alt Hacker NewsReminds me of the 2024 blog post Look ma, I wrote a new JIT compiler for PostgreSQL [0]. Both articles lament that Postgres's LLVM-based JIT [1] takes a while to generate code.
> The rarity of JIT compilers makes me believe that implementing a JIT compiler historically was too difficult for it to be worthwhile.
That's only true of writing a JIT from scratch. There's no rarity of JITs, it's just that LLVM (and other frameworks) are often used. Every major interpreter has a JIT compiler. PCRE2 has a JIT compiler. There are JIT frameworks out there with much faster code-generation than LLVM: Cranelift, GNU Lightning, Mir. I doubt they could do code-generation faster than a custom copy-and-patch JIT, but they'd be much faster than LLVM.
[0] https://www.pinaraf.info/2024/03/look-ma-i-wrote-a-new-jit-c... , discussed: https://news.ycombinator.com/item?id=39742916
Replies
> There's no rarity of JITs, it's just that LLVM (and other frameworks) are often used
Except that using LLVM has high latency limitting it's applicability. Postgres just disabled LLVM by default because of this[0].
[0] https://www.postgresql.org/message-id/E1w8GWU-002bSL-31%40ge...
The original Dartmouth BASIC had a JIT like approach, the REPL would compile to machine code before execution.
It was the limits of 8 bit home computers hardware that made the interpreter version be more widely known.
Same to Lisp, Smalltalk, and many other languages.
Fully agree with you.
A few other small and fast JITs: https://github.com/zherczeg/sljit (used by libpcre), https://github.com/asmjit/asmjit (RPCS3 and FBGEMM) and https://webkit.org/blog/5852/introducing-the-b3-jit-compiler... (only used by JSC in Webkit, I think)
Not sure where the idea comes from that Cranelift is much faster than LLVM -O0, at least in our experiments in 2024 it wasn't, see [1] Fig. 6.
Template-based code generators suffer from bad code quality due to missing register allocation.
Our TPDE-based compilers compile a bit slower than template-based code generation but the generated code is much smaller and faster ([2] Fig. 2). Also for database workloads ([2] Fig. 6).
All that said, Postgres' main limitation is that it (IIRC) only compiles single expressions from operators, not pipelines. This fundamentally limits the achievable performance improvement compared to databases that perform more extensive query compilation.
[1]: https://aengelke.net/pubs/2403-cgo.pdf [2]: https://aengelke.net/pubs/2602-cgo1.pdf
PS: sorry for the promotion of my own research here, just couldn't resist.