JIT compiler for expressions

Hello hackers,

We'd like to present our work on adding LLVM JIT compilation of expressions
in SQL queries for PostgreSQL. The source code (based on 9.6.1) along with
brief manual is available at our github: https://github.com/ispras/postgres .
Сurrent speedup for TPC-H Q1 is 20% (on 40GB workload). Please feel free to
test it and tell us what you think.

Currently, our JIT is used to compile expressions in every query, so for
short-running queries JIT compilation may take longer than query execution
itself. We plan to address it by using planner estimate to decide whether
it worth JIT compiling, also we may try parallel JIT compilation. But for
now we recommend testing it on a large workload in order to pay off the
compilation (we've tested on 40GB database for Q1).

The changes in PostgreSQL code itself are rather small, while the biggest
part of new code in our repository is autogenerated (it's LLVM IR
generators for PostgreSQL backend functions). The only real reason for
shipping prebuild_llvm_backend.cpp is that it takes patched LLVM version to
generate, otherwise it's generated right from PostgreSQL source code
(please see more on automatic backend generation at our github site). With
pre-generated cpp file, building our github PGSQL version w/JIT requires
only clean, non-patched LLVM 3.7.

JIT compilation was tested on Linux, and currently we have 5 actual tests
failing (which results in 24 errors in a regtest). It requires LLVM 3.7
(3.7.1) as build dependency (you can specify path to proper llvm-config
with --with-llvm-config= configure option, e.g. it could be named
llvm-config-3.7 on your system). Mac support is highly experimental, and
wasn't tested much, but if you like to give it a try, you can do it with
LLVM 3.7 from MacPorts or Homebrew.

This work is a part of our greater effort on implementing full JIT compiler
in PostgreSQL, where along with JITting expressions we've changed the
iteration model from Volcano-style to push-model and reimplemented code
generation with LLVM for most of Scan/Aggregation/Join methods. That
approach gives much better speedup (x4-5 times on Q1), but it takes many
code changes, so we're developing it as PostgreSQL extension. It's not
ready for release yet, but we're now working on performance, compatibility,
as well as how to make it easier to maintain by making it possible to build
both JIT compiler and the interpreter from the same source code. More
information about our full JIT compiler and related work is available in
presentation at LLVM Cauldron (http://llvm.org/devmtg/2016-09/slides/Melnik-
PostgreSQLLLVM.pdf ) and PGCon (https://www.pgcon.org/2016/
schedule/attachments/411_ISPRAS%20LLVM+Postgres%20Presentation.pdf ).
Also we're going to give a lightning talk at upcoming PGConf.EU in Tallinn,
and discuss the further development with PostgreSQL community. We'd
appreciate any feedback!

--
Best regards,
Dmitry Melnik
Institute for System Programming of the Russian Academy of Sciences
ISP RAS (www.ispras.ru/en/)