannes Weiß via swift-dev
> <swift-dev@swift.org> wrote:
>
> 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 ---
>
> _______________________________________________
> swift-dev mailing list
> swift-dev@swift.org
> https://lists.swift.org/mailman/listinfo/swift-dev
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