Swift - Under the Hood
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The document discusses the Swift programming language, its history, and developments, presenting at the QCon London conference. It highlights Swift's performance optimizations, including memory management and static versus dynamic dispatch. The talk underscores Swift's evolution, current capabilities, and potential future directions in software development.
![@alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd
Static and Dynamic?
• Static dispatch (used by C, C++, Swift)
• Function calls are known precisely
• Compiler generates call/callq to direct symbol
• Fastest, and allows for optimisations
• Dynamic dispatch (used by Objective-C, Swift)
• Messages are dispatched through objc_msgSend
• Effectively call(cache["methodName"])
Swift can generate
Objective-C classes
and use runtime](https://image.slidesharecdn.com/untitled-150330123017-conversion-gate01/85/Swift-Under-the-Hood-9-320.jpg)
![Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd
Static Dispatch
a() -> b() -> c()
a b c
Dynamic Dispatch
[a:] -> [b:] -> [c:]
a b c
objc_msgSend objc_msgSend
Optimises
to abc
Cannot be
optimised](https://image.slidesharecdn.com/untitled-150330123017-conversion-gate01/85/Swift-Under-the-Hood-11-320.jpg)
![@alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd
Example C based IR
• The ubiquitous Hello World program…
#include <stdio.h>
!
int main() {
puts("Hello World")
}
@.str = private unnamed_addr constant [12 x i8] ⤦
c"Hello World00", align 1
!
define i32 @main() #0 {
%1 = call i32 @puts(i8* getelementptr inbounds
([12 x i8]* @.str, i32 0, i32 0))
ret i32 0
}
clang helloWorld.c -emit-llvm -c -S -o -](https://image.slidesharecdn.com/untitled-150330123017-conversion-gate01/85/Swift-Under-the-Hood-17-320.jpg)
![Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd
@.str = private unnamed_addr constant [12 x i8] ⤦
c"Hello World00", align 1
!
define i32 @main() #0 {
%1 = call i32 @puts(i8* getelementptr inbounds
([12 x i8]* @.str, i32 0, i32 0))
ret i32 0
}
clang helloWorld.c -emit-assembly -S -o -
_main
pushq %rbp
movq %rsp, %rbp
leaq L_.str(%rip), %rdi
callq _puts
xorl %eax, %eax
popq %rbp
retq
.section __TEXT
L_.str: ## was @.str
.asciz "Hello World"
stack management
rdi = &L_.str
puts(rdi)
eax = 0
return(eax)
L_.str = "Hello World"
main function](https://image.slidesharecdn.com/untitled-150330123017-conversion-gate01/85/Swift-Under-the-Hood-18-320.jpg)
![@alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd
Example Swift based IR
• The ubiquitous Hello World program…
println("Hello World")
@0 = private unnamed_addr constant [12 x i8] ⤦
c"Hello World00"
!
define i32 @main(i32, i8**) {
…
call void @_TFSs7printlnU__FQ_T_(⤦
%swift.opaque* %5, ⤦
%swift.type* getelementptr inbounds ⤦
(%swift.full_type* @_TMdSS, i64 0, i32 1))
ret i32 0
}
swiftc helloWorld.swift -emit-ir —o -](https://image.slidesharecdn.com/untitled-150330123017-conversion-gate01/85/Swift-Under-the-Hood-20-320.jpg)
Swift - Under the Hood
- 1. Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Swift Under the Hood
- 2. InfoQ.com: News & Community Site • 750,000 unique visitors/month • Published in 4 languages (English, Chinese, Japanese and Brazilian Portuguese) • Post content from our QCon conferences • News 15-20 / week • Articles 3-4 / week • Presentations (videos) 12-15 / week • Interviews 2-3 / week • Books 1 / month Watch the video with slide synchronization on InfoQ.com! http://www.infoq.com/presentations /swift-llvm
- 3. Presented at QCon London www.qconlondon.com Purpose of QCon - to empower software development by facilitating the spread of knowledge and innovation Strategy - practitioner-driven conference designed for YOU: influencers of change and innovation in your teams - speakers and topics driving the evolution and innovation - connecting and catalyzing the influencers and innovators Highlights - attended by more than 12,000 delegates since 2007 - held in 9 cities worldwide
- 4. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd ▸ About This Talk • Overview • Where did Swift come from? • What makes Swift fast? • Where is Swift going? • Alex Blewitt @alblue • NeXT owner and veteran Objective-C programmer • Author of Swift Essentials http://swiftessentials.org Based on Swift 1.1, the public release in March 2015
- 5. Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Where did Swift come from?
- 6. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Pre-history • Story starts in 1983 with Objective-C • Created as a Smalltalk like runtime on top of C • NeXT licensed Objective-C in 1988 • NextStep released in 1989 (and NS prefix) • Apple bought NeXT in 1996 • OSX Server in 1999 • OSX 10.0 Beta in 2000, released in 2001
- 7. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Objective-C • Originally implemented as a pre-processor for C • Rewrote Objective-C code as C code • Enhanced and merged into GCC • Compiler integrated under GPL • Runtime libraries open source (and GNUStep) http://www.opensource.apple.com/source/ objc4/objc4-208/runtime/objc.h /*! * Copyright (c) 1999 Apple Computer, Inc. All rights reserved.! * objc.h! * Copyright 1988-1996, NeXT Software, Inc.! */!
- 8. Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Timeline C 1972 Objective-C 1983 Smalltalk 1972 Objective-C 2.0 2007 C++ 1983 C++07 2007 C++11 2011 C++14 2014 LLVM 1.0 2003 Clang 1.0 2009 Swift 1.0 2014 Static dispatch Dynamic dispatch
- 9. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Static and Dynamic? • Static dispatch (used by C, C++, Swift) • Function calls are known precisely • Compiler generates call/callq to direct symbol • Fastest, and allows for optimisations • Dynamic dispatch (used by Objective-C, Swift) • Messages are dispatched through objc_msgSend • Effectively call(cache["methodName"]) Swift can generate Objective-C classes and use runtime
- 10. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd objc_msgSend • Every Objective-C message calls objc_msgSend • Hand tuned assembly – fast, but still overhead 40 50 60 70 80 90 100 110 Leopard Snow Leopard Lion Mountain Lion Mavericks 107 104 47 47 44 Removal of special- case GC handling CPU, registers (_cmd, self), energy🔋
- 11. Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Static Dispatch a() -> b() -> c() a b c Dynamic Dispatch [a:] -> [b:] -> [c:] a b c objc_msgSend objc_msgSend Optimises to abc Cannot be optimised
- 12. Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd What makes Swift fast?
- 13. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Memory optimisation • Contiguous arrays of data vs objects • NSArray • Diverse • Memory fragmentation • Limited memory load benefits for locality • Array<…> • Iteration is more performant over memory
- 14. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Optimisations • Most optimisations rely on inlining • Instead of a() -> b(), have ab() instead • Reduces function prologue/epilog (stack/reg spill) • Reduces branch miss and memory jumps • May unlock peephole optimisations • func foo(i:Int) {if i<0 {return}…} • foo(-1) foo(negative) can be optimised away completely Increases code size
- 15. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Link Time Optimisation • LTO performs whole-program optimisation • Instead of writing out x86 .o files, writes LLVM • LLVM linker reads all files, optimises • Can see optimisations where single file cannot • final methods and data structures can be inlined • Structs are always final (no subclassing) • private (same file) internal (same module)
- 16. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Swift and LLVM • Swift and clang are both built on LLVM • Originally stood for Low Level Virtual Machine • Family of tools (compiler, debugger, linker etc.) • Abstract assembly language • Intermediate Representation (IR), Bitcode (BC) • Infinite register RISC typed instruction set • Call and return convention agnostic Bad name, wasn't really VMs
- 17. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Example C based IR • The ubiquitous Hello World program… #include <stdio.h> ! int main() { puts("Hello World") } @.str = private unnamed_addr constant [12 x i8] ⤦ c"Hello World00", align 1 ! define i32 @main() #0 { %1 = call i32 @puts(i8* getelementptr inbounds ([12 x i8]* @.str, i32 0, i32 0)) ret i32 0 } clang helloWorld.c -emit-llvm -c -S -o -
- 18. Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd @.str = private unnamed_addr constant [12 x i8] ⤦ c"Hello World00", align 1 ! define i32 @main() #0 { %1 = call i32 @puts(i8* getelementptr inbounds ([12 x i8]* @.str, i32 0, i32 0)) ret i32 0 } clang helloWorld.c -emit-assembly -S -o - _main pushq %rbp movq %rsp, %rbp leaq L_.str(%rip), %rdi callq _puts xorl %eax, %eax popq %rbp retq .section __TEXT L_.str: ## was @.str .asciz "Hello World" stack management rdi = &L_.str puts(rdi) eax = 0 return(eax) L_.str = "Hello World" main function
- 19. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Advantages of IR • LLVM IR can still be understood when compiled • Allows for more accurate transformations • Inlining across method/function calls • Elimination of unused code paths • Optimisation phases that are language agnostic
- 20. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Example Swift based IR • The ubiquitous Hello World program… println("Hello World") @0 = private unnamed_addr constant [12 x i8] ⤦ c"Hello World00" ! define i32 @main(i32, i8**) { … call void @_TFSs7printlnU__FQ_T_(⤦ %swift.opaque* %5, ⤦ %swift.type* getelementptr inbounds ⤦ (%swift.full_type* @_TMdSS, i64 0, i32 1)) ret i32 0 } swiftc helloWorld.swift -emit-ir —o -
- 21. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Name Mangling • Name Mangling is source → assembly identifiers • C name mangling: main → _main • C++ name mangling: main → __Z4mainiPPc • __Z = C++ name • 4 = 4 characters following for name (main) • i = int • PPc = pointer to pointer to char (i.e. char**)
- 22. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Swift Name Mangling • With the Swift symbol _TFSs7printlnU__FQ_T_ • _T = Swift symbol • F = function • Ss = "Swift" (module, as in Swift.println) • 7println = "println" (function name) • U__ = single generic type argument, unbound • F = function • Q_ = generic argument • T_ = empty tuple () (return type)
- 23. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Swift Name Mangling • With the Swift symbol _TFSs7printlnU__FQ_T_ • _T = Swift symbol • F = function • Ss = "Swift" (module, as in Swift.println) • 7println = "println" (function name) • U__ = single generic type argument, unbound • F = function • Q_ = generic argument • T_ = empty tuple () (return type) $ xcrun swift-demangle _TFSs7printlnU__FQ_T_ _TFSs7printlnU__FQ_T_ ---> Swift.println <A>(A) -> ()
- 24. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Swift Intermediate Language • Similar to IL, but with some Swift specifics println("Hello World") sil_stage canonical ! import Builtin import Swift import SwiftShims ! // top_level_code sil private @top_level_code : $@thin () -> () { bb0: // function_ref Swift.println <A>(A) -> () %0 = function_ref @_TFSs7printlnU__FQ_T_ : ⤦ $@thin <τ_0_0> (@in τ_0_0) -> () // user: %9 swiftc helloWorld.swift -emit-sil —o -
- 25. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Swift vTables • Method lookup in Swift is like C++ with vTable class World { func hello() {…} } sil_stage canonical import Builtin; import Swift; import SwiftShims sil private @top_level_code : $@thin () -> () { // function_ref Swift.println <A>(A) -> () %0 = function_ref @_TFSs7printlnU__FQ_T_ : ⤦ $@thin <τ_0_0> (@in τ_0_0) -> () // user: %9 … sil_vtable World { #World.hello!1: _TFC4main5World5hellofS0_FT_T_ // main.World.hello (main.World)() -> () #World.init!initializer.1: _TFC4main5WorldcfMS0_FT_S0_ // main.World.init (main.World.Type)() -> main.World } swiftc helloWorld.swift -emit-sil —o -
- 26. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd SwiftObject and ObjC • Swift objects can also be used in Objective-C • Swift instance in memory has an isa pointer • Objective-C can call Swift code with no changes • Swift classes have @objc to use dynamic dispatch • Reduces optimisations • Automatically applied when using ObjC • Protocols, Superclasses
- 27. Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Where is Swift going?
- 28. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Is Swift swift yet? • Is Swift as fast as C? • Wrong question • Is Swift as fast, or faster than Objective-C? • Can be as fast for Objective-C interoperability • Can be faster for data/struct processing • More optimisation possibilities in future
- 29. @alblueDr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Swift • Being heavily developed – 3 releases in a year • Provides a transitional mechanism from ObjC • Existing libraries/frameworks will continue to work • Can drop down to native calls when necessary • Used as replacement language in LLDB • Future of iOS development?
- 30. Dr Alex Blewitt @alblue Swift Under the Hood ©Bandlem Ltd Q&A
- 31. Watch the video with slide synchronization on InfoQ.com! http://www.infoq.com/presentations/swift- llvm