Horton

Re: JIT compiling with LLVM v11

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>, Pg Hackers <pgsql-hackers@postgresql.org>, Pierre Ducroquet <p.psql@pinaraf.info>
Date: 2018-03-15T02:56:52Z
Lists: pgsql-hackers 
Hi,

On 2018-03-14 22:36:52 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2018-03-13 15:29:33 -0700, Andres Freund wrote:
> >> On 2018-03-14 10:32:40 +1300, Thomas Munro wrote:
> >>> It looks
> >>> like -fexcess-precision-standard is coming from a configure test that
> >>> was run against ${CC}, not against ${CLANG}, so I hacked the generated
> >>> src/Makefile.global to remove that too, just to see if I could get
> >>> past that.
> 
> >> Yea, I'd hoped we could avoid duplicating all the configure tests, but
> >> maybe not :(.
> 
> > I've mostly done that now (not pushed).  I've created new
> > PGAC_PROG_VARCC_VARFLAGS_OPT(compiler variable, flag variable, testflag)
> > function, which now is used to implement PGAC_PROG_CC_CFLAGS_OPT and
> > PGAC_PROG_CC_VAR_OPT (similar for CXX).  That makes it reasonable to
> > test the variables clang recognizes separately.
> 
> Meh.

Why?  The necessary configure code isn't that large:

# Test for behaviour changing compiler flags, to keep compatibility
# with compiler used for normal postgres code. XXX expand
if test "$with_llvm" = yes ; then
  PGAC_PROG_VARCC_VARFLAGS_OPT(CLANG, BITCODE_CFLAGS, [-fno-strict-aliasing])
  PGAC_PROG_VARCC_VARFLAGS_OPT(CLANG, BITCODE_CFLAGS, [-fwrapv])
  PGAC_PROG_VARCC_VARFLAGS_OPT(CLANG, BITCODE_CFLAGS, [-fexcess-precision=standard])

  AC_SUBST(BITCODE_CFLAGS, $BITCODE_CFLAGS)
fi

If the relevant clang version doesn't understand, say
-fno-strict-aliasing, then we'd in trouble already if it's
required. After all we do support compiling postgres with clang.


> I agree with Thomas' concern that it's not clear we can or should
> just ignore discrepancies between the -f options supported by the C
> and CLANG compilers.

What's the precise concern here? We pass these flags to work around
compiler issues / "defining our standard". As I said above, if we do not
know the right flags to make clang behave sensibly, we're in trouble
already.

For a good part of the code we already want to be compatible with
compiling postgres with one compiler, and linking to libraries compiled
with something else.


> Is it really so necessary to bring a second compiler into the mix for
> this?  Why not just insist that JIT is only supported if the main build
> is done with clang, too?  My experience with mixing results from different
> compilers is, eh, not positive.

I don't like that option. It doesn't really buy us much, a few lines of
config code, and one additional configure option that should normally be
autodected from the environment.  Requiring a specific compiler will be
terrible on windows, seems out of line how we do development, requires
using clang which is still generates a bit slower code, prevent getting
gcc warnings etc.


Greetings,

Andres Freund

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".