minivm

![The MiniVM logo, a blueish grey brick (or maybe a can of tuna if you are a cat)](res/MiniVM.svg)

# MiniVM

MiniVM is an interpreter for Lua 5.5, written in C11. It has no JIT and needs nothing beyond libc, libm and libdl, so it runs on platforms that forbid generating machine code at runtime. This is version 0.1.0.

It specializes code on the types it actually sees, using lazy basic block versioning (the technique behind Ruby's YJIT), and runs the specialized code as direct-threaded code. On the benchmarks below it is 1.9x faster than reference Lua 5.5.1 on small programs and 1.2x faster on real ones.

## Performance

Each number compares MiniVM's time with another interpreter's, as a geometric mean over the benchmarks. Memory is peak RSS, compared the same way.

| Compared with | Micro-benchmarks | Real programs | All | Peak memory |
| --- | --- | --- | --- | --- |
| Lua 5.5.1 | 1.9x faster | 1.2x faster | 1.7x faster | 1.6x as much |
| LuaJIT 2.1 interpreter (`-joff`) | 1.2x faster | 1.3x slower | 1.1x faster | 1.5x as much |
| LuaJIT 2.1 with its JIT | 2.8x slower | 1.3x slower | 2.5x slower | 1.4x as much |

There are 29 micro-benchmarks and 7 real programs: the fennel compiler, the tl (Teal) type checker, json.lua, lulpeg, serpent, inspect and etlua. LuaJIT runs 28 and 4 of them. inspect, lulpeg and serpent use Lua 5.3 syntax, and `deep-rec` recurses deeper than LuaJIT's stack allows.

`strcat-accum` builds a string with `s = s .. x` in a loop, which MiniVM does with ropes in about 1% of the time. Without it, MiniVM is 1.7x faster than Lua 5.5.1 on micro-benchmarks, as fast as LuaJIT's interpreter, and 3.5x slower than LuaJIT's JIT.

tl is the one program where MiniVM is slower than Lua 5.5.1.

Times in milliseconds, best of 5 runs:

| Benchmark | MiniVM | Lua 5.5.1 | LuaJIT `-joff` | LuaJIT |
| --- | ---: | ---: | ---: | ---: |
| `branchy` | 344 | 546 | 668 | 324 |
| `calls-callable` | 341 | 319 | 239 | 30 |
| `calls-mono` | 390 | 784 | 395 | 46 |
| `calls-poly` | 157 | 284 | 233 | 35 |
| `closure-churn` | 187 | 409 | 325 | 284 |
| `concat` | 162 | 334 | 251 | 76 |
| `coro-pingpong` | 471 | 501 | 168 | 208 |
| `deep-rec` | 355 | 395 | - | - |
| `fannkuch` | 101 | 111 | 86 | 20 |
| `gc-churn` | 198 | 283 | 178 | 15 |
| `gc-strings` | 178 | 369 | 243 | 168 |
| `gc-survivors` | 187 | 334 | 122 | 113 |
| `list-deep` | 412 | 414 | 374 | 374 |
| `loop-math` | 491 | 1221 | 799 | 86 |
| `mandel` | 96 | 243 | 168 | 20 |
| `mixed-arith` | 233 | 406 | 197 | 71 |
| `nbody` | 1356 | 1760 | 941 | 111 |
| `poly-return` | 162 | 283 | 137 | 30 |
| `rec-mutual` | 416 | 588 | 398 | 40 |
| `spectralnorm` | 264 | 319 | 207 | 10 |
| `strbuild` | 51 | 76 | 61 | 51 |
| `strcat-accum` | 10 | 933 | 655 | 1731 |
| `table-churn` | 354 | 755 | 577 | 461 |
| `table-hash` | 106 | 248 | 137 | 30 |
| `table-oldmut` | 132 | 229 | 162 | 15 |
| `table-pairs` | 158 | 339 | 66 | 61 |
| `table-rw` | 86 | 148 | 116 | 15 |
| `trees-large` | 13174 | 50996 | 14236 | 12749 |
| `tr