Thanks very much Arnold, also for filing the bug!
> On 7 Jul 2017, at 8:07 pm, Arnold Schwaighofer <aschwaigho...@apple.com>
> wrote:
>
> 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
>
>
>> 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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