Re: JIT compiling with LLVM v9.0

Hi,

On 2018-01-29 10:28:18 -0800, Jeff Davis wrote:
> OK. How about this: are you open to changes that move us in the
> direction of extensibility later? (By this I do *not* mean imposing a
> bunch of requirements on you... either small changes to your patches
> or something part of another commit.)

I'm good with that.

> Or are you determined that this always should be a part of core?

I do think JIT compilation should be in core, yes. And after quite some
looking around that currently means either using LLVM or building our
own from scratch, and the latter doesn't seem attractive. But that
doesn't mean there *also* can be extensibility. If somebody wants to
experiment with a more advanced version of JIT compilation, develop a
gcc backed version (which can't be in core due to licensing), ... - I'm
happy to provide hooks that only require a reasonable effort and don't
affect the overall stability of the system (i.e. no callback from
PostgresMain()'s sigsetjmp() block).

> I don't want to stand in your way, but I am also hesitant to dive head
> first into LLVM and not look back. Postgres has always been lean, fast
> building, and with few dependencies.

It's an optional dependency, and it doesn't increase build time that
much... If we were to move the llvm interfacing code to a .so, there'd
not even be a packaging issue, you can just package that .so separately
and get errors if somebody tries to enable LLVM without that .so being
installed.

> In other words, are you "strongly against [extensbility being a
> requirement for the first commit]" or "strongly against [extensible
> JIT]"?

I'm strongly against there not being an in-core JIT. I'm not at all
against adding APIs that allow to do different JIT implementations out
of core.

> If the source for functions is in the catalog, we could build the
> bitcode at runtime and still do the inlining. We wouldn't need to do
> anything at build time. (Again, this would be "cool stuff for the
> future", I am not asking you for it now.)

Well, the source would require an actual compiler around. And the
inlining *just* for the function code itself isn't actually that
interesting, you e.g. want to also be able to

Greetings,

Andres Freund