LLVM Internal Architecture par Michel Guillet
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LLVM is a collection of modular compiler and toolchain technologies including a backend, assembly code generator for over 10 CPUs, the LLDB debugger, and the Clang C/C++/Objective-C frontend and static analyzer. It originated in the 2000s and was adopted by Apple in 2011 as it is more modular than GCC, integrates better into IDEs, and was more suitable for improving Objective-C support. Compilation involves transforming source code into a lower level representation like assembly, and LLVM's design with frontend, optimized intermediate representation, and backend makes it easier to add new languages and CPU targets compared to GCC.
LLVM Internal Architecture par Michel Guillet
- 2. What is LLVM ? • LLVM stood for: Low Level Virtual Machine • “The LLVM Project is a collection of modular and reusable compiler and toolchain technologies” — llvm.org • LLVM Core: backend, assembly code generator for 10+ CPUs (60+ for GCC) • LLDB: debugger • Clang: C/C++/Objective-C frontend + static analyser 2
- 3. History • March 22, 1987: GGC 0.9 — First beta version • Circa 2000: LLVM Project started • Circa 2005: C. Lattner hired by Apple • July 18, 2007: GGC 4.2.1 — Last GPLv2 version • Crica 2010: C. Lattner began working on Swift • October 12, 2011: Xcode 4.1 — LLVM only 3
- 4. Why GCC became an issue ? • GPLv3: Apple would have been forced to open Xcode and other build tool related to GCC • Not modular: Poor integration into the IDE • 7+ MLOC: Too big, too old • Objective-C: Not a priority for GCC team, slow improvements 4
- 5. But what is “compilation” ? • Compilation: transformation of a source code into another source code (of a lesser abstraction) • Example: C into x86 assembly or Ada into PowerPC assembly • Transpilation: compilation into a langage of similar abstraction • Example: CoffeeScript into Javascript or LESS into CSS 5
- 6. You ! Front end (Clang) Back end (LLVM) But what is “compilation” ? CPU (x86, ARM) 1) Algorithm 2) Implementation (C, Java) 3) IR (GIMPLE, LLVM IR) 4) Assembly (x86-64, PPC, ARM) 5) Microcode 6
- 7. – 2 bar 3 = int foo + 4 x Front End 1. Lexical analysis 2. Syntax analysis 3. Semantic analysis int foo = 4 + 2 * ( bar - 3 ) int foo = 4 + 2 * ( bar - 3 ) do we know “int” ? does “foo” already exist ? do we know “x” ? is this an int ? do we know “bar” as this stage ? is this an int ?7
- 8. – 2 bar 3 = int foo + 4 x Front End 4. Optimization 5. IR generation int foo = 4 + 2 * ( bar - 3 ) bar 2 = int foo - x 2 %tmp = mul i32 %bar, 2 %foo1 = sub i32 %tmp, 2 8
- 9. 11110010 11100101 11101011 11010000 11000100 11000110 00110000 00100000 01101111 00111100 00111100 00100000 Back End • Optimization • Assembly generation %tmp = mul i32 %bar, 2 %foo1 = sub i32 %tmp, 2 %0 = add i32 %bar, %bar %foo1 = sub i32 %0, 2 9
- 10. This is not new ! • GCC has a similar design… • … but it’s not modular • You cannot hack you way in • LLVM was built modularly from the beginning
- 11. Why Modularity is a big deal • No need to deal with CPU specificities • Creating a new language is easier (reduce skill set) • Adding new CPU support is easier (do it one time) • Creating tools around languages is easier (Clang Static Analyzer, Refactoring)
- 13. Q&A 13
- 14. Thanks! 14