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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
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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
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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
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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
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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
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– 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
– 2
bar 3
=
int foo +
4 x
Front End4. 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
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11110010 11100101 1110101111010000 11000100 1100011000110000 00100000 0110111100111100 00111100 0010000001101110 01100111 01110100
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
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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
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)
Webkit FTL
https://www.webkit.org/blog/3362/introducing-the-webkit-ftl-jit/
Q&A
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Thanks!
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