Re: JIT compiling with LLVM v9.0

On Wed, Jan 31, 2018 at 3:05 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2018-01-31 14:42:26 +1300, Thomas Munro wrote:
>> I'm just starting to look at this (amazing) work, and I don't have a
>> strong opinion yet. But certainly, making it easy for packagers to
>> put the -jit stuff into a separate package for the reasons already
>> given sounds sensible to me. Some systems package LLVM as one
>> gigantic package that'll get you 1GB of compiler/debugger/other stuff
>> and perhaps violate local rules by installing a compiler when you
>> really just wanted libLLVM{whatever}.so. I guess it should be made
>> very clear to users (explain plans, maybe startup message, ...?)
>
> I'm not quite sure I understand. You mean have it display whether
> available? I think my plan is to "just" set jit_expressions=on (or
> whatever we're going to name it) fail if the prerequisites aren't
> available. I personally don't think this should be enabled by default,
> definitely not in the first release.

I assumed (incorrectly) that you wanted it to default to on if
available, so I was suggesting making it obvious to end users if
they've accidentally forgotten to install -jit. If it's not enabled
until you actually ask for it and trying to enable it when it's not
installed barfs, then that seems sensible.

>> This seems to be a valid complaint. I don't think you should be
>> (indirectly) wrapping Types.h in extern "C". At a guess, your
>> llvmjit.h should be doing its own #ifdef __cplusplus'd linkage
>> specifiers, so you can use it from C or C++, but making sure that you
>> don't #include LLVM's headers from a bizarro context where __cplusplus
>> is defined but the linkage is unexpectedly already "C"?
>
> Hm, this seems like a bit of pointless nitpickery by the compiler to me,
> but I guess...

Well that got me curious about how GCC could possibly be accepting
that (it certainly doesn't like extern "C" template ... any more than
the next compiler). I dug a bit and realised that it's the stdlib
that's different: libstdc++ has its own extern "C++" in <cmath>,
while libc++ doesn't.

--
Thomas Munro
http://www.enterprisedb.com