Horton

Re: JIT compiling with LLVM v9.0

Jeff Davis <pgsql@j-davis.com>

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From: Jeff Davis <pgsql@j-davis.com>
To: Andres Freund <andres@anarazel.de>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-01-27T02:26:03Z
Lists: pgsql-hackers 
On Wed, Jan 24, 2018 at 11:02 PM, Andres Freund <andres@anarazel.de> wrote:
> Not entirely sure what you mean. You mean why I don't inline
> slot_getsomeattrs() etc and instead generate code manually?  The reason
> is that the generated code is a *lot* smarter due to knowing the
> specific tupledesc.

I would like to see if we can get a combination of JIT and LTO to work
together to specialize generic code at runtime.

Let's say you have a function f(int x, int y, int z). You want to be
able to specialize it on y at runtime, so that a loop gets unrolled in
the common case where y is small.

1. At build time, create bitcode for the generic implementation of f().
2. At run time, load the generic bitcode into a module (let's call it
the "generic module")
3. At run time, create a new module (let's call it the "bind module")
that only does the following things:
   a. declares a global variable bind_y, and initialize it to the value 3
   b. declares a wrapper function f_wrapper(int x, int z), and all the
function does is call f(x, bind_y, z)
4. Link the generic module and the bind module together (let's call
the result the "linked module")
5. Optimize the linked module

After sorting out a few details about symbols and inlining, what will
happen is that the generic f() will be inlined into f_wrapper, and it
will see that bind_y is a constant, and then unroll a "for" loop over
y.

I experimented a bit before and it works for basic cases, but I'm not
sure if it's as good as your hand-generated LLVM.

If we can make this work, it would be a big win for
readability/maintainability. The hand-generated LLVM is limited to the
bind module, which is very simple, and doesn't need to be changed when
the implementation of f() changes.

Regards,
     Jeff Davis

Commits

  1. Improve JIT docs.

  2. Add documentation for the JIT feature.

  3. Add EXPLAIN support for JIT.

  4. Add inlining support to LLVM JIT provider.

  5. JIT tuple deforming in LLVM JIT provider.

  6. Add FIELDNO_* macro designating offset into structs required for JIT.

  7. Add expression compilation support to LLVM JIT provider.

  8. Expand list of synchronized types and functions in LLVM JIT provider.

  9. Add helpers for emitting LLVM IR.

  10. Basic planner and executor integration for JIT.

  11. Debugging and profiling support for LLVM JIT provider.

  12. Support for optimizing and emitting code in LLVM JIT provider.

  13. Add file containing extensions of the LLVM C API.

  14. Basic JIT provider and error handling infrastructure.

  15. Add configure infrastructure (--with-llvm) to enable LLVM support.

  16. Add C++ support to configure.

  17. Add PGAC_PROG_VARCC_VARFLAGS_OPT autoconf macro.

  18. Fix VM buffer pin management in heap_lock_updated_tuple_rec().

  19. Allow tupleslots to have a fixed tupledesc, use in executor nodes.

  20. Expression evaluation based aggregate transition invocation.

  21. Perform slot validity checks in a separate pass over expression.

  22. Rely on executor utils to build targetlist for DML RETURNING.

  23. Refer to OS X as "macOS", except for the port name which is still "darwin".