Swift compiler 101 & How async function works

The introduction of the Swift compiler and an explanation of how it implements asynchronous functions

Transcript

1. 2025/01/17 w a iw a i swiftc shiz Swift compiler 101 How async function works &
2. Self-introduction •shiz(sizu) X: @stzn3 GitHub: @stzn •iOS engineer in Fukuoka, Japan .Z
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3. Contents •Swift compiler 101 •Two features of Swift async function (overview) •Two features of Swift async function (detail)
4. Used Swift version •swift-DEVELOPMENT-SNAPSHOT-2024-12-22- a(Swift6.2)
5. Contents •Swift compiler 101 •Two features of Swift async function (overview) •Two features of Swift async function (detail)
6. What complier does?
7. Compiler •A program that translate source code from a high-level programming language to a low- level programming language
8. Mainly compiler does •Parsing • Divide the source code into the smallest meaningful units "tokens." •Lexical analysis • Check syntax rules based on tokens and analyze the structure of the program. Generate AST •Semantic analysis • Check the type and scope of the variable and check if the program is semantically correct •IR generation • Generate code to make the compiler optimized •Optimization • Transform IR into functionally equivalent but faster (or smaller) forms •Code generation • Translate transformed IR into machine language ⚠Depends on the compiler and so on….
9. The main pipelines of compiler • Parsing • Divide the source code into the smallest meaningful units "tokens." • Lexical analysis • Check syntax rules based on tokens and analyze the structure of the program. Generate AST • Semantic analysis • Check the type and scope of the variable and check if the program is semantically correct •IR generation • Generate code to make the compiler optimized • Optimization • Transform IR into functionally equivalent but faster (or smaller) forms • Code generation • Translate transformed IR into machine language ⚠Depends on the compiler and so on….
10. IR •Intermediate Representation •Intermediate code generated in the process of converting programming language code to machine language
11. IR • Intermediate Representation • Intermediate code generated in the process of converting programming language code to machine language •Why needed? • The structure is simple and the code is easy to optimize • Generate code for various targets (e.g. Apple Silicon's MacOS) • Development e ff i ciency by separating compiler processes
12. What Swi ft compiler does?
13. The main pipelines of Swift compiler 1. Parse(Parsing, Lexical analysis) 2. Sema(Semantic Analysis) 3. SILGen(IR generation) 4.SILOptimizer(Optimization) 5. IRGen(IR generation) 6.LLVM(Optimization, Code generation) https://qiita.com/rintaro/items/3ad640e3938207218c20 ❶ ❷ ❸ ❹ ❺ ❻
14. More info about Swift compiler(Japanese) https://qiita.com/rintaro/items/3ad640e3938207218c20
15. More info about Swift compiler https://www.swift.org/documentation/swift-compiler/
16. Some supplements
17. The main pipelines of Swift compiler 1. Parse(Parsing, Lexical analysis) 2. Sema(Semantic Analysis) 3.SILGen(IR generation) 4. SILOptimizer(Optimization) 5.IRGen(IR generation) 6. LLVM(Optimization, Code generation) https://qiita.com/rintaro/items/3ad640e3938207218c20 ⚠More IRs are generated (e.g. AST)
18. SILGen •Generate SIL •SIL: Swift Intermediate Language •High-level, Swift-speci fi c intermediate language suitable for further analysis and optimization of Swift code TIPS: Documentation about SIL was updated recently
19. IRGen •Generate LLVM IR •IR for LLVM generated to facilitate optimization in the process of generating the fi nal machine language
20. LLVMʁ
21. The main pipelines of Swift compiler 1. Parse(Parsing, Lexical analysis) 2. Sema(Semantic Analysis) 3. SILGen(IR generation) 4. SILOptimizer(Optimization) 5. IRGen(IR generation) 6.LLVM(Optimization, Code generation) https://qiita.com/rintaro/items/3ad640e3938207218c20
22. LLVM •Not Swift •Compiler backend •Generate machine languages for various targets from LLVM IR •Optimization according to the target •Used universally(C++ɺRustɺ ActionScript…)
23. More info about LLVM https://github.com/learn-llvm/awesome-llvm
24. SIL & LLVM IR Swift SIL LLVM IR
25. SIL & LLVM IR Swift SIL LLVM IR string integer Array i64 i32 ptr
26. Contents •Swift compiler 101 •Two features of Swift async function (overview) •Two features of Swift async function (detail)
27. Contents •Swift compiler 101 •Two features of Swift async function (overview) •Two features of Swift async function (detail)
28. Explore Swift performance Two features of Swift async function
29. Two features of Swift async function •Keep local state on a special stack •Split functions into partial functions that run between suspensions
30. Two features of Swift async function •Keep local state on a special stack •Split functions into partial functions that run between suspensions
31. Stack memory Sync function Call stack(※) on stack memory Call stack func1 func2 func3 Thread ※ A data structure that stores info about active functions
32. Stack(data structure) Stack Element LIFO(Last-In, First-Out) Element Stack Element Element Last-In First-Out push pop
33. Stack(data structure) Stack Element LIFO(Last-In, First-Out) Element Stack Element Element Last-In First-Out ⚠Stack can also be used in heap memory
34. Call stack func1 func2 func3 Thread Stack frame Parameters Local variables Return address …and so on
35. Thread Sync function Start Finish Call stack func1 func2 foo Call stack func1 func2 Thread foo Pop Push disappear
36. Let's think about it with the async function
37. Start is the same Start Call stack func1 func2 Thread async
38. Call stack func1 func2 Thread async Need to pop the frame on suspends Await Call stack func1 func2 Thread async async Run other operations Start disappear
39. Call stack func1 func2 Thread async Need the frame again Call stack func1 func2 Thread async Call stack func1 func2 func3 Thread Resume async Hmm…? Await ⚠Might be a di ff erent thread Start
40. So
41. Keep local state on a special stack
42. async Heap memory Stack memory Store to another stack Thread func1 func2 async Call stack Stack Stack on heap memory
43. Only info across suspend points async Stack Call stack async a b Suspend point
44. async Stack Call stack async Async frame Information without any suspend points in- between Information across suspend points
45. Stack memory Return to call stack on resume Stack Thread ⚠might be a di ff erent thread func4 func5 async Call stack Heap memory
46. Task Task manages async process
47. Task TaskAllocator manages memory TaskAllocator
48. Heap memory Hold memory areas Task TaskAllocator Memory 💡It is done when creating a Task Task TaskAllocator Memory
49. Heap memory Task TaskAllocator Memory Task TaskAllocator Memory async Allocate as needed
50. Heap memory Task TaskAllocator Memory async Memory Allocate a new memory if needed
51. Heap memory Task More detail TaskAllocator Memory async StackAllocator(TaskAllocator is a kind of typealias) Slab Continuous memory Stack discipline + Bump pointer allocation Mechanism for e ff i cient memory allocation
52. Two features of Swift async function •Keep local state on a special stack •Split functions into partial functions that run between suspensions
53. Two features of Swift async function •Keep local state on a special stack •Split functions into partial functions that run between suspensions
54. Split functions?
55. Split function between await Partial function hoge1 hoge2 Partial function await = potential suspend point
56. Appendix: ramp & resume function •ramp function: By the fi rst await •resume function: After await
57. hoge1 hoge2 resume function created await by await hoge3
58. hoge1 Call the process a ft er await on resume await hoge2 https://github.com/swiftlang/swift/blob/main/docs/SIL/Types.md#async-functions async frame Process on resume
59. hoge1 await hoge2 https://github.com/swiftlang/swift/blob/main/docs/SIL/Types.md#async-functions async frame Process on resume 💡Information where to run on the Call stack Call the process a ft er await on resume
60. Two more features •Tail call •AsyncContext
61. Two more features •Tail call •AsyncContext
62. hoge1 await await hoge2 async frame Process on resume Function Pointer Pass address directly
63. hoge1 Call when complete the process await await hoge2 async frame Process on resume Function Pointer call
64. hoge1 await await hoge2 async frame Process on resume Function Pointer call Tail call 💡No need to use stack(Memory optimization)
65. Two more features •Tail call •AsyncContext
66. Task Entire task info async frame async frame async frame One async function info AsyncContext
67. hoge1 await hoge2 async frame Process on resume AsyncContext pointer Get info from AsyncContext via pointer AsyncContext pointer AsyncContext pointer
68. Where split function?
69. LLVM! 1. Parse(Parsing, Lexical analysis) 2. Sema(Semantic Analysis) 3. SILGen(IR generation) 4. SILOptimizer(Optimization) 5. IRGen(IR generation) 6.LLVM(Optimization, Code generation) https://qiita.com/rintaro/items/3ad640e3938207218c20
70. Async lowering https://llvm.org/docs/Coroutines.html#async-lowering
71. Originally implemented for Swi f
72. Why needed? •Minimize overhead • Omit unnecessary process • Originally suitable for C++ & not fi t for the usage of Swift(e.g. for loop) • Emphasis on advanced optimization on the Swift side • Compatible with Swift's unique calling conventions • Supports future language features • Flexible memory management by the caller(via Task)
73. Coroutine Representations and ABIs in LLVM
74. ⚠ Returned-Continuation Lowering → Async Lowering LLVMͷϓϩϙʔβϧ
75. Contents •Swift compiler 101 •Two features of Swift async function (overview) •Two features of Swift async function (detail)
76. Contents •Swift compiler 101 •Two features of Swift async function (overview) •Two features of Swift async function (detail)
77. Mostly about LLVM
78. LLVM IR 101
79. Module Structure of LLVM IR Function BasicBlock Instruction Manage the entire program One program List of instructions executed in order Actual instruction to the machine
80. Function Function Module Almost like this if … else BasicBlock Instruction Instruction … BasicBlock Instruction Instruction … if … else BasicBlock Instruction Instruction … BasicBlock Instruction Instruction …
81. Two features of Swift async function •Keep local state on a special stack •Split functions into partial functions that run between suspensions
82. Heap memory Stack async b Only info across suspend points
83. How to know whether it is necessary or not?
84. •Common parent classes for all values (variables, constants, functions, instructions, etc.) •def-use chain: Hold a list of other Values that use this value(UseList) Value class https://llvm.org/doxygen/classllvm_1_1Value.html LLVM
85. •If two BasicBlocks A and B are given, check whether the path from A to B passes through the suspend point (whether a suspend instruction is included) SuspendCrossingInfo class https://github.com/swiftlang/llvm-project/blob/next/llvm/include/llvm/Transforms/Coroutines/SuspendCrossingInfo.h LLVM
86. BasicBlock Value What does it do? UseList BasicBlock Value BasicBlock Value BasicBlock Value … LLVM Across suspend point? SuspendCrossingInfo compares BasicBlocks after splitting functions
87. Can reduce memory usage
88. ...It should be
89. Bug https://github.com/swiftlang/swift/issues/72289
90. Unnecessary async frame… Nothing across suspend point Async frame is created but https://github.com/swiftlang/swift/issues/72289
91. Investigation Need a memory if a value is included in Spills Is it because swift_task_alloc(※) is called? ※ Called when AsyncContext requires a dynamic size https://github.com/swiftlang/swift/issues/72289
92. All async functions calls it = AsyncContext size ❌static https://github.com/swiftlang/swift/blob/main/lib/IRGen/GenCall.cpp#L166 https://github.com/swiftlang/swift/issues/72289 Investigation
93. •My understanding is correct? •How solve it? •If not across suspend point, use a static AsyncContextSize? (Not sure how to judge) •Erase swift_task_alloc on SILOptimizer or LLVM? Hmm…🤔 https://github.com/swiftlang/swift/issues/72289
94. BTW
95. Two features of Swift async function •Keep local state on a special stack •Split functions into partial functions that run between suspensions
96. Code optimization in LLVM ม׵લ LLVM IR ม׵ޙ LLVM IR
97. Optimization with multiple passes ม׵લ LLVM IR Pass Pass ม׵ޙ LLVM IR … Parse & Optimization
98. Functions split is one of them ม׵લ LLVM IR Pass Pass ม׵ޙ LLVM IR … Function split
99. Coroutine Transformation Passes •CoroEarly •CoroSplit •CoroAnnotationElide •CoroElide •CoroCleanup https://llvm.org/docs/Coroutines.html#coroutine-transformation-passes LLVM Coro = Coroutine Mechanism to handle async process (etc.) in LLVM
100. Function split in CoroSplit •CoroEarly •CoroSplit •CoroAnnotationElide •CoroElide •CoroCleanup https://llvm.org/docs/Coroutines.html#coroutine-transformation-passes LLVM
101. Split BasicBlocks Function BasicBlock … Suspend Instruction … BasicBlock … BasicBlock Suspend Instruction … ✂ https://github.com/swiftlang/llvm-project/blob/next/llvm/lib/Transforms/Coroutines/CoroSplit.cpp
102. Contents •Swift compiler 101 •Two features of Swift async function (overview) •Two features of Swift async function (detail)
103. One last thing
104. It's best to touch it yourself!
105. https://zenn.dev/omochimetaru/articles/8a7f35c5e23a24 ⚠It’s Japanese, but we can follow commands in it.
106. How to debug https://github.com/swiftlang/swift/blob/main/docs/DebuggingTheCompiler.md
107. •Even if you are not familiar with C++, it's okay at fi rst •Debug and grasp the processing fl ow (it’s important to fi nd the entry point) •Get used to frequent words and abbreviations •Don't be misled by casting For those who are interested in Swift compilers
108. • Swift ίϯύΠϥͷΞʔΩςΫνϟ(The architecture of Swift compiler) • Swift Compiler • Awesome LLVM • SwiftίϯύΠϥ։ൃೖ໳(The introduction of Swift compiler dvelopment) • Explore Swift performance • Swift concurrency: Behind the scenes • Swift repository • LLVM repository(forked by Apple) • Async lowering • Issue about unnecessary async frame References
109. Thank you!