Vandad Nahavandipoor
http://www.oreilly.com/pub/au/4596
Email: [email protected]
Blog: http://vandadnp.wordpress.com
Skype: vandad.np
In Swift, typecasting works in a different way than in Objective-C. We need to use the is keyword really to typecast and downcast, as you will see later. i am not going to teach you how to typecast in Swift. you can learn that here. i will however teach you the low-level details of how Swift deals with typecasting and downcasting. as always, if you find something wrong with this article, you can just send a pull request and correct it.
i will be using the release scheme for the builds. i will also use the following xcodebuild version:
Xcode 6.2
Build version 6C107aif you are curious as to knowing which version of the SDK you have on your own machine, you can run the xcrun xcodebuild -version command to find out.
So let's say you have this array:
let items = [
0xabcdefa,
"Foo",
0xabcdefb,
"Bar"
]and you want to go through the items in the array and find the ones which are integers and the ones which are strings like so:
func example1(){
for v in items{
if v is Int{
println(0xabcdefc)
}
else if v is String{
println(0xabcdefd)
}
}
}so we are conditionally typecasting the values and if we yield a result, then we print a value to the console. let's see how the compiler assembled the example1() function:
0000000100001280 push rbp ; XREF=-[_TtC12swift_weekly14ViewController example1]+23, __TFC12swift_weekly14ViewController11viewDidLoadfS0_FT_T_+111, -[_TtC12swift_weekly14ViewController viewDidLoad]+119
0000000100001281 mov rbp, rsp
0000000100001284 push r15
0000000100001286 push r14
0000000100001288 push r13
000000010000128a push r12
000000010000128c push rbx
000000010000128d sub rsp, 0x68
0000000100001291 mov qword [ss:rbp+var_90], rdi
0000000100001298 mov rax, qword [ds:__TWvdvC12swift_weekly14ViewController5itemsGSaCSo8NSObject_] ; __TWvdvC12swift_weekly14ViewController5itemsGSaCSo8NSObject_
000000010000129f mov r12, qword [ds:rdi+rax]
00000001000012a3 test r12, r12
00000001000012a6 je 0x100001617
00000001000012ac mov al, byte [ds:r12+0x19]
00000001000012b1 mov rbx, qword [ds:r12+0x10]
00000001000012b6 test al, al
00000001000012b8 je 0x10000130a
00000001000012d2 mov r14, rax
00000001000012d5 test rbx, rbx
00000001000012d8 je 0x100001619
00000001000012ee mov rsi, qword [ds:0x100018558] ; @selector(count), argument "selector" for method imp___stubs__objc_msgSend
00000001000012f5 mov rdi, rbx ; argument "instance" for method imp___stubs__objc_msgSend
00000001000012f8 call imp___stubs__objc_msgSend
00000001000012fd mov r15, rax
0000000100001308 jmp 0x100001339
0000000100001322 mov r14, rax
0000000100001325 xor r15d, r15d
0000000100001328 test rbx, rbx
000000010000132b je 0x100001339
000000010000132d mov r15, qword [ds:rbx+0x10]
0000000100001341 test r15, r15
0000000100001344 je 0x1000015e2
000000010000134a xor r15d, r15d
000000010000134d mov rax, qword [ds:imp___got___TMdSi] ; imp___got___TMdSi
0000000100001354 add rax, 0x8
0000000100001358 mov qword [ss:rbp+var_80], rax
000000010000135c mov rax, qword [ds:imp___got___TMdSS] ; imp___got___TMdSS
0000000100001363 add rax, 0x8
0000000100001367 mov qword [ss:rbp+var_88], rax
000000010000136e nop
0000000100001370 lea rax, qword [ds:r15+0x1] ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+860
0000000100001374 mov qword [ss:rbp+var_78], rax
0000000100001378 mov al, byte [ds:r12+0x19]
000000010000137d test al, al
000000010000137f je 0x1000013e0
0000000100001399 mov rbx, rax
000000010000139c test r15, r15
000000010000139f js 0x100001605
00000001000013ad mov r14, rax
00000001000013b0 mov rdi, r12
00000001000013b3 call __TTSCSo8NSObject___TFVSs12_ArrayBufferg5countSi
00000001000013b8 mov rbx, rax
00000001000013cb cmp r15, rbx
00000001000013ce jl 0x10000142f
00000001000013d0 jmp 0x100001617
00000001000013d5 nop qword [cs:rax+rax+0x0]
00000001000013e0 mov rbx, qword [ds:r12+0x10] ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+255
00000001000013fd mov r14, rax
0000000100001400 test rbx, rbx
0000000100001403 je 0x100001617
0000000100001409 mov r13, qword [ds:rbx+0x10]
000000010000141d test r15, r15
0000000100001420 js 0x100001617
0000000100001426 cmp r15, r13
0000000100001429 jge 0x100001617
000000010000142f lea rdi, qword [ss:rbp+var_30] ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+334
0000000100001433 mov rsi, r15
0000000100001436 mov rdx, r12
0000000100001439 call __TTSCSo8NSObject___TFVSs12_ArrayBufferg9subscriptFSiQ_
000000010000143e mov r15, qword [ss:rbp+var_30]
0000000100001442 mov qword [ss:rbp+var_38], r15
0000000100001446 mov rbx, qword [ds:__TMLCSo8NSObject] ; __TMLCSo8NSObject
0000000100001455 test rbx, rbx
0000000100001458 jne 0x100001478
000000010000145a mov rdi, qword [ds:imp___got__OBJC_CLASS_$_NSObject] ; imp___got__OBJC_CLASS_$_NSObject
0000000100001461 call imp___stubs__swift_getInitializedObjCClass
0000000100001466 mov rdi, rax
0000000100001469 call imp___stubs__swift_getObjCClassMetadata
000000010000146e mov rbx, rax
0000000100001471 mov qword [ds:__TMLCSo8NSObject], rbx ; __TMLCSo8NSObject
0000000100001478 mov r8d, 0x6 ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+472
000000010000147e lea rdi, qword [ss:rbp+var_40]
0000000100001482 lea rsi, qword [ss:rbp+var_38]
0000000100001486 mov rdx, rbx
0000000100001489 mov rcx, qword [ss:rbp+var_80]
000000010000148d call imp___stubs__swift_dynamicCast
0000000100001492 test al, 0x1
0000000100001494 je 0x1000014b0
0000000100001496 mov qword [ss:rbp+var_70], 0xabcdefc
000000010000149e lea rdi, qword [ss:rbp+var_70]
00000001000014a2 jmp 0x1000014f2
00000001000014a4 nop qword [cs:rax+rax+0x0]
00000001000014b0 mov qword [ss:rbp+var_48], r15 ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+532
00000001000014bc mov r8d, 0x6
00000001000014c2 lea rdi, qword [ss:rbp+var_60]
00000001000014c6 lea rsi, qword [ss:rbp+var_48]
00000001000014ca mov rdx, rbx
00000001000014cd mov rcx, qword [ss:rbp+var_88]
00000001000014d4 call imp___stubs__swift_dynamicCast
00000001000014d9 test al, 0x1
00000001000014db je 0x100001540
00000001000014e6 mov qword [ss:rbp+var_68], 0xabcdefd
00000001000014ee lea rdi, qword [ss:rbp+var_68]
00000001000014f2 call __TTSSi_VSs7_StdoutS_Ss16OutputStreamType___TFSs5printU_Ss16OutputStreamType__FTQ_RQ0__T_ ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+546
000000010000152f mov edi, 0xa ; argument "c" for method imp___stubs__putchar
0000000100001534 call imp___stubs__putchar
0000000100001548 mov al, byte [ds:r12+0x19]
000000010000154d mov rbx, qword [ds:r12+0x10]
0000000100001552 test al, al
0000000100001554 je 0x1000015a0
0000000100001566 mov r14, rax
0000000100001569 test rbx, rbx
000000010000156c je 0x100001619
0000000100001582 mov rsi, qword [ds:0x100018558] ; @selector(count), argument "selector" for method imp___stubs__objc_msgSend
0000000100001589 mov rdi, rbx ; argument "instance" for method imp___stubs__objc_msgSend
000000010000158c call imp___stubs__objc_msgSend
0000000100001591 mov r15, rax
000000010000159c jmp 0x1000015ca
000000010000159e nop
00000001000015b0 mov r14, rax
00000001000015b3 test rbx, rbx
00000001000015b6 mov r15d, 0x0
00000001000015bc je 0x1000015ca
00000001000015be mov r15, qword [ds:rbx+0x10]
00000001000015d2 mov rax, qword [ss:rbp+var_78]
00000001000015d6 cmp rax, r15
00000001000015d9 mov r15, rax
00000001000015dc jne 0x100001370
00000001000015f6 add rsp, 0x68
00000001000015fa pop rbx
00000001000015fb pop r12
00000001000015fd pop r13
00000001000015ff pop r14
0000000100001601 pop r15
0000000100001603 pop rbp
0000000100001604 ret
0000000100001615 ud2
0000000100001617 ud2 ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+38, __TFC12swift_weekly14ViewController8example1fS0_FT_T_+336, __TFC12swift_weekly14ViewController8example1fS0_FT_T_+387, __TFC12swift_weekly14ViewController8example1fS0_FT_T_+416, __TFC12swift_weekly14ViewController8example1fS0_FT_T_+425
0000000100001619 ud2 ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+88, __TFC12swift_weekly14ViewController8example1fS0_FT_T_+748
000000010000161b nop qword [ds:rax+rax+0x0] -[_TtC12swift_weekly14ViewController example1]:
0000000100001620 push rbp ; Objective C Implementation defined at 0x100017708 (instance)
0000000100001621 mov rbp, rsp
0000000100001624 push rbx
0000000100001625 push rax
000000010000162e mov rdi, rbx ; argument #1 for method __TFC12swift_weekly14ViewController8example1fS0_FT_T_
0000000100001631 add rsp, 0x8
0000000100001635 pop rbx
0000000100001636 pop rbp
0000000100001637 jmp __TFC12swift_weekly14ViewController8example1fS0_FT_T_
; endp
000000010000163c nop qword [ds:rax+0x0]note that i have removed big chunks of the output assembly because they are all about retain/release codes which aren't relevant to what we are trying to analyse. so this is not the full assembly, but the most interesting part!
this is what the code is doing:
-
in order to do the fast enumeration over our
itemsarray, swift first has to find out how many items are available in the array. this is how our program finds that out:00000001000012ee mov rsi, qword [ds:0x100018558] ; @selector(count), argument "selector" for method imp___stubs__objc_msgSend 00000001000012f5 mov rdi, rbx ; argument "instance" for method imp___stubs__objc_msgSend 00000001000012f8 call imp___stubs__objc_msgSend 00000001000012fd mov r15, rax
according to the System V calling convention, the first parameter to a method call is passed in
rdiand the next inrsiregisters. hererdiis being set to the address of ouritemsarray andrsiis set to thecountmethod of the array and once we get the count, we store it in ther15general purpose register. -
after we store the count of
itemsin ther15register, we jump to0x100001339which eventually checks if the array is empty and if it is, jumps out of the loop...0000000100001341 test r15, r15 0000000100001344 je 0x1000015e2
-
we know that our array is not empty so if we carry on, swift will attempt to get the
nth object out of theitemsarray wherenis our current implicit loop counter:0000000100001b6f lea rdi, qword [ss:rbp+var_30] ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+334 0000000100001b73 mov rsi, r15 0000000100001b76 mov rdx, r12 0000000100001b79 call __TTSCSo8NSObject___TFVSs12_ArrayBufferg9subscriptFSiQ_
according to the System V calling convention,
rdi,rsiandrdxcontain the parameters that are passed to a method in that particular order. hererdiis being set toqword [ss:rbp+0xffffffffffffffd0](i decoded thevar_30value in the address). i have no clue what this is doing. do you? please send a pull request.then the
rsiregister is being set to the value inside ther15register. that register was previously set to the count of ouritemsarray so nowrsiis equal to the count of the array. then the third parameter which isrdxis set tor12which, if you recall from the beginning of the asm code, was set to the address of theitemsarray like so:00000001000019d1 mov qword [ss:rbp+var_90], rdi 00000001000019d8 mov rax, qword [ds:__TWvdvC12swift_weekly14ViewController5itemsGSaCSo8NSObject_] ; __TWvdvC12swift_weekly14ViewController5itemsGSaCSo8NSObject_ 00000001000019df mov r12, qword [ds:rdi+rax] 00000001000019e3 test r12, r12
so:
rdiis set to (i don't know, do you? send a pull request please)rsiis set toitems.countrdxis pointing to theitemsarray
after these values are set, swift is calling the
__TTSCSo8NSObject___TFVSs12_ArrayBufferg9subscriptFSiQ_internal function which i am not going to get into. this is basically a function that takes an array of items, and then extracts a given value out of it. so could it be thatrdiin this case is the index into theitemsarray before we call the aforementioned function? maybe! after this call is made,raxwill be set to the extracted item. -
then i think what is happening next (submit a pull request please if this is incorrect) is that swift is trying to find out if the retrieved item inside the array is an instance of
NSObjector not and if yes, then it is trying to get its metadata to find out if its of the type that we are conditionally looking for:0000000100001b7e mov r15, qword [ss:rbp+0xffffffffffffffd0] 0000000100001b82 mov qword [ss:rbp+var_38], r15 0000000100001b86 mov rbx, qword [ds:__TMLCSo8NSObject] ; __TMLCSo8NSObject 0000000100001b95 test rbx, rbx 0000000100001b98 jne 0x100001bb8 0000000100001b9a mov rdi, qword [ds:imp___got__OBJC_CLASS_$_NSObject] ; imp___got__OBJC_CLASS_$_NSObject 0000000100001ba1 call imp___stubs__swift_getInitializedObjCClass 0000000100001ba6 mov rdi, rax 0000000100001ba9 call imp___stubs__swift_getObjCClassMetadata 0000000100001bae mov rbx, rax 0000000100001bb1 mov qword [ds:__TMLCSo8NSObject], rbx ; __TMLCSo8NSObject
as you can see, a call is being made to a private internal method called
imp___stubs__swift_getObjCClassMetadataand as its name explains, i believe it receives an instance ofNSObjectand then retrives information about that object, such as its class name. -
now the juicy part is going to happen. this is where swift will try to typecast the enumerated value to
Int0000000100001bb8 mov r8d, 0x6 ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+472 0000000100001bbe lea rdi, qword [ss:rbp+0xffffffffffffffc0] 0000000100001bc2 lea rsi, qword [ss:rbp+0xffffffffffffffc8] 0000000100001bc6 mov rdx, rbx 0000000100001bc9 mov rcx, qword [ss:rbp+0xffffffffffffff80] 0000000100001bcd call imp___stubs__swift_dynamicCast 0000000100001bd2 test al, 0x1 0000000100001bd4 je 0x100001bf0
parameters are passed in
rdi,rsi,rdxandrcxas explained in System V AMD64 ABI calling convention. then a magic function calledimp___stubs__swift_dynamicCastis executed to the typecasting. oh shit. so typecasting is done by making a function call in swift. that's not good news if you ask me. imagine performing typecasts in a loop that repeats thousands of times during the lifetime of your app. can you live with the performance implications? it's up to you.then at the end of this call, we check if
alhas atrueor not and if no, we jump over to0x100001bf0. but wait a minute. before discussing that jump, let's see how swift understood that we are typecasting our value toInt... i've looked at the code for a while (considering that it is quite late at night here), and i cannot, for the life of me, find out how swift is asking for a specific type from theimp___stubs__swift_dynamicCastfunction. so if you do know how this function works, send a pull request and enlighten everybody. thanks. -
as you saw in the previous step, if we could find an
Int, then we continued on without jumping over tocs:0x100001bf0and that could would be:0000000100001bd6 mov qword [ss:rbp+var_70], 0xabcdefc 0000000100001bde lea rdi, qword [ss:rbp+var_70] 0000000100001be2 jmp 0x100001c32 0000000100001be4 nop qword [cs:rax+rax+0x0]
so here our
0xabcdefcvalue is loaded into the stack and then therdiregister will point to it and we will jump over tocs:0x100001c32which is this:0000000100001c32 call __TTSSi_VSs7_StdoutS_Ss16OutputStreamType___TFSs5printU_Ss16OutputStreamType__FTQ_RQ0__T_ ; XREF=__TFC12swift_weekly14ViewController8example1fS0_FT_T_+546
and this is a call to a long-ass function named
__TTSSi_VSs7_StdoutS_Ss16OutputStreamType___TFSs5printU_Ss16OutputStreamType__FTQ_RQ0__T_. this will eventually print our0xabcdefcint value to the screen. -
if we weren't successful in finding the
Intvalue, then we would have this code in front of us:0000000100001bfc mov r8d, 0x6 0000000100001c02 lea rdi, qword [ss:rbp+0xffffffffffffffa0] 0000000100001c06 lea rsi, qword [ss:rbp+0xffffffffffffffb8] 0000000100001c0a mov rdx, rbx 0000000100001c0d mov rcx, qword [ss:rbp+0xffffffffffffff78] 0000000100001c14 call imp___stubs__swift_dynamicCast 0000000100001c19 test al, 0x1 0000000100001c1b je 0x100001c80 0000000100001c26 mov qword [ss:rbp+var_68], 0xabcdefd 0000000100001c2e lea rdi, qword [ss:rbp+var_68]
this is similar to the previous typecasting in that it is using the
imp___stubs__swift_dynamicCastfunction to do the typecasting from ourNSObjecttoString. (note, for some reason, Swift decided to turn our wholeitemsarray into an array of[NSObject], instead of[AnyObject]and maybe some of thisNSObjectmonkey-business was because of that. Do you know why swift did that? send a pull request to fix this up for everybody).
the rest of this code is not magic. so really the important thing that you would want to take away with you is that typecasting in swift is done by an actual function call to an internal and private function called imp___stubs__swift_dynamicCast.
Let's have a look at unconditional downcasting in swift. as i mentioned before, you can learn about downcasting on apple's website so i won't teach it here. imagine you have the following two classes:
class Vehicle{
func id() -> Int{
return 0xabcdefa
}
}
class Car : Vehicle{
override func id() -> Int {
return 0xabcdefc
}
}and then we do this:
func example2(){
let v: Vehicle = Car()
let c = [v][0] as Car
println(c)
}so the variable c is unconditionally downcasted from the first item in the array that only has v as its member. this array is [Vehicle] so if we want to get a Car object out of it, we have to downcast it, which is what we are doing. So let's see how Swift will compile this code:
0000000100001740 push rbp ; XREF=__TFC12swift_weekly14ViewController11viewDidLoadfS0_FT_T_+111, __TToFC12swift_weekly14ViewController11viewDidLoadfS0_FT_T_+119
0000000100001741 mov rbp, rsp
0000000100001744 push r15
0000000100001746 push r14
0000000100001748 push r13
000000010000174a push r12
000000010000174c push rbx
000000010000174d sub rsp, 0x28
0000000100001751 mov qword [ss:rbp+var_48], rdi
0000000100001755 mov r12, qword [ds:__TMLCC12swift_weekly14ViewController3Car] ; __TMLCC12swift_weekly14ViewController3Car
000000010000175c test r12, r12
000000010000175f jne 0x100001777
0000000100001761 lea rdi, qword [ds:objc_class__TtCC12swift_weekly14ViewController3Car] ; objc_class__TtCC12swift_weekly14ViewController3Car
0000000100001768 call imp___stubs__swift_getInitializedObjCClass
000000010000176d mov r12, rax
0000000100001770 mov qword [ds:__TMLCC12swift_weekly14ViewController3Car], r12 ; __TMLCC12swift_weekly14ViewController3Car
0000000100001777 mov esi, 0x10 ; XREF=__TFC12swift_weekly14ViewController8example2fS0_FT_T_+31
000000010000177c mov edx, 0x7
0000000100001781 mov rdi, r12
0000000100001784 call imp___stubs__swift_allocObject
0000000100001789 mov r14, rax
000000010000178c lea rdi, qword [ds:0x10001e210] ; 0x10001e210 (_metadata + 0x10)
0000000100001793 mov esi, 0x20
0000000100001798 mov edx, 0x7
000000010000179d call imp___stubs__swift_allocObject
00000001000017a2 mov r13, rax
00000001000017a5 mov qword [ds:r13+0x10], 0x1
00000001000017ad mov qword [ds:r13+0x18], r14
00000001000017b1 mov rbx, qword [ds:__TMLGCSs23_ContiguousArrayStorageCC12swift_weekly14ViewController7Vehicle_] ; __TMLGCSs23_ContiguousArrayStorageCC12swift_weekly14ViewController7Vehicle_
00000001000017b8 test rbx, rbx
00000001000017bb jne 0x1000017f5
00000001000017bd mov rsi, qword [ds:__TMLCC12swift_weekly14ViewController7Vehicle] ; __TMLCC12swift_weekly14ViewController7Vehicle
00000001000017c4 test rsi, rsi
00000001000017c7 jne 0x1000017df
00000001000017c9 lea rdi, qword [ds:objc_class__TtCC12swift_weekly14ViewController7Vehicle] ; objc_class__TtCC12swift_weekly14ViewController7Vehicle
00000001000017d0 call imp___stubs__swift_getInitializedObjCClass
00000001000017d5 mov rsi, rax
00000001000017d8 mov qword [ds:__TMLCC12swift_weekly14ViewController7Vehicle], rsi ; __TMLCC12swift_weekly14ViewController7Vehicle
00000001000017df mov rdi, qword [ds:imp___got___TMPdCSs23_ContiguousArrayStorage] ; imp___got___TMPdCSs23_ContiguousArrayStorage, XREF=__TFC12swift_weekly14ViewController8example2fS0_FT_T_+135
00000001000017e6 call imp___stubs__swift_getGenericMetadata1
00000001000017eb mov rbx, rax
00000001000017ee mov qword [ds:__TMLGCSs23_ContiguousArrayStorageCC12swift_weekly14ViewController7Vehicle_], rbx ; __TMLGCSs23_ContiguousArrayStorageCC12swift_weekly14ViewController7Vehicle_
00000001000017fd mov qword [ss:rbp+var_50], rax
0000000100001801 mov esi, 0x28
0000000100001806 mov edx, 0x7
000000010000180b mov rdi, rbx
000000010000180e call imp___stubs__swift_bufferAllocate
0000000100001813 mov r14, rax
0000000100001816 mov qword [ds:r14+0x18], 0x0
000000010000181e mov qword [ds:r14+0x10], 0x0
0000000100001826 mov rdi, r14 ; argument "ptr" for method imp___stubs__malloc_size
0000000100001829 call imp___stubs__malloc_size
000000010000182e sub rax, 0x20
0000000100001832 jo 0x1000019d5
0000000100001838 cmp rax, 0xfffffffffffffff9
000000010000183c jl 0x1000019d5
0000000100001842 mov rcx, rax
0000000100001845 sar rcx, 0x3f
0000000100001849 shr rcx, 0x3d
000000010000184d add rcx, rax
0000000100001850 sar rcx, 0x3
0000000100001854 lea rax, qword [ds:rcx+rcx+0x1]
0000000100001859 mov qword [ds:r14+0x10], 0x1
0000000100001861 mov qword [ds:r14+0x18], rax
0000000100001865 mov r15, qword [ds:r13+0x18]
0000000100001869 mov qword [ds:r14+0x20], r15
000000010000186d call imp___stubs___TMaCSs20_IndirectArrayBuffer
0000000100001872 mov esi, 0x1b
0000000100001877 mov edx, 0x7
000000010000187c mov rdi, rax
000000010000187f call imp___stubs__swift_allocObject
0000000100001884 mov rbx, rax
000000010000188f xor eax, eax
0000000100001891 test r14, r14
0000000100001894 je 0x100001899
0000000100001896 mov rax, r14
0000000100001899 mov qword [ds:rbx+0x10], rax ; XREF=__TFC12swift_weekly14ViewController8example2fS0_FT_T_+340
000000010000189d mov byte [ds:rbx+0x18], 0x1
00000001000018a1 mov byte [ds:rbx+0x19], 0x0
00000001000018a5 mov byte [ds:rbx+0x1a], 0x0
00000001000018a9 mov qword [ss:rbp+var_30], r13
00000001000018ad lea rdi, qword [ss:rbp+var_30]
00000001000018b1 call imp___stubs__swift_fixLifetime
00000001000018bf mov al, byte [ds:rbx+0x19]
00000001000018c2 test al, al
00000001000018c4 je 0x100001906
00000001000018de mov r14, rax
00000001000018e1 mov rdi, rbx
00000001000018e4 call __TTSCC12swift_weekly14ViewController7Vehicle___TFVSs12_ArrayBufferg5countSi
00000001000018e9 mov r15, rax
00000001000018fc test r15, r15
00000001000018ff jg 0x100001931
0000000100001901 jmp 0x1000019d5
0000000100001906 mov r14, qword [ds:rbx+0x10] ; XREF=__TFC12swift_weekly14ViewController8example2fS0_FT_T_+388
000000010000190a test r14, r14
000000010000190d je 0x1000019d5
0000000100001913 mov r15, qword [ds:r14+0x10]
0000000100001927 cmp r15, 0x0
000000010000192b jle 0x1000019d5
0000000100001931 lea rdi, qword [ss:rbp+var_38] ; XREF=__TFC12swift_weekly14ViewController8example2fS0_FT_T_+447
0000000100001935 xor esi, esi
0000000100001937 mov rdx, rbx
000000010000193a call __TTSCC12swift_weekly14ViewController7Vehicle___TFVSs12_ArrayBufferg9subscriptFSiQ_
000000010000193f mov rdi, qword [ss:rbp+var_38]
0000000100001943 mov rsi, r12
0000000100001946 call imp___stubs__swift_dynamicCastClassUnconditional
000000010000194b mov qword [ss:rbp+var_40], rax
0000000100001957 mov rbx, rax
000000010000195a lea rdi, qword [ss:rbp+var_40]
000000010000195e call __TTSCC12swift_weekly14ViewController3Car_VSs7_StdoutS2_Ss16OutputStreamType___TFSs5printU_Ss16OutputStreamType__FTQ_RQ0__T_
000000010000199b mov edi, 0xa ; argument "c" for method imp___stubs__putchar
00000001000019a0 call imp___stubs__putchar
00000001000019c6 add rsp, 0x28
00000001000019ca pop rbx
00000001000019cb pop r12
00000001000019cd pop r13
00000001000019cf pop r14
00000001000019d1 pop r15
00000001000019d3 pop rbp
00000001000019d4 reti've cut some of the asm code out of this output since they were doing the retain/release cycles for us, i thought they were quite useless.
now let's focus on this part of our swift code:
let c = [v][0] as Carthis is translated to asm like so:
0000000100001937 mov rdx, rbx
000000010000193a call __TTSCC12swift_weekly14ViewController7Vehicle___TFVSs12_ArrayBufferg9subscriptFSiQ_
000000010000193f mov rdi, qword [ss:rbp+var_38]
0000000100001943 mov rsi, r12
0000000100001946 call imp___stubs__swift_dynamicCastClassUnconditionalas you can see, swift called a private hidden method called imp___stubs__swift_dynamicCastClassUnconditional to downcast the [Vehicle] array's first item into a Car instance
let's add another class to our combination from the previous section:
class Bicycle : Vehicle{
override func id() -> Int {
return 0xabcdefb
}
}so now we have a Car and a Bicycle class and both of them inherit from the Vehicle class. now we can put instances of all these classes inside an array, enumerate the array, and conditionally pick the bicycles and the cars:
func example3(){
let items = [Bicycle(), Car(), Bicycle()]
for i in items{
if let b = i as? Bicycle{
println(0xabcdefd)
}
else if let c = i as? Car{
println(0xabcdefe)
}
}
}let's look at the asm:
0000000100001c80 push rbp ; XREF=-[_TtC12swift_weekly14ViewController example3]+23
0000000100001c81 mov rbp, rsp
0000000100001c84 push r15
0000000100001c86 push r14
0000000100001c88 push r13
0000000100001c8a push r12
0000000100001c8c push rbx
0000000100001c8d sub rsp, 0x58
0000000100001c91 mov qword [ss:rbp+var_78], rdi
0000000100001c95 mov rbx, qword [ds:__TMLCC12swift_weekly14ViewController7Bicycle] ; __TMLCC12swift_weekly14ViewController7Bicycle
0000000100001c9c test rbx, rbx
0000000100001c9f jne 0x100001cb7
0000000100001ca1 lea rdi, qword [ds:objc_class__TtCC12swift_weekly14ViewController7Bicycle] ; objc_class__TtCC12swift_weekly14ViewController7Bicycle
0000000100001ca8 call imp___stubs__swift_getInitializedObjCClass
0000000100001cad mov rbx, rax
0000000100001cb0 mov qword [ds:__TMLCC12swift_weekly14ViewController7Bicycle], rbx ; __TMLCC12swift_weekly14ViewController7Bicycle
0000000100001cb7 mov esi, 0x10 ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+31
0000000100001cbc mov edx, 0x7
0000000100001cc1 mov rdi, rbx
0000000100001cc4 call imp___stubs__swift_allocObject
0000000100001cc9 mov r15, rax
0000000100001ccc mov rdi, qword [ds:__TMLCC12swift_weekly14ViewController3Car] ; __TMLCC12swift_weekly14ViewController3Car
0000000100001cd3 test rdi, rdi
0000000100001cd6 jne 0x100001cee
0000000100001cd8 lea rdi, qword [ds:objc_class__TtCC12swift_weekly14ViewController3Car] ; objc_class__TtCC12swift_weekly14ViewController3Car
0000000100001cdf call imp___stubs__swift_getInitializedObjCClass
0000000100001ce4 mov rdi, rax
0000000100001ce7 mov qword [ds:__TMLCC12swift_weekly14ViewController3Car], rdi ; __TMLCC12swift_weekly14ViewController3Car
0000000100001cee mov qword [ss:rbp+var_70], rdi ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+86
0000000100001cf2 mov esi, 0x10
0000000100001cf7 mov edx, 0x7
0000000100001cfc call imp___stubs__swift_allocObject
0000000100001d01 mov r12, rax
0000000100001d04 mov esi, 0x10
0000000100001d09 mov edx, 0x7
0000000100001d0e mov rdi, rbx
0000000100001d11 mov qword [ss:rbp+var_60], rbx
0000000100001d15 call imp___stubs__swift_allocObject
0000000100001d1a mov rbx, rax
0000000100001d1d lea rdi, qword [ds:0x10001e250] ; 0x10001e250 (_metadata4 + 0x10)
0000000100001d24 mov esi, 0x30
0000000100001d29 mov edx, 0x7
0000000100001d2e call imp___stubs__swift_allocObject
0000000100001d33 mov r14, rax
0000000100001d36 mov qword [ds:r14+0x10], 0x3
0000000100001d3e mov qword [ds:r14+0x20], 0x0
0000000100001d46 mov qword [ds:r14+0x18], 0x0
0000000100001d4e mov qword [ds:r14+0x18], r15
0000000100001d52 mov qword [ds:r14+0x20], r12
0000000100001d56 mov qword [ds:r14+0x28], rbx
0000000100001d5a mov rdi, qword [ds:__TMLGCSs23_ContiguousArrayStorageCC12swift_weekly14ViewController7Vehicle_] ; __TMLGCSs23_ContiguousArrayStorageCC12swift_weekly14ViewController7Vehicle_
0000000100001d61 test rdi, rdi
0000000100001d64 jne 0x100001d9e
0000000100001d66 mov rsi, qword [ds:__TMLCC12swift_weekly14ViewController7Vehicle] ; __TMLCC12swift_weekly14ViewController7Vehicle
0000000100001d6d test rsi, rsi
0000000100001d70 jne 0x100001d88
0000000100001d72 lea rdi, qword [ds:objc_class__TtCC12swift_weekly14ViewController7Vehicle] ; objc_class__TtCC12swift_weekly14ViewController7Vehicle
0000000100001d79 call imp___stubs__swift_getInitializedObjCClass
0000000100001d7e mov rsi, rax
0000000100001d81 mov qword [ds:__TMLCC12swift_weekly14ViewController7Vehicle], rsi ; __TMLCC12swift_weekly14ViewController7Vehicle
0000000100001d88 mov rdi, qword [ds:imp___got___TMPdCSs23_ContiguousArrayStorage] ; imp___got___TMPdCSs23_ContiguousArrayStorage, XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+240
0000000100001d8f call imp___stubs__swift_getGenericMetadata1
0000000100001d94 mov rdi, rax
0000000100001d97 mov qword [ds:__TMLGCSs23_ContiguousArrayStorageCC12swift_weekly14ViewController7Vehicle_], rdi ; __TMLGCSs23_ContiguousArrayStorageCC12swift_weekly14ViewController7Vehicle_
0000000100001d9e mov esi, 0x38 ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+228
0000000100001da3 mov edx, 0x7
0000000100001da8 call imp___stubs__swift_bufferAllocate
0000000100001dad mov rbx, rax
0000000100001db0 mov qword [ds:rbx+0x18], 0x0
0000000100001db8 mov qword [ds:rbx+0x10], 0x0
0000000100001dc0 mov rdi, rbx ; argument "ptr" for method imp___stubs__malloc_size
0000000100001dc3 call imp___stubs__malloc_size
0000000100001dc8 sub rax, 0x20
0000000100001dcc jo 0x100002219
0000000100001dd2 cmp rax, 0xfffffffffffffff9
0000000100001dd6 jl 0x100002219
0000000100001ddc mov rcx, r14
0000000100001ddf add rcx, 0x18
0000000100001de3 mov rdx, rax
0000000100001de6 sar rdx, 0x3f
0000000100001dea shr rdx, 0x3d
0000000100001dee add rdx, rax
0000000100001df1 sar rdx, 0x3
0000000100001df5 lea rax, qword [ds:rdx+rdx+0x1]
0000000100001dfa mov qword [ds:rbx+0x10], 0x3
0000000100001e02 mov qword [ds:rbx+0x18], rax
0000000100001e06 mov r15, qword [ds:rcx]
0000000100001e09 mov qword [ds:rbx+0x20], r15
0000000100001e0d mov r12, qword [ds:rcx+0x8]
0000000100001e11 mov qword [ds:rbx+0x28], r12
0000000100001e15 mov r13, qword [ds:rcx+0x10]
0000000100001e19 mov qword [ds:rbx+0x30], r13
0000000100001e1d call imp___stubs___TMaCSs20_IndirectArrayBuffer
0000000100001e22 mov esi, 0x1b
0000000100001e27 mov edx, 0x7
0000000100001e2c mov rdi, rax
0000000100001e2f call imp___stubs__swift_allocObject
0000000100001e34 mov qword [ss:rbp+var_50], rax
0000000100001e50 xor eax, eax
0000000100001e52 test rbx, rbx
0000000100001e55 je 0x100001e5a
0000000100001e57 mov rax, rbx
0000000100001e5a mov r12, qword [ss:rbp+var_50] ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+469
0000000100001e5e mov qword [ds:r12+0x10], rax
0000000100001e63 mov byte [ds:r12+0x18], 0x1
0000000100001e69 mov byte [ds:r12+0x19], 0x0
0000000100001e6f mov byte [ds:r12+0x1a], 0x0
0000000100001e75 mov qword [ss:rbp+var_30], r14
0000000100001e79 lea rdi, qword [ss:rbp+var_30]
0000000100001e7d call imp___stubs__swift_fixLifetime
0000000100001e8b mov al, byte [ds:r12+0x19]
0000000100001e90 test al, al
0000000100001e92 je 0x100001ef2
0000000100001eac mov r15, rax
0000000100001eaf test r12, r12
0000000100001eb2 je 0x100002219
0000000100001eb8 mov rbx, qword [ds:r12+0x10]
0000000100001ebd test rbx, rbx
0000000100001ec0 je 0x100002219
0000000100001ed6 mov rsi, qword [ds:0x10001f668] ; @selector(count), argument "selector" for method imp___stubs__objc_msgSend
0000000100001edd mov rdi, rbx ; argument "instance" for method imp___stubs__objc_msgSend
0000000100001ee0 call imp___stubs__objc_msgSend
0000000100001ee5 mov r14, rax
0000000100001ef0 jmp 0x100001f27
0000000100001ef2 mov rbx, qword [ds:r12+0x10] ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+530
0000000100001ef7 test rbx, rbx
0000000100001efa je 0x1000021cf
0000000100001f00 mov r14, qword [ds:rbx+0x10]
0000000100001f2f test r14, r14
0000000100001f32 je 0x1000021ca
0000000100001f38 xor eax, eax
0000000100001f3a mov r14, r15
0000000100001f3d mov qword [ss:rbp+var_68], r15
0000000100001f41 nop qword [cs:rax+rax+0x0]
0000000100001f50 mov r13, rax ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+1346
0000000100001f53 lea rax, qword [ds:r13+0x1]
0000000100001f57 mov qword [ss:rbp+var_50], rax
0000000100001f5b mov al, byte [ds:r12+0x19]
0000000100001f60 test al, al
0000000100001f62 je 0x100001fc0
0000000100001f7c mov rbx, rax
0000000100001f7f test r13, r13
0000000100001f82 js 0x100002209
0000000100001f90 mov rbx, rax
0000000100001f93 mov rdi, r12
0000000100001f96 mov r15, r12
0000000100001f99 call __TTSCC12swift_weekly14ViewController7Vehicle___TFVSs12_ArrayBufferg5countSi
0000000100001f9e mov r14, rax
0000000100001fb1 cmp r13, r14
0000000100001fb4 jl 0x10000201a
0000000100001fb6 jmp 0x100002219
0000000100001fbb nop qword [ds:rax+rax+0x0]
0000000100001fc0 mov rax, r12 ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+738
0000000100001fc3 mov r12, qword [ds:rax+0x10]
0000000100001fc7 mov r15, rax
0000000100001fca test r12, r12
0000000100001fcd je 0x100002219
0000000100001fd3 mov rax, qword [ds:r12+0x10]
0000000100001fd8 mov qword [ss:rbp+var_58], rax
0000000100001fec mov rbx, rax
0000000100002007 test r13, r13
000000010000200a js 0x100002219
0000000100002010 cmp r13, qword [ss:rbp+var_58]
0000000100002014 jge 0x100002219
000000010000201a mov qword [ss:rbp+var_58], rbx ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+820
000000010000201e lea rdi, qword [ss:rbp+var_38]
0000000100002022 mov rsi, r13
0000000100002025 mov r12, r15
0000000100002028 mov rdx, r12
000000010000202b call __TTSCC12swift_weekly14ViewController7Vehicle___TFVSs12_ArrayBufferg9subscriptFSiQ_
0000000100002030 mov r14, qword [ss:rbp+var_38]
0000000100002034 mov rdi, r14
0000000100002037 mov rsi, qword [ss:rbp+var_60]
000000010000203b call imp___stubs__swift_dynamicCastClass
0000000100002040 mov rbx, rax
0000000100002043 test rbx, rbx
0000000100002046 je 0x100002070
0000000100002048 mov qword [ss:rbp+var_48], 0xabcdefd
0000000100002058 mov r14, rax
000000010000205b lea rdi, qword [ss:rbp+var_48]
000000010000205f jmp 0x10000209b
0000000100002061 nop qword [cs:rax+rax+0x0]
0000000100002070 mov rdi, r14 ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+966
0000000100002073 mov rsi, qword [ss:rbp+var_70]
0000000100002077 call imp___stubs__swift_dynamicCastClass
000000010000207c mov rbx, rax
000000010000207f test rbx, rbx
0000000100002082 je 0x1000020f1
0000000100002084 mov qword [ss:rbp+var_40], 0xabcdefe
0000000100002094 mov r14, rax
0000000100002097 lea rdi, qword [ss:rbp+var_40]
000000010000209b call __TTSSi_VSs7_StdoutS_Ss16OutputStreamType___TFSs5printU_Ss16OutputStreamType__FTQ_RQ0__T_ ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+991
00000001000020d8 mov edi, 0xa ; argument "c" for method imp___stubs__putchar
00000001000020dd call imp___stubs__putchar
00000001000020f9 mov al, byte [ds:r12+0x19]
00000001000020fe test al, al
0000000100002100 je 0x100002160
0000000100002113 mov r14, rax
0000000100002116 cmp qword [ss:rbp+var_68], 0x0
000000010000211b je 0x100002219
0000000100002121 mov rbx, qword [ds:r12+0x10]
0000000100002126 mov r13, r12
0000000100002129 test rbx, rbx
000000010000212c je 0x100002219
0000000100002142 mov rsi, qword [ds:0x10001f668] ; @selector(count), argument "selector" for method imp___stubs__objc_msgSend
0000000100002149 mov rdi, rbx ; argument "instance" for method imp___stubs__objc_msgSend
000000010000214c call imp___stubs__objc_msgSend
0000000100002151 mov r12, rax
000000010000215c jmp 0x1000021b0
000000010000215e nop
0000000100002160 mov rbx, qword [ds:r12+0x10] ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+1152
0000000100002165 mov r13, r12
0000000100002168 test rbx, rbx
000000010000216b mov r14, qword [ss:rbp+var_58]
000000010000216f je 0x100002192
0000000100002171 mov r12, qword [ds:rbx+0x10]
0000000100002190 jmp 0x1000021b0
000000010000219a mov r14, rax
00000001000021a4 xor r12d, r12d
00000001000021a7 nop qword [ds:rax+rax+0x0]
00000001000021b8 mov rax, qword [ss:rbp+var_50]
00000001000021bc cmp rax, r12
00000001000021bf mov r12, r13
00000001000021c2 jne 0x100001f50
00000001000021c8 jmp 0x1000021e1
00000001000021ca mov r14, r15 ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+690
00000001000021cd jmp 0x1000021e1
00000001000021de mov r14, rax
00000001000021fa add rsp, 0x58
00000001000021fe pop rbx
00000001000021ff pop r12
0000000100002201 pop r13
0000000100002203 pop r14
0000000100002205 pop r15
0000000100002207 pop rbp
0000000100002208 retso let's see what happens in the loop:
-
first we get the current item our of the array:
000000010000201a mov qword [ss:rbp+var_58], rbx ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+820 000000010000201e lea rdi, qword [ss:rbp+var_38] 0000000100002022 mov rsi, r13 0000000100002025 mov r12, r15 0000000100002028 mov rdx, r12 000000010000202b call __TTSCC12swift_weekly14ViewController7Vehicle___TFVSs12_ArrayBufferg9subscriptFSiQ_
-
then it is attempting to downcast the object into an object of type
Bicycle0000000100002030 mov r14, qword [ss:rbp+var_38] 0000000100002034 mov rdi, r14 0000000100002037 mov rsi, qword [ss:rbp+var_60] 000000010000203b call imp___stubs__swift_dynamicCastClass
weird with the
r14registermovand then straight into therdiregister. this could be optimized to:mov rdi, qword [ss:rbp+var_38]
so that we could get rid of the
movtor14. do you know why swift did this? send a pull request and enhance this article. it turns out conditional downcasting is done with a different method that is used for unconditional downcasting. theimp___stubs__swift_dynamicCastClassfunction seems to return a boolean as we will see soon. -
the return value of the
imp___stubs__swift_dynamicCastClasswill then be checked as a boolean to see if the object could be dynamically downcasted toBicycle:0000000100002040 mov rbx, rax 0000000100002043 test rbx, rbx 0000000100002046 je 0x100002070
-
if the downcasting was successful, we will print our value to the console:
0000000100002048 mov qword [ss:rbp+var_48], 0xabcdefd 0000000100002058 mov r14, rax 000000010000205b lea rdi, qword [ss:rbp+var_48] 000000010000205f jmp 0x10000209b 0000000100002061 nop qword [cs:rax+rax+0x0]
the jump is obviously to a
println()statement -
if the downcasting didn't work, we jumped to
0x100002070:0000000100002070 mov rdi, r14 ; XREF=__TFC12swift_weekly14ViewController8example3fS0_FT_T_+966 0000000100002073 mov rsi, qword [ss:rbp+var_70] 0000000100002077 call imp___stubs__swift_dynamicCastClass 000000010000207c mov rbx, rax 000000010000207f test rbx, rbx 0000000100002082 je 0x1000020f1
you get the picture. basically, conditional downcasts work in this way:
- the
imp___stubs__swift_dynamicCastClassinternal function is called to attempt to downcast the value to the desired class - this function returns a bool, indicating whether the downcasting was successful or not.
- Typecasting of values in Swift is done through an internal function called
imp___stubs__swift_dynamicCast. Swift tends to typecast dynamically at runtime, rather than compile-time. This obviously has performance implications so keep that in mind. - An internal function called
__TTSSi_VSs7_StdoutS_Ss16OutputStreamType___TFSs5printU_Ss16OutputStreamType__FTQ_RQ0__T_does the work forprintln()ofIntvalues to the console. - Unconditional downcasts in Swift are done with a call to an internal function called
imp___stubs__swift_dynamicCastClassUnconditional, at runtime. - Swift tends to use an internal function called
ArrayBufferg9subscriptFSiQ_to extract objects out of an array. - Conditional downcasts are done with an internal function called
imp___stubs__swift_dynamicCastClass. - Conditional downcasts are different from unconditional ones since they also have to check the return value of the
imp___stubs__swift_dynamicCastClassfor true/false. They are therefore slower than unconditional downcasts.
- The Swift Programming Language - Type Casting
- Intel® 64 and IA-32 Architectures Software Developer’s Manual Combined Volumes: 1, 2A, 2B, 2C, 3A, 3B, and 3C
X86CallingConv.tdfile, a part of LLVM compiler's open source code- System V AMD64 ABI calling convention