LuaJIT

LuaJIT is a tracing just-in-time compiler for the Lua programming language. Mike Pall, a primary maintainer of the project had resigned in 2015, resorting only to occasional patching to the future 2.1 version.^[3]

LuaJIT

The logo featured on the LuaJIT website.
Original author(s)	Mike Pall
Stable release	2.0.5 (later v2.1.ROLLING is also updated, e.g. in 2023) / May 1, 2017; 6 years ago (2017-05-01)
Repository	github.com/LuaJIT/LuaJIT
Written in	C, Lua
Operating system	Unix-like, MacOS, Windows, iOS, Android, PlayStation
Platform	x86, X86-64, PowerPC, ARM, MIPS^[1]
Type	Just-in-time compiler
License	MIT License^[2]
Website	luajit.org

History

The LuaJIT project was started in 2005 by developer Mike Pall, released under the MIT open source license.^[4]

The second major release of the compiler, 2.0.0, featured major performance increases.^[5]

The latest release, 2.0.5 is released in 2017. However, Mike Pall, the creator and maintainer recommends using the tip of the v2.1 branch, and does not believe in releases.^[6]

Notable users

• CERN, for their Methodical Accelerator Design 'next-generation' software for describing and simulating particle accelerators^[7]
• OpenResty, a fork of nginx with Lua scripting^[8]
• Kong, a web API gateway^[9]
• Cloudflare, who use LuaJIT in their web application firewall service^[10]

Performance

LuaJIT is often the fastest Lua runtime.^[11] LuaJIT has also been named the fastest implementation of a dynamic programming language.^[12][13]

LuaJIT includes a Foreign Function Interface compatible with C data structures. Its use is encouraged for numerical computation.^[14]

Tracing

LuaJIT is a tracing just-in-time compiler. LuaJIT chooses loops and function calls as trace anchors to begin recording possible hot paths. Function calls will require twice as many invocations to begin recording as a loop. Once LuaJIT begins recording, all control flow, including jumps and calls, are inlined to form a linear trace. All executed bytecode instructions are stored and incrementally converted into LuaJIT's Static single-assignment Intermediate representation. LuaJIT's trace compiler is often capable of inlining and removing dispatches from object orientation, operators, and type modifications.^[15]

Internal representation

LuaJIT uses two types of internal representation. A stack-based bytecode is used for the interpreter, and a static single-assignment form is used for the just-in-time compiler. The interpreter bytecode is frequently patched by the JIT compiler, often to begin executing a compiled trace or to mark a segment of bytecode for causing too many trace aborts.^[13]

-- Loop with if-statement local x = 0 for i=1,1e4 do x = x + 11 if i%10 == 0 then -- if-statement x = x + 22 end x = x + 33 end 

---- TRACE 1 start Ex.lua:5 ---- TRACE 1 IR 0001 int SLOAD #2 CI 0002 > num SLOAD #1 T 0003 num ADD 0002 +11 0004 int MOD 0001 +10 0005 > int NE 0004 +0 0006 + num ADD 0003 +33 0007 + int ADD 0001 +1 0008 > int LE 0007 [phone] ------ LOOP ------------ 0010 num ADD 0006 +11 0011 int MOD 0007 +10 0012 > int NE 0011 +0 0013 + num ADD 0010 +33 0014 + int ADD 0007 +1 0015 > int LE 0014 [phone] int PHI 0007 0014 0017 num PHI 0006 0013 ---- TRACE 1 stop -> loop ---- TRACE 2 start 1/4 Ex.lua:8 ---- TRACE 2 IR 0001 num SLOAD #1 PI 0002 int SLOAD #2 PI 0003 num ADD 0001 +22 0004 num ADD 0003 +33 0005 int ADD 0002 +1 0006 > int LE 0005 [phone] num CONV 0005 num.int ---- TRACE 2 stop -> 1 

Extensions

LuaJIT adds several extensions to its base implementation, Lua 5.1, most of which do not break compatibility.^[16]

• "BitOp" for binary operations on unsigned 32-bit integers (these operations are also compiled by the just-in-time compiler)^[17]
• "CoCo", which allows the VM to be fully resumable across all contexts^[18]
• A foreign function interface^[19]
• Portable bytecode (regardless of architecture, word size, or endianness, not version)^[20]

DynASM

DynASM

Developer(s)	Mike Pall
Stable release	2.0.5 / May 1, 2017; 6 years ago (2017-05-01)
Preview release	2.1.0 beta3 GC64
Repository	github.com/LuaJIT/LuaJIT
Written in	Lua, C^[21]
Platform	x86, X86-64, PowerPC, ARM, MIPS
Type	Preprocessor, Linker
License	MIT License^[2]
Website	luajit.org/dynasm.html

DynASM is a lightweight preprocessor for C which was created for LuaJIT 1.0.0 to make developing the just-in-time compiler easier.^{[citation needed]} DynASM replaces assembly code in C files with runtime writes to a 'code buffer', such that a developer may generate and then evoke code at runtime from a C program.

DynASM was phased out in LuaJIT 2.0.0 after a complete rewrite of the assembler^{[citation needed]}, but remains in use by the LuaJIT contributors as a better assembly syntax for the LuaJIT interpreter.

DynASM includes a bare-bones C header file which is used at compile time for logic the preprocessor generates. The actual preprocessor is written in Lua.

Example

|.type L, lua_State, esi // L. |.type BASE, TValue, ebx // L->base. |.type TOP, TValue, edi // L->top. |.type CI, CallInfo, ecx // L->ci. |.type LCL, LClosure, eax // L->ci->func->value. |.type UPVAL, UpVal |.macro copyslot, D, S, R1, R2, R3 | mov R1, S.value; mov R2, S.value.na[1]; mov R3, S.tt | mov D.value, R1; mov D.value.na[1], R2; mov D.tt, R3 |.endmacro |.macro copyslot, D, S; copyslot D, S, ecx, edx, eax; .endmacro |.macro getLCL, reg ||if (!J->pt->is_vararg) { | mov LCL:reg, BASE[-1].value ||} else { | mov CI, L->ci | mov TOP, CI->func | mov LCL:reg, TOP->value ||} |.endmacro |.macro getLCL; getLCL eax; .endmacro [...] static void jit_op_getupval(jit_State *J, int dest, int uvidx) {  | getLCL  | mov UPVAL:ecx, LCL->upvals[uvidx]  | mov TOP, UPVAL:ecx->v  | copyslot BASE[dest], TOP[0] } 

References

1. "LuaJIT". LuaJIT. Retrieved 25 February 2022.
2. "LuaJIT/COPYRIGHT at v2.1 · LuaJIT/LuaJIT". GitHub. 7 January 2022.
3. "[ANN] Looking for new LuaJIT maintainers - luajit - FreeLists". www.freelists.org. Retrieved 2023-03-29.
4. "The LuaJIT Project". luajit.org. Retrieved 2023-06-17.
5. Pall, Mike. "Re: [ANN] llvm-lua 1.0". lua-users.org. Retrieved 25 February 2022.
6. "Project status - Issue #665 - LuaJIT/LuaJIT". GitHub. Retrieved 3 February 2023.
7. Deniau, Laurent. "Lua(Jit) for computing accelerator beam physics". CERN Document Server. CERN. Retrieved 25 February 2022.
8. "OpenResty® - Official Site". openresty.org.
9. "Kong/kong". GitHub. Kong. 25 February 2022. Retrieved 25 February 2022.
10. "Helping to make Luajit faster". blog.cloudflare.com. 19 October 2017. Retrieved 25 February 2022.
11. "LuaJIT Performance".
12. "Laurence Tratt: The Impact of Meta-Tracing on VM Design and Implementation". tratt.net. Retrieved 2 March 2022.
13. d'Andrea, Laurent (2019). Behavioural Analysis of Tracing JIT Compiler Embedded in the Methodical Accelerator Design Software (Thesis). CERN. Retrieved 31 July 2022.
14. Pall, Mike. "Tuning numerical computations for LuaJIT (was Re: [ANN] Sci-1.0-beta1) - luajit - FreeLists". www.freelists.org.
15. Rottenkolber, Max. "Later Binding: Just-in-Time Compilation of a Younger Dynamic Programming Language." ELS. 2020
16. "Extensions". LuaJIT. Retrieved 25 February 2022.
17. "BitOp Semantics". LuaJIT. Retrieved 25 February 2022.
18. "Coco - True C Coroutines". LuaJIT. Retrieved 25 February 2022.
19. "FFI Library". LuaJIT. Retrieved 25 February 2022.
20. "Extensions". luajit.org. Retrieved 2022-08-25.
21. "DynASM Features". DynASM. Retrieved 25 February 2022.