コンパイル結果を観察してSwift の効率的なエラー処理 メカニズムを紐解く

Swift言語におけるエラー処理のメカニズムは、他のプログラミング言語と共通する命名や概念を持ちながら、その実装と性能の特性において独自のアプローチを取っています。本トークでは、Swiftのエラー処理の仕組みを具体的なコード例とそのコンパイル結果を通じて解説し、他の言語との比較を行います。

発表では、まずシンプルな例を通じて、エラー処理付きの関数がどのようにコンパイルされるかを示します。特に、throwキーワードの周辺で行われる処理、つまり通常の戻り値処理に加えたエラーオブジェクトの伝達、およびtry周辺でのエラー検知と分岐処理に焦点を当てます。

そしてコンパイル結果の観察から、Swiftが如何にして他言語と類似の構文を提供しながら、実行時の負荷を軽減しているかを明らかにします。比較対象としてJavaを挙げて、類似の構文を提供しながらもその実装方法と実行時の性能に違いがある点を簡潔に解説します。

このトークはさまざまなエラー処理方法を一通り学んだSwift中級者へ特にお勧めできるものです。
このトークを通じてtryを利用したエラー処理の性質をより深く理解し、さまざまあるエラー処理方法の中から状況に応じた適切な方法を選択できるようになると考えています。

Transcript

1. Ryu Noguchi ίϯύΠϧ݁ՌΛ؍࡯ͯ͠4XJGU
   ͷޮ཰తͳΤϥʔॲཧ
   ϝΧχζϜΛඥղ͘
2. 3ZV
   /PHVDIJ J04ΤϯδχΞ
   !SBQU@EFW
   -*/&Ϡϑʔגࣜձࣾ
   :BIPPϚοϓ
   J04։ൃ
3. 4XJGUͷΤϥʔॲཧ ͓͞Β͍ func someFunc() throws { throw SomeError() } do { try someFunc() } catch { print(error) } UISPXT
   ΤϥʔΛૹग़͢Δؔ਺ͷϚʔΫ UISPX
   ΤϥʔΛૹग़͢Δ EP
   ΤϥʔͷՄೳੑ͕͋ΔॲཧϒϩοΫ USZ
   ΤϥʔΛૹग़͢ΔࣜͷϚʔΫ DBUDI
   ΤϥʔॲཧͷϒϩοΫ
4. 4XJGUͷΤϥʔॲཧ͸ಛघ υΩϡϝϯτʹ΋ͦ͏ॻ͔Ε͍ͯΔ Error handling in Swift resembles exception handling in other languages, with the use of the try, catch and throw keywords. Unlike exception handling in many languages — including Objective-C — error handling in Swift doesn’t involve unwinding the call stack, a process that can be computationally expensive. As such, the performance characteristics of a throw statement are comparable to those of a return statement. IUUQTEPDTTXJGUPSHTXJGUCPPLEPDVNFOUBUJPOUIFTXJGUQSPHSBNNJOHMBOHVBHFFSSPSIBOEMJOH 4XJGUͰͷΤϥʔॲཧ͸USZ΍DBUDI
   UISPXͷར༻ͳͲଞͷݴޠͱࣅ͍ͯΔ͕ɺ 0CKFDUJWF$ΛؚΉଞͷݴޠͱҧͬͯ
   தུ
   ܭࢉྔͷଟ͍ॲཧΛߦΘͳ͍ɻ
   ͦͷͨΊUISPXจͷੑೳಛੑ͸SFUVSOจͷͦΕʹඖఢ͢Δɻ
5. SFUVSOจʹඖఢ
   Ͳ͏͍͏͜ͱͩ
6. ࣮ࡍʹݟͯΈΑ͏
7. αϯϓϧίʔυ struct MyError: Error {} func randomlyThrows() throws -> Int { if Double.random(in: 0..<1) < 0.5 { throw MyError() } else { return 0 } } func main() -> Int { do { try randomlyThrows() } catch { return -1 } return 0 }
8. *3·ͰίϯύΠϧ͢Δ 4*-ͱ*3ͱBTN w 4XJGUͷίʔυ͸ҎԼͷॱͰίϯύΠϧ͞ΕΔ
   ‎4XJGUதؒݴޠ
   4*-
   ‎--7.தؒදݱ
   *3
   ‎ωΠςΟϒίʔυ
   BTN
   w 4*-ͷஈ֊Ͱ͸Τϥʔ͸·ͩந৅Խ͞Εͨ··
   㾎*3Ͱ͸͡ΊͯͦͷϕʔϧͷཪଆΛ೷͚Δ --7.ɿ4XJGUίϯύΠϥͷόοΫΤϯυίϯύΠϥ BTNɿ"4TF.CMZ
   ΞηϯϒϦݴޠ
9. %swift.error = type opaque %TSd = type <{ double }> %TSnySdG = type <{ %TSd, %TSd }> %TSn = type <{}> %TSn.0 = type <{}> %swift.metadata_response = type { %swift.type*, i64 } %swift.vwtable = type { i8*, i8*, i8*, i8*, i8*, i8*, i8*, i8*, i64, i64, i32, i32 } %swift.tuple_type = type { %swift.type, i64, i8*, [0 x %swift.tuple_element_type] } %swift.tuple_element_type = type { %swift.type*, i32 } @"$sSdN" = external global %swift.type, align 8 @"$sSdSLsWP" = external global i8*, align 8 @".str.39.Range requires lowerBound <= upperBound" = private unnamed_addr constant [40 x i8] c"Range requires lowerBound <= upperBound\00" @".str.39.Swift/arm64e-apple-macos.swiftinterface" = private unnamed_addr constant [40 x i8] c"Swift/arm64e-apple- macos.swiftinterface\00" @".str.11.Fatal error" = private unnamed_addr constant [12 x i8] c"Fatal error\00" @"$sS2dSBsWL" = linkonce_odr hidden global i8** null, align 8 @"$sSdSBsMc" = external global %swift.protocol_conformance_descriptor, align 4 @"$ss6UInt64VABs17FixedWidthIntegersWL" = linkonce_odr hidden global i8** null, align 8 @"$ss6UInt64Vs17FixedWidthIntegersMc" = external global %swift.protocol_conformance_descriptor, align 4 @"$ss6UInt64VN" = external global %swift.type, align 8 @"$s19ErrorHandlingSample02MyA0VACs0A0AAWL" = linkonce_odr hidden global i8** null, align 8 @"$ss5ErrorMp" = external global %swift.protocol, align 4 @"got.$ss5ErrorMp" = private unnamed_addr constant %swift.protocol* @"$ss5ErrorMp" @"$ss5ErrorP7_domainSSvgTq" = external global %swift.method_descriptor, align 4 @"got.$ss5ErrorP7_domainSSvgTq" = private unnamed_addr constant %swift.method_descriptor* @"$ss5ErrorP7_domainSSvgTq" @"$ss5ErrorP5_codeSivgTq" = external global %swift.method_descriptor, align 4 @"got.$ss5ErrorP5_codeSivgTq" = private unnamed_addr constant %swift.method_descriptor* @"$ss5ErrorP5_codeSivgTq" @"$ss5ErrorP9_userInfoyXlSgvgTq" = external global %swift.method_descriptor, align 4 @"got.$ss5ErrorP9_userInfoyXlSgvgTq" = private unnamed_addr constant %swift.method_descriptor* @"$ss5ErrorP9_userInfoyXlSgvgTq" @"$ss5ErrorP19_getEmbeddedNSErroryXlSgyFTq" = external global %swift.method_descriptor, align 4 @"got.$ss5ErrorP19_getEmbeddedNSErroryXlSgyFTq" = private unnamed_addr constant %swift.method_descriptor* @"$ss5ErrorP19_getEmbeddedNSErroryXlSgyFTq" @"$s19ErrorHandlingSample02MyA0Vs0A0AAMcMK" = internal global [16 x i8*] zeroinitializer @"$s19ErrorHandlingSample02MyA0Vs0A0AAMc" = hidden constant { i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i16, i32, i32 } { i32 add (i32 trunc (i64 sub (i64 ptrtoint (%swift.protocol** @"got.$ss5ErrorMp" to i64), i64 ptrtoint ({ i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i16, i16, i32, i32 }* @"$s19ErrorHandlingSample02MyA0Vs0A0AAMc" to i64)) i32), i32 1), i32 trunc (i64 sub (i64 ptrtoint (<{ i32, i32, i32, i32, i32, i32, i32 }>* @"$s19ErrorHandlingSample02MyA0VMn" to
10. *3Λൈਮ ॏཁͳ෦෼͸͜͜
    
    UISPX 46: ; preds = %23 call swiftcc void @"$s19ErrorHandlingSample02MyA0VACycfC"() %47 = call i8** @"$s19ErrorHandlingSample02MyA0VACs0A0AAWl"() #10 %48 = call swiftcc { %swift.error*, %swift.opaque* } @swift_allocError(%swift.type* bitcast (i64* getelementptr inbounds (<{ i8*, i8**, i64, <{ i32, i32, i32, i32, i32, i32, i32 }>* }>, <{ i8*, i8**, i64, <{ i32, i32, i32, i32, i32, i32, i32 }>* }>* @"$s19ErrorHandlingSample02MyA0VMf", i32 0, i32 2) to %swift.type*), i8** %47, %swift.opaque* null, i1 false) #6 %49 = extractvalue { %swift.error*, %swift.opaque* } %48, 0 %50 = extractvalue { %swift.error*, %swift.opaque* } %48, 1 %51 = bitcast %swift.opaque* %50 to %T19ErrorHandlingSample02MyA0V* store %swift.error* %49, %swift.error** %1, align 8 call swiftcc void @swift_willThrow(i8* swiftself undef, %swift.error** noalias nocapture readonly swifterror dereferenceable(8) %1) #6 store %swift.error* null, %swift.error** %1, align 8 store %swift.error* %49, %swift.error** %1, align 8 ret i64 undef func randomlyThrows() throws -> Int { if Double.random(in: 0..<1) < 0.5 { throw MyError() } else { return 0 } }
11. *3Λൈਮ ॏཁͳ෦෼͸͜͜
    
    UISPX func randomlyThrows() throws -> Int { if Double.random(in: 0..<1) < 0.5 { throw MyError() } else { return 0 } } let throwingError: any Error = MyError() STORE_REGISTER(nil, SWIFT_ERROR_REGISTER) STORE_REGISTER(throwingError, SWIFT_ERROR_REGISTER) return UNDEFINED STORE_REGISTER(value, registerId): Ϩδελʹ஋Λ୅ೖ ٖࣅ4XJGUίʔυ ٖࣅؔ਺ ٖࣅఆ਺ SWIFT_ERROR_REGISTER: SwiftͰࢦఆ͞Ε͍ͯΔΤϥʔ༻Ϩδελ UNDEFINED: ࢦఆͷܕͰ͸͋Δ͕ɺ஋͕ෆఆͳ΋ͷ ΤϥʔΛࢦఆͷϨδελʹ୅ೖ ͠ɺద౰ͳ஋ΛSFUVSO͍ͯ͠Δ ͚ͩʂ
12. *3Λൈਮ ॏཁͳ෦෼͸͜͜
    
    USZ entry: %swifterror = alloca swifterror %swift.error*, align 8 store %swift.error* null, %swift.error** %swifterror, align 8 %error.debug = alloca %swift.error*, align 8 %0 = bitcast %swift.error** %error.debug to i8* call void @llvm.memset.p0i8.i64(i8* align 8 %0, i8 0, i64 8, i1 false) %1 = call swiftcc i64 @"$s19ErrorHandlingSample14randomlyThrowsSiyKF"(%swift.refcounte d* swiftself undef, %swift.error** noalias nocapture swifterror dereferenceable(8) %swifterror) %2 = load %swift.error*, %swift.error** %swifterror, align 8 %3 = icmp ne %swift.error* %2, null br i1 %3, label %8, label %4 4: ; preds = %entry %5 = phi i64 [ %1, %entry ] br label %6 8: ; preds = %entry %9 = phi %swift.error* [ %2, %entry ] store %swift.error* null, %swift.error** %swifterror, align 8 %10 = call %swift.error* @swift_errorRetain(%swift.error* %9) #6 store %swift.error* %9, %swift.error** %error.debug, align 8 call void @swift_errorRelease(%swift.error* %9) #6 call void @swift_errorRelease(%swift.error* %9) #6 br label %6 func main() -> Int { do { try randomlyThrows() } catch { return -1 } return 0 }
13. *3Λൈਮ ॏཁͳ෦෼͸͜͜
    
    USZ func main() -> Int { do { try randomlyThrows() } catch { return -1 } return 0 } _ = randomlyThrows() let thrownError = LOAD_REGISTER(SWIFT_ERROR_REGISTER) if let thrownError { return -1 } return 0 ٖࣅ4XJGUίʔυ LOAD_REGISTER(registerId): Ϩδελͷ஋Λऔಘ ٖࣅؔ਺ ࢦఆͷϨδελʹΤϥʔ͕ೖͬ ͍ͯͨΒΤϥʔॲཧΛ࣮ߦ
14. *3ΛݟͯΘ͔ͬͨ͜ͱ UISPXͱUSZͷϕʔϧͷཪଆ w UISPX
    ˠ
    ΤϥʔΛϨδελʹೖΕͯద౰ͳ஋ΛSFUVSO͢Δ
    w USZ
    ˠ
    ϨδελʹΤϥʔ͕ೖ͍ͬͯΕ͹ΤϥʔॲཧΛ࣮ߦ͢Δ ͍ͣΕ΋ܰྔ
    ͔ͩΒSFUVSOʹඖఢ͢Δͱॻ͍͍ͯΔʂ
15. ଞͷݴޠ͸Ͳ͏ͩΖ͏ʁ +BWBͱൺֱ IUUQTEPDTPSBDMFDPNKBWBTFUVUPSJBMFTTFOUJBMFYDFQUJPOTEF fi OJUJPOIUNM When an error occurs within a method, the method creates an object and hands it off to the runtime system. … The runtime system searches the call stack for a method that contains a block of code that can handle the exception. … When an appropriate handler is found, the runtime system passes the exception to the handler. Τϥʔ͕ൃੜ͢ΔͱϥϯλΠϜʹΤϥʔΦϒδΣΫτ͕౉͞ΕΔɻ தུ
    ϥϯ λΠϜ͸ίʔϧελοΫΛḪΓɺΤϥʔॲཧͰ͖Δద੾ͳϒϩοΫΛݕࡧ͠ɺ தུ
    ݟ͔ͭΕ͹ͦͷϒϩοΫʹྫ֎
    ΤϥʔΦϒδΣΫτ
    Λ౉͢ɻ DBUDIՄೳͳϒϩοΫΛݕࡧ͢ΔϥϯλΠϜΦʔόʔϔου͕͋Δʂ
16. ·ͱΊ ͜ͷௐ͔ࠪΒԿΛֶ΂Δ͔ w 4XJGUͷΤϥʔॲཧ͸୯७ͳSFUVSOͱJGจʹల։͞ΕΔͷͰߴ଎
    w +BWBͰ͸ϥϯλΠϜ͕հೖ͢Δඞཁ͕͋Γ௿଎ʹͳΓ΍͍͢
    wϝϦοτ΋͋ΔͷͰҰ֓ʹѱ͍ͱ͸͍͑ͳ͍
    w Ωʔϫʔυ͕ࣅ͍ͯͯ΋ผ෺ͳͷͰੑೳ໘ͰUSZΛආ͚Δඞཁ͸ͳ͍ʂ
    w USZΛ׆༻͍ͯ͜͠͏ʂ
17. 1MVT
    6MUSB ؾʹͳΔ͋Ε͜Ε w 5ZQFEUISPXTͩͱͲ͏ͳΔͷʁ
    ˠ
    ࡉ͔͍ڍಈ͸มΘΓ·͕͢େ࿮͸Ұॹ
    w DBUDIͰ࠶౓UISPX͢Δͱʁ
    ˠ
    มΘΒͣΤϥʔΛϨδελʹೖΕͯSFUVSO
    w Ϋϩʔδϟͷ৔߹͸ʁ
    ˠ
    ී௨ͷؔ਺ͱಉ͡Α͏ʹ࣮ݱ
    w ύλʔϯࢦఆͷ͋ΔDBUDIͷ৔߹͸ʁ
    ˠ
    ॱ൪ʹύλʔϯϚον "TL
    UIF
    4QFBLFSͰޠΓ߹͍·͠ΐ͏ʂʂʂ ଞʹ΋ؾʹͳΔʁ
18. 3ZV
    /PHVDIJ J04ΤϯδχΞ
    !SBQU@EFW
    -*/&Ϡϑʔגࣜձࣾ
    :BIPPϚοϓ
    J04։ൃ