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Ruby JIT
Hacking Guide
@k0kubun
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@k0kubun
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RJIT ERB
Haml Slim
I maintain
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What's JIT?
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Ruby JITs
MJIT
Ruby 2.6
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Ruby JITs
YJIT
Ruby 3.1
MJIT
Ruby 2.6
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Ruby JITs
YJIT
Ruby 3.1
MJIT
Ruby 2.6
RJIT
Ruby 3.3
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Railsbench
Speedup relative to 2.5 No JIT (s/s)
0
0.5
1
1.5
2
Ruby 2.5 Ruby 2.6 Ruby 2.7 Ruby 3.0 Ruby 3.1 Ruby 3.2 Ruby 3.3
1.93
1.85
1.54 1.59
1.2
1.25
1.08
1.05
1.07
1.26
1.24
1.12
1.09
1.05
1.07
1
No JIT MJIT / RJIT YJIT
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Ruby 3.2 YJIT
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Ruby 3.3 YJIT
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Using YJIT
• Install Rust and then build Ruby
• Use --yjit or export RUBY_YJIT_ENABLE=1
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Ruby JIT Hacking Guide
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Ruby Hacking Guide
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Ruby JIT Hacking Guide
JIT
JIT
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How Ruby JIT works
Ruby
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How Ruby JIT works
1
+
2
Ruby
Abstract
Syntax
Tree
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How Ruby JIT works
1
+
2
putobject 1
putobject 2
opt_plus
leave
Ruby
Abstract
Syntax
Tree
Instruction
Sequence
(Bytecode)
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How Ruby JIT works
1
+
2
putobject 1
putobject 2
opt_plus
leave
Ruby
Abstract
Syntax
Tree
Instruction
Sequence
(Bytecode)
Machine
Code
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x86_64 assembly
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• Read asm comments
•--yjit-dump-disasm
•--rjit-dump-disasm
x86_64 assembly
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• mov: assignment instruction
• esi: register for stack[0]
x86_64 assembly
stack[0] = 3
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x86_64 assembly
• mov: assignment instruction
• edi: register for stack[1]
stack[0] = 3
stack[1] = 5
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x86_64 assembly
• add,sub: arithmetic instruction
• rax: temporary register
stack[0] = 3
stack[1] = 5
temp = stack[0]
temp -= 1
temp += stack[1]
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x86_64 assembly
• jo: jump if over
fl
ow
• rsi: register for stack[0]
stack[0] = 3
stack[1] = 5
temp = stack[0]
temp -= 1
temp += stack[1]
jump if overflow
stack[0] = temp
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x86_64 encoding
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x86_64 encoding
opv86: https://hikalium.github.io/opv86/
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x86_64 encoding
opv86: https://hikalium.github.io/opv86/
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Calling a custom JIT
Ruby 3.2 Ruby 3.3+
1. Run Ruby with --mjit=pause --rjit=pause
2. Override RubyVM::MJIT.compile RubyVM::RJIT#compile
3. Call RubyVM::MJIT.resume RubyVM::RJIT.resume
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Building JIT is fun
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k0kubun/ruby-jit-challenge
Ruby JIT Challenge
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Ruby JIT Challenge
Hashtag: #ruby-jit-challenge
Speedup relative to No JIT (s/s)
0
3
6
9
12
No JIT RJIT YJIT Ruby JIT
11.08
6.31
3.75
1
Fibonatti benchmark
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Optimizing Ruby JIT
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Side exits
side exit
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Method rede
fi
nition
Rede
fi
nition Hook
Invalidate
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Method rede
fi
nition
Rede
fi
nition Hook
Invalidate
side exit
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Constant rede
fi
nition
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Register allocation: Stack
0: rsi
1: rdi
4: r10
2: r8
3: r9
VM stack
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Register allocation: Stack
0: rsi
1: rdi
4: r10
2: r8
3: r9
VM stack
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Register allocation: Local variables
• Spill registers on C function calls
• Binding
• debug_inspector API
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Polymorphic method cache
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Splitting
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Method inlining
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Conclusion
• Enjoy custom JIT development
• Let's make YJIT the best Ruby JIT