June 19, 2024

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Reinterpreting The Lua Interpreter | Hackaday

2 min read
Reinterpreting The Lua Interpreter | Hackaday

The plan behind Lua is a stunning 1. A straightforward and concise syntax delivers pretty much all of the niceties of a very first-course language. Additionally, a naive implementation of an interpreter with a large switch circumstance can be carried out in an afternoon. But assembly is your go-to to get first rate overall performance in a JIT-type interpreter. So [Haoran Xu] commenced to inquire himself if he could obtain much better general performance with no hand-rolled assembly, and right after a few months of perform, he posted a function-in-development named LuaJIT Remake (LJR).

At the moment, it supports Lua 5.1, and on a smattering of 34 benchmarks, LJR beats the primary swiftest Lua, LuaJIT, by about 28% and the official Lua engine by 3x. [Haoran] features a wonderful rationalization of interpreters that offers great qualifications and context for the challenge.

But the prolonged and short of it is that swap circumstances are pricey and challenging to optimize for compilers, so utilizing tail calling is a reasonable option that arrives with some major disadvantages. With tail calls, each case assertion turns into a “function” that is jumped to and then jumped out of without mucking with the stack or the registers much too much.

Nevertheless, the calling conference necessitates any callee-saved registers to be preserved, which implies you reduce some registers as there is no way to convey to the compiler that this purpose is permitted to crack the calling convention. Clang is at this time the only compiler that features a certain tail-contact annotation ([[clang::musttail]]). There are other limits also, for occasion requiring the caller and callee to have equivalent function prototypes to protect against unbounded stack growth.

So [Haoran] went back again to the drawing board and wrote two new applications: C++ bytecode semantical description and a exclusive compiler referred to as Deegen. The C++ bytecode appears like this:

void Insert(TValue lhs, TValue rhs) 
DEEGEN_Outline_BYTECODE(Insert) 
  Operands(
    BytecodeSlotOrConstant("lhs"),
    BytecodeSlotOrConstant("rhs")
  )
  End result(BytecodeValue)
  Implementation(Add)
  Variant(
    Op("lhs").IsBytecodeSlot(),
    Op("rhs").IsBytecodeSlot()
  )
  Variant(
    Op("lhs").IsConstant(),
    Op("rhs").IsBytecodeSlot()
  )
  Variant(
    Op("lhs").IsBytecodeSlot(),
    Op("rhs").IsConstant()
  )

Take note that this is not the C keyword return. Instead, there is a definition of the bytecode and then an implementation. This bytecode is transformed into LLVM IR and then fed into Deegen, which can rework the features to do tail phone calls accurately, use the GHC calling conventions, and a couple other optimizations like inline caching as a result of a intelligent C++ lambda system. The site write-up is extremely properly-written and features a amazing glimpse into the wild globe of interpreters.

The code is on Github. But if you’re interested in a more whimsical interpreter, here’s a Brainf**k interpreter published in Befunge.

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