Utilizing LLVM LTO for Swift

わいわいswiftc #34 オンライン
https://iosdiscord.connpass.com/event/239673/

Transcript

1. Utilizing LLVM LTO for Swift
   Experimental Hermetic seal at link
   Θ͍Θ͍swiftc #34
   @kateinoigakukun
   1
   

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2. ۙگ
   • લճͷొஃ͸൒೥લ
   !
   ʢ2021೥9݄ʣ
   •
   "
   Swiftίϛολʹͳͬͨ
   •
   "
   Rubyίϛολʹ΋ͳͬͨ
   2
   

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3. SwiftίϯύΠϥͷLTOʹਐḿ͕͋ͬͨ
   3
   

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4. Swiftͷ࠷దԽύε
   • SIL Optimizer
   • Thin Cross Module Optimizer (a.k.a Swift LTO)
   • LLVM Optimizer
   • LLVM LTO/ThinLTO
   • ࠓ೔͸ίί
   4
   

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5. ௨ৗͷϦϯΫ
   5
   

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6. Link Time OptimizationʢLTOʣ
   6
   

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7. Link Time OptimizationʢLTOʣ
   • ϦϯΧͷதͰ࠷దԽ
   • ίϯύΠϥͰ͸ग़དྷͳ͍ɺ຋༁୯ҐΛލ͍ͩ࠷దԽ͕Ͱ͖Δ
   • ຋༁୯Ґ: 1ΦϒδΣΫτϑΝΠϧͷੜ੒ݩͱͳΔιʔε
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8. SwiftͱLLVM LTOͷาΈ
   • 2020: LLVM LTOͷ༗ޮԽ by katei
   • 2021~: Hermetic seal at link by kubamracek
   • LLVM LTOͱSwiftͷ࿈ܞڧԽ
   8
   

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9. Ϟνϕʔγϣϯ
   Ͳ͏ͯ͠LTO͕ඞཁͳͷ͔ʁʢਪఆʣ
   • Ͳ͏΍ΒFreestanding؀ڥͷαϙʔτΛ ͰਐΊ͍ͯΔ༷ࢠ
   • Freestanding: OSແ͠ͷ؀ڥʢCɺC++ͷ༻ޠΒ͍͠ʣ1
   • ΧʔωϧͷதͰSwift͕ಈ͘ʁ
   1 https://ja.wikipedia.org/wiki/ϑϦʔελϯσΟϯά؀ڥ
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10. Ϟνϕʔγϣϯ
    • OS͕ແ͍ͷͰɺಈతϦϯΫ͕Ͱ͖ͳ͍
    • stdlib΋੩తϦϯΫ
    • αΠζ͕ංେԽͯ͠ࠔͬͨ
    • LTOͳΒstdlibͷαΠζ΋ݮΒͤΔʂ
    10
    

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11. LLVM LTO
    11
    

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12. LLVM LTOͰग़དྷΔ࠷దԽ
    DFEͱVFE͸αΠζʹΑ͘ޮ͘
    !
    • Dead Function Elimination (DFE)
    • Virtual Function Elimination (VFE)
    • Whole Program Devirtualization
    • Inlining
    • etc...
    12
    

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13. Dead Function Elimination
    • Ͳ͔͜Β΋ʮ࢖༻ʯ͞Ε͍ͯͳ͍ؔ਺Λ࡟আ͢Δ
    • ຋༁୯ҐͰ΋ద༻͞ΕͯΔ
    • ՄࢹੑʹΑͬͯ͸ɺϦϯΫ࣌ʹશͯͷʮ࢖༻ʯ͕ݟ͑Δ͜ͱ
    ͕อূ͞ΕΔͷͰɺফͤΔՄೳੑ͕૿͑Δ
    13
    

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14. ୯७ͳέʔε
    // LibX.swift
    public func unusedFunc() {} // ফͤΔ
    public func usedFunc() {}
    // main.swift
    import LibX
    usedFunc()
    14
    

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15. ୯७ͳέʔε
    # LTOͳ͠
    $ swiftc LibX.swift -emit-library -static -emit-module
    $ swiftc main.swift -I. -L. -lLibX
    $ nm main | swift demangle
    0000000100003fa0 T LibX.unusedFunc() -> ()
    0000000100003fa4 T LibX.usedFunc() -> ()
    0000000100003fac s ___swift_reflection_version
    0000000100000000 T __mh_execute_header
    0000000100003f88 T _main
    # LTO͋Γ
    $ swiftc LibX.swift -lto=llvm-thin -Xfrontend -internalize-at-link -emit-library -static -emit-module
    $ swiftc main.swift -lto=llvm-thin -Xfrontend -internalize-at-link -I. -L. -lLibX
    $ nm main | swift demangle
    0000000100003fac T LibX.usedFunc() -> ()
    0000000100003fb0 s ___swift_reflection_version
    0000000100000000 T __mh_execute_header
    0000000100003fa4 T _main
    15
    

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16. DFEͰফͤΔ৚݅
    γϯϘϧX͕ফͤΔ৚݅
    • ֎෦͔Βݟ͑ΔγϯϘϧ͔ΒɺXͷʮ࢖༻ʯʹ౸ୡ͠ͳ͍
    • Xͷʮ࢖༻ʯ͕શͯݟ͑Δ͜ͱ͕อূͰ͖Δ
    16
    

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17. ʮ࢖༻ʯ͕શͯݟ͑ΔϦϯέʔδ
    • linkonce, internal, private, available_externally 2
    /// Whether the definition of this global may be discarded if it is not used
    /// in its compilation unit.
    static bool isDiscardableIfUnused(LinkageTypes Linkage) {
    return isLinkOnceLinkage(Linkage) || isLocalLinkage(Linkage) ||
    isAvailableExternallyLinkage(Linkage);
    }
    2 https://github.com/llvm/llvm-project/blob/62bcfcb5a588e5e844f8e4e42a2e4d15c907a746/llvm/include/llvm/IR/
    GlobalValue.h#L369-L374
    17
    

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18. linkonce: Thunkؔ਺ͳͲʢγάωνϟ͕ಉ͡ͳΒ࢖͍ճͤ
    Δʣ
    • ΋͠֎෦Ͱ࢖ΘΕ͍ͯͯ΋ɺ࢖͏ଆ͕ग़ྗ͢ΔͷͰɺ࢖ΘΕ
    ͍ͯͳ͍࠷దԽ୯ҐͰ͸ফͯ͠OK
    18
    

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19. internal, private: ಉ͡LLVM module಺ͰͷΈ࢖༻Մೳ
    • ͍͍ͩͨSwiftͷprivateؔ਺
    19
    

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20. available_externally: ΠϯϥΠϯԽͷͨΊʹ֎෦͔Β࣋ͪग़
    ͞Εͨؔ਺ఆٛ
    • ͦ΋ͦ΋ΦϒδΣΫτϑΝΠϧʹग़ྗ͞Εͳ͍΋ͷͳͷͰɺ
    ࢖ΘΕͯແͯ͘ɺ࠷దԽνϟϯε͕ແ͚Ε͹ফͯ͠OK
    20
    

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21. LTOͰϦϯέʔδΛڱΊΒΕΔέʔε
    • LTO࣌ʹɺϏοτίʔυԽ͞Ε͍ͯͳ͍֎ଆͷΦϒδΣΫτ
    ϑΝΠϧ͕ཁٻ͢ΔγϯϘϧ͕෼͔Δ
    • ֎ଆͷΦϒδΣΫτϑΝΠϧ͕ཁٻ͍ͯ͠ͳ͚Ε͹ɺ
    internalʹͰ͖Δɻ
    21
    

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22. DFEͰফͤͳ͍έʔε
    // LibX.swift
    public class A {
    public func unusedFunc() {} // ফͤͳ͍
    public func usedFunc() {}
    public init() {}
    }
    public class B : A {
    override func usedFunc() {}
    }
    // main.swift
    import LibX
    let a: A = B()
    a.usedFunc()
    22
    

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23. DFEͰফͤͳ͍έʔε
    $ swiftc -lto=llvm-thin -Xfrontend -internalize-at-link -emit-library -static -emit-module LibX.swift
    $ swiftc -lto=llvm-thin -Xfrontend -internalize-at-link main.swift -I. -L. -lLibX
    $ nm main | swift demangle
    0000000100003eac t LibX.A.unusedFunc() -> ()
    0000000100003eb0 t LibX.A.usedFunc() -> ()
    0000000100003eb4 T LibX.A.__allocating_init() -> LibX.A
    0000000100003ec4 T LibX.A.init() -> LibX.A
    23
    

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24. Ͳ͏ͯ͠ফͤͳ͍ʁ
    • Type DescriptorͱType MetadataͷVTable͔Βࢀর͞ΕͯΔ
    ͔Β
    24
    

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25. ͓͞Β͍ɿ VTable
    class A {
    func foo() {}
    func bar() {}
    }
    class B : A {
    override func bar() {}
    }
    class C : B {
    override func foo() {}
    func fizz() {}
    }
    ಈతϙϦϞʔϑΟζϜΛ࣮ݱ
    ͢ΔͨΊͷσʔλߏ଄
    Class Slot[0] Slot[1] Slot[2]
    A A.foo A.bar
    B A.foo B.bar
    C C.foo B.bar C.fizz
    25
    

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26. VTable in Type Metadata
    ; full type metadata for LibX.A
    @"$s4LibX1ACMf" = internal global <{ ... }> <{
    ; A.__deallocating_deinit
    void (%T4LibX1AC*)* @"$s4LibX1ACfD",
    ; value witness table for Builtin.NativeObject
    i8** @"$sBoWV",
    ; MetadataKind::Class
    i64 0,
    ; superclass
    %swift.type* null,
    ; ClassFlags::UsesSwiftRefcounting, InstanceAddressPoint
    i32 2, i32 0,
    ; InstanceSize, InstanceAlignMask, RuntimeReservedBits
    i32 16, i16 7, i16 0,
    ; ClassSize, ClassAddressPoint
    i32 96, i32 16,
    ; nominal type descriptor for LibX.A
    <{ ... }>* @"$s4LibX1ACMn",
    ; IVarDestroyer
    i8* null,
    ; VTable[0]: LibX.A.unusedFunc() -> ()
    void (%T4LibX1AC*)* @"$s4LibX1AC10unusedFuncyyF",
    ; VTable[1]: LibX.A.usedFunc() -> ()
    void (%T4LibX1AC*)* @"$s4LibX1AC8usedFuncyyF",
    ; VTable[2]: LibX.A.__allocating_init() -> LibX.A
    %T4LibX1AC* (%swift.type*)* @"$s4LibX1ACACycfC"
    }>, align 8
    26
    

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27. VTable in Type Descriptor
    ; nominal type descriptor for LibX.A
    @"$s4LibX1ACMn" = constant <{ ... }> <{
    ; ContextDescriptorFlags
    i32 -2147483568,
    ; Parent Descriptor: module descriptor LibX
    i32 cast_relptr <{ ... }> @"$s4LibXMXM",
    ; Name: "A\00"
    i32 cast_relptr [2 x i8] @.str.1,
    ; type metadata accessor for LibX.A
    i32 cast_relptr %swift.metadata_response (i64)* @"$s4LibX1ACMa",
    ; reflection metadata field descriptor LibX.A
    i32 cast_relptr { i32, i32, i16, i16, i32 }* @"$s4LibX1ACMF",
    ; SuperclassType, MetadataNegativeSizeInWords
    i32 0, i32 2,
    ; MetadataPositiveSizeInWords, NumImmediateMembers
    i32 10, i32 3,
    ; NumFields, FieldOffsetVectorOffset
    i32 0, i32 7,
    ; VTable Offset, VTable Size
    i32 7, i32 3,
    ; VTable
    %swift.method_descriptor {
    ; Kind::Method, LibX.A.unusedFunc() -> ()
    i32 16, i32 cast_relptr void (%T4LibX1AC*)* @"$s4LibX1AC10unusedFuncyyF",
    },
    %swift.method_descriptor {
    ; Kind::Method, LibX.A.usedFunc() -> ()
    i32 16, i32 cast_relptr void (%T4LibX1AC*)* @"$s4LibX1AC8usedFuncyyF",
    },
    %swift.method_descriptor {
    ; Kind::Init, LibX.A.__allocating_init() -> LibX.A
    i32 1, i32 cast_relptr void (%T4LibX1AC*)* @"$s4LibX1ACACycfC",
    }
    }>, section "__TEXT,__const", align 4
    • ؔ਺ϙΠϯλ͕૬ରϙΠϯ
    λͱͯ͠ຒΊࠐ·ΕͯΔ
    27
    

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28. Virtual Function Elimination
    • 2019೥10݄ʹLLVMʹಋೖ3
    • ݁ߏ࠷ۙ
    !
    • VTableͷߏ଄ʹLLVMϝλσʔλΛ෇͚ͯɺ
    ࢖ΘΕ͍ͯͳ͍εϩοτͷϝιουΛ࡟আ
    • ΋ͱ΋ͱ͸C++޲͚
    3 https://reviews.llvm.org/D63932
    28
    

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29. VTableϝλσʔλ૷০ͷྫ ʢItanium C++ ABIʣ
    • લఏ: γϯϘϧ͸hidden4
    • ݺ͹ΕΔՄೳੑ͕͋Δͷ͸
    B::fooͱA::bar͚ͩ
    Class Slot[0] Slot[1]
    A A::foo A::bar
    B B::foo A::bar
    struct A {
    A() = default;
    virtual int foo(int);
    virtual int bar(float);
    };
    struct B : A {
    B() = default;
    virtual int foo(int);
    };
    int A::foo(int) { return 1; }
    int A::bar(float) { return 2; }
    int B::foo(int) { return 3; }
    extern "C" int test(B *p) {
    return p->foo(42);
    }
    extern "C" int test2(A *p) {
    return p->bar(24);
    }
    4 ίϚϯυ clang++ -cc1 -flto -flto-unit -fvirtual-function-elimination -fwhole-program-vtables -fvisibility hidden
    29
    

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30. !type͸ޓ׵ͳVTableΛදݱ
    ; vtable for A
    @_ZTV1A = dso_local unnamed_addr constant { [4 x i8*] } { [4 x i8*] [
    ; [ 0]
    i8* null,
    ; [ 8] typeinfo for A
    i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*),
    ; [16] A::foo(int)
    i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A3fooEi to i8*),
    ; [24] A::bar(float)
    i8* bitcast (i32 (%struct.A*, float)* @_ZN1A3barEf to i8*)
    ] }, align 8, !type !0
    ; { vtable offset, typeid }
    !0 = !{i64 16, !"_ZTS1A" }
    30
    

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31. BͷVTable͸AͱBͷVTableͱޓ׵
    ; vtable for B
    @_ZTV1B = dso_local unnamed_addr constant { [4 x i8*] } { [4 x i8*] [
    ; [ 0]
    i8* null,
    ; [ 8] typeinfo for B
    i8* bitcast ({ i8*, i8*, i8* }* @_ZTI1B to i8*),
    ; [16] B::foo(int)
    i8* bitcast (i32 (%struct.B*, i32)* @_ZN1B3fooEi to i8*),
    ; [24] A::bar(float)
    i8* bitcast (i32 (%struct.A*, float)* @_ZN1A3barEf to i8*)
    ] }, align 8, !type !0, !type !1
    ; { vtable offset, typeid }
    !0 = !{i64 16, !"_ZTS1A" }
    !1 = !{i64 16, !"_ZTS1B" }
    31
    

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33. @llvm.type.checked.load(i8* %vtbl, i32 0, metadata !"_ZTS1B")
    ͰBޓ׵VTableͷΦϑηοτ16 + 0ͷεϩοτ(foo)Λϩʔυ
    define hidden i32 @test(%struct.B* %p) #0 {
    entry:
    %p.addr = alloca %struct.B*, align 8
    store %struct.B* %p, %struct.B** %p.addr, align 8
    %0 = load %struct.B*, %struct.B** %p.addr, align 8
    %1 = bitcast %struct.B* %0 to i32 (%struct.B*, i32)***
    %vtable = load i32 (%struct.B*, i32)**, i32 (%struct.B*, i32)*** %1, align 8
    %2 = bitcast i32 (%struct.B*, i32)** %vtable to i8*
    %3 = call { i8*, i1 } @llvm.type.checked.load(i8* %2, i32 0, metadata !"_ZTS1B")
    %4 = extractvalue { i8*, i1 } %3, 1
    %5 = extractvalue { i8*, i1 } %3, 0
    %6 = bitcast i8* %5 to i32 (%struct.B*, i32)*
    %call = call i32 %6(%struct.B* nonnull dereferenceable(8) %0, i32 42)
    ret i32 %call
    }
    33
    

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34. @llvm.type.checked.load(i8* %vtbl, i32 0, metadata !"_ZTS1B")
    ͰAޓ׵VTableͷΦϑηοτ16 + 8ͷεϩοτ(bar)Λϩʔυ
    define hidden i32 @test2(%struct.B* %p) #0 {
    entry:
    %p.addr = alloca %struct.B*, align 8
    store %struct.B* %p, %struct.B** %p.addr, align 8
    %0 = load %struct.B*, %struct.B** %p.addr, align 8
    %1 = bitcast %struct.B* %0 to %struct.A*
    %2 = bitcast %struct.A* %1 to i32 (%struct.A*, float)***
    %vtable = load i32 (%struct.A*, float)**, i32 (%struct.A*, float)*** %2, align 8
    %3 = bitcast i32 (%struct.A*, float)** %vtable to i8*
    %4 = call { i8*, i1 } @llvm.type.checked.load(i8* %3, i32 8, metadata !"_ZTS1A")
    %5 = extractvalue { i8*, i1 } %4, 1
    %6 = extractvalue { i8*, i1 } %4, 0
    %7 = bitcast i8* %6 to i32 (%struct.A*, float)*
    %call = call i32 %7(%struct.A* nonnull dereferenceable(8) %1, float 2.400000e+01)
    ret i32 %call
    }
    34
    

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35. • ݺͼग़͠ʹ࢖ΘΕ͏Δ
    VTableͱͦͷεϩοτ͕੩
    తʹ෼͔Δ
    • ࢖ΘΕͳ͍VTableͷεϩο
    τΛnullptrʹஔ׵Ͱ͖Δ
    • ࠓճ͸ᶄͱᶅ͚͕ͩ࢖ΘΕ
    ͯΔͷͰɺA::foo͕ফͤΔ
    35
    

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36. Hermetic seal at linkʢLLVM LTOͱSwiftͷ࿈ܞڧԽʣ
    Swiftͷؔ਺ςʔϒϧʹLLVMϝλσʔλΛ༩͑Ε͹ɺC++ͱಉ༷
    ʹςʔϒϧ͔ΒͷࢀরΛফͤΔ
    • VFE: Virtual Function Elimination
    • WME: Witness Method Elimination
    36
    

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37. Virtual Function Elimination for Swift
    https://github.com/apple/swift/pull/39128
    • -enable-llvm-vfe
    • VTableܦ༝ͷϝιουσΟεύονʹ
    @llvm.type.checked.loadΛ࢖͏Α͏ʹ
    • Type MetadataͱType DescriptorΛVTableɺ
    ϕʔεϝιουΛtypeidͱͯ͠!typeϝλσʔλΛ௥Ճ
    37
    

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38. LLVMʹඞཁͳมߋ
    VTableͷΤϯτϦͱͯ͠૬ରϙΠϯλΛαϙʔτ
    • https://reviews.llvm.org/D107645
    • https://reviews.llvm.org/D109114
    • https://reviews.llvm.org/D109169
    ΞυϗοΫʹ૬ରϙΠϯλͷύλʔϯΛղऍ
    @symbol = ... {
    i32 trunc (i64 sub (i64 ptrtoint ( @target to i64), i64 ptrtoint (... @symbol to i64)) to i32)
    }
    38
    

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39. LLVMʹඞཁͳมߋ
    VTableͷؔ਺ΤϯτϦͷൣғΛࢦఆͰ͖Δ
    !vcall_visibility֦ு
    • https://reviews.llvm.org/D108741
    • SwiftͷType Descriptorʹ͸ϝλσʔλΞΫηαͳͲɺVFEͰ
    ফͪ͠Ό͍͚ͳ͍ؔ਺͕ೖ͍ͬͯΔͨΊ
    • C++ͷVTable͸ଞͷϝλσʔλ͔Βಠཱͨ͠ςʔϒϧͰɺ
    ೖ͍ͬͯΔؔ਺ϙΠϯλ͸શͯVFEͰফͤͨ
    39
    

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40. ద༻ͯ͠ΈΔ
    class A {
    func unusedFunc() {}
    func usedFunc() {}
    init() {}
    }
    class B : A {
    override func usedFunc() {}
    }
    func test() {
    let a: A = B()
    a.usedFunc()
    }
    $ swiftc -emit-ir LibX.swift \
    -Xfrontend -disable-objc-interop \
    -Xfrontend -enable-llvm-vfe
    40
    

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41. ; nominal type descriptor for LibX.A
    @"$s4LibX1ACMn" = constant <{ ... }> <{
    ; ...
    ; VTable
    %swift.method_descriptor {
    ; [52] Kind::Method, [56] LibX.A.unusedFunc() -> ()
    i32 16, i32 cast_relptr void (%T4LibX1AC*)* @"$s4LibX1AC10unusedFuncyyF",
    },
    %swift.method_descriptor {
    ; [60] Kind::Method, [64] LibX.A.usedFunc() -> ()
    i32 16, i32 cast_relptr void (%T4LibX1AC*)* @"$s4LibX1AC8usedFuncyyF",
    },
    %swift.method_descriptor {
    ; [68] Kind::Init, [72] LibX.A.__allocating_init() -> LibX.A
    i32 1, i32 cast_relptr void (%T4LibX1AC*)* @"$s4LibX1ACACycfC",
    }
    }>, section "__TEXT,__const", align 4, !type !0, !type !1, !type !2, !vcall_visibility !3
    ; method descriptor for LibX.A.unusedFunc() -> ()
    !0 = !{i64 56, !"$s4LibX1AC10unusedFuncyyFTq"}
    ; method descriptor for LibX.A.usedFunc() -> ()
    !1 = !{i64 64, !"$s4LibX1AC8usedFuncyyFTq"}
    ; method descriptor for LibX.A.__allocating_init() -> LibX.A
    !2 = !{i64 72, !"$s4LibX1ACACycfCTq"}
    ; { VCallVisibilityPublic, rangeStart, rangeEnd }
    !3 = !{i64 0, i64 56, i64 76}
    41
    

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42. ; full type metadata for LibX.A
    @"$s4LibX1ACMf" = internal global <{ ... }> <{
    ; ...
    ; [56] nominal type descriptor for LibX.A
    <{ ... }>* @"$s4LibX1ACMn",
    ; [64] IVarDestroyer
    i8* null,
    ; [72] VTable[0]: LibX.A.unusedFunc() -> ()
    void (%T4LibX1AC*)* @"$s4LibX1AC10unusedFuncyyF",
    ; [80] VTable[1]: LibX.A.usedFunc() -> ()
    void (%T4LibX1AC*)* @"$s4LibX1AC8usedFuncyyF",
    ; [88] VTable[2]: LibX.A.__allocating_init() -> LibX.A
    %T4LibX1AC* (%swift.type*)* @"$s4LibX1ACACycfC"
    }>, align 8, !type !5, !type !6, !type !7, !vcall_visibility !8
    ; method descriptor for LibX.A.unusedFunc() -> ()
    !5 = !{i64 72, !"$s4LibX1AC10unusedFuncyyFTq"}
    ; method descriptor for LibX.A.usedFunc() -> ()
    !6 = !{i64 80, !"$s4LibX1AC8usedFuncyyFTq"}
    ; method descriptor for LibX.A.__allocating_init() -> LibX.A
    !7 = !{i64 88, !"$s4LibX1ACACycfCTq"}
    ; { VCallVisibilityPublic, rangeStart, rangeEnd }
    !8 = !{i64 0, i64 72, i64 92}
    42
    

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43. typeid=$s4LibX1AC8usedFuncyyFTqͷςʔϒϧͷΦϑηοτ0ͷεϩοτΛϩʔυ
    ʢΦϑηοτ0͸ݻఆʣ
    define hidden swiftcc void @"$s4LibX4testyyF"() #0 {
    entry:
    %a.debug = alloca %T4LibX1AC*, align 8
    %0 = bitcast %T4LibX1AC** %a.debug to i8*
    call void @llvm.memset.p0i8.i64(i8* align 8 %0, i8 0, i64 8, i1 false)
    %1 = call swiftcc %swift.metadata_response @"$s4LibX1BCMa"(i64 0) #8
    %2 = extractvalue %swift.metadata_response %1, 0
    %3 = call swiftcc %T4LibX1BC* @"$s4LibX1BCACycfC"(%swift.type* swiftself %2)
    %4 = bitcast %T4LibX1BC* %3 to %T4LibX1AC*
    store %T4LibX1AC* %4, %T4LibX1AC** %a.debug, align 8
    %5 = getelementptr inbounds %T4LibX1AC, %T4LibX1AC* %4, i32 0, i32 0, i32 0
    %6 = load %swift.type*, %swift.type** %5, align 8
    %7 = bitcast %swift.type* %6 to void (%T4LibX1AC*)**
    %8 = getelementptr inbounds void (%T4LibX1AC*)*, void (%T4LibX1AC*)** %7, i64 8
    %9 = bitcast void (%T4LibX1AC*)** %8 to i8*
    ; method descriptor for LibX.A.usedFunc() -> ()
    %10 = call { i8*, i1 } @llvm.type.checked.load(i8* %9, i32 0, metadata !"$s4LibX1AC8usedFuncyyFTq")
    %11 = extractvalue { i8*, i1 } %10, 0
    %12 = bitcast i8* %11 to void (%T4LibX1AC*)*
    call swiftcc void %12(%T4LibX1AC* swiftself %4)
    call void bitcast (void (%swift.refcounted*)* @swift_release to void (%T4LibX1AC*)*)(%T4LibX1AC* %4) #2
    ret void
    }
    43
    

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44. View Slide

45. DFEͰফͤͳ͔ͬͨέʔεΛVFEʹ͔͚ͯΈΔ
    unusedFunc ফ͑ͯͨ ✌
    $ swiftc -lto=llvm-full \
    -Xfrontend -internalize-at-link \
    -Xfrontend -enable-llvm-vfe \
    -emit-library -static -emit-module LibX.swift
    $ swiftc -lto=llvm-full \
    -Xfrontend -internalize-at-link \
    -Xfrontend -enable-llvm-vfe \
    -I. -L. -lLibX main.swift
    $ nm main | swift demangle
    0000000100003ed4 t LibX.A.usedFunc() -> ()
    0000000100003ed8 t LibX.A.__allocating_init() -> LibX.A
    0000000100003f9c s reflection metadata field descriptor LibX.A
    0000000100003eb8 t type metadata accessor for LibX.A
    ...
    45
    

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46. Witness Method Elimination
    https://github.com/apple/swift/pull/39287
    • -enable-llvm-wme
    • WitnessςʔϒϧͷΤϯτϦ΋VTableͱಉ༷ʹϚʔΩϯά
    • Witness MethodΛtypeidͱ͢Δ
    46
    

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47. WMEͰফͤΔέʔε
    // LibX.swift
    public protocol TheProtocol {
    func unusedFunc()
    func usedFunc()
    }
    public struct A : TheProtocol {
    public func unusedFunc() {} // ফ͑Δ
    !
    public func usedFunc() {}
    public init() {}
    }
    // main.swift
    import LibX
    let a: TheProtocol = A()
    // @llvm.type.checked.load(
    // i8* %usedFuncSlot, i32 0,
    // ; method descriptor for LibX.TheProtocol.usedFunc() -> ()
    // metadata !"$s4LibX11TheProtocolP8usedFuncyyFTq"
    // )
    a.usedFunc()
    47
    

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48. ; protocol witness table for LibX.A : LibX.TheProtocol in LibX
    @"$s4LibX1AVAA11TheProtocolAAWP" = constant [3 x i8*] [
    ; [ 0] protocol conformance descriptor for LibX.A : LibX.TheProtocol in LibX
    i8* bitcast (%swift.protocol_conformance_descriptor* @"$s4LibX1AVAA11TheProtocolAAMc" to i8*),
    ; [ 8] protocol witness for LibX.TheProtocol.unusedFunc() -> () in conformance LibX.A : LibX.TheProtocol in LibX
    i8* bitcast (void (%T4LibX1AV*, %swift.type*, i8**)* @"$s4LibX1AVAA11TheProtocolA2aDP10unusedFuncyyFTW" to i8*),
    ; [16] protocol witness for LibX.TheProtocol.usedFunc() -> () in conformance LibX.A : LibX.TheProtocol in LibX
    i8* bitcast (void (%T4LibX1AV*, %swift.type*, i8**)* @"$s4LibX1AVAA11TheProtocolA2aDP8usedFuncyyFTW" to i8*)
    ], align 8, !type !0, !type !1, !vcall_visibility !2, !typed_global_not_for_cfi !3
    ; method descriptor for LibX.TheProtocol.unusedFunc() -> ()
    !0 = !{i64 8, !"$s4LibX11TheProtocolP10unusedFuncyyFTq"}
    ; method descriptor for LibX.TheProtocol.usedFunc() -> ()
    !1 = !{i64 16, !"$s4LibX11TheProtocolP8usedFuncyyFTq"}
    ; { VCallVisibilityLinkageUnit, rangeStart, rangeEnd }
    !2 = !{i64 1, i64 8, i64 20 }
    48
    

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49. ࠷దԽͱফͤΔର৅
    ࠷దԽ ফͤΔର৅
    DFE ؔ਺, staticؔ਺, finalϝιου
    VFE Ϋϥεͷϝιου
    WME ϓϩτίϧͷϝιου
    49
    

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50. -experimental-hermetic-seal-at-link
    • Virtual Method EliminationɺWitness Method Eliminationɺ
    +α
    ͕༗ޮʹͳΔΦϓγϣϯ
    • ͜ͷΦϓγϣϯͰϏϧυ͞ΕͨϞδϡʔϧͷʮ࢖༻ʯ͸
    ϦϯΫ࣌ʹશͯݟ͍͑ͯΔඞཁ͕͋Δɻ
    • Ϗϧυ͞ΕͨϞδϡʔϧ͸ɺ-experimental-hermetic-
    seal-at-linkΛ෇͚͔ͨ࣌͠importͰ͖ͳ͍ɻ
    50
    

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51. ϕϯνϚʔΫ
    SwiftyJSONͰܭଌ
    51
    

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52. ϕϯνϚʔΫ
    ʢࢀߟ஋ʣSwift LTOͷϕϯνϚʔΫʢ౰࣌ʣ for SwiftyJSON
    52
    

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53. ϦϯΫ
    • Add a 'standalone_minimal' preset to build a minimal, static,
    OS independent, self-contained binaries of stdlib. by
    kubamracek · Pull Request #33286 · apple/swift
    • Implement LLVM IR Virtual Function Elimination for Swift
    classes. by kubamracek · Pull Request #39128 · apple/swift
    • Implement LLVM IR Witness Method Elimination for Swift
    witness tables. by kubamracek · Pull Request #39287 ·
    53
    

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