Re: New LLVM JIT Features

On 2019-04-03 10:44:06 +0530, preejackie wrote:
> Hi Andres,
> 
> Thanks for the reply! Please see my comments inline.
> 
> On 03/04/19 3:20 AM, Andres Freund wrote:
> > Hi,
> > 
> > On 2019-04-02 00:51:51 +0530, preejackie wrote:
> > > As LLVM ORC supports compiling in multiple backend threads, it would be
> > > effective if we compile the functions speculatively before they are called
> > > by the executing function. So when we request JIT to compile a function, JIT
> > > will immediately returns the function address for raw executable bits. This
> > > will greatly reduce the JIT latencies in modern multi-core machines.
> > I personally think this should be approached somewhat differently -
> > putting patchpoints into code reduces the efficiency of the generated
> > code, so I don't think that's the right approach. What I think we should
>  What do you mean by patch points here? To my knowledge, LLVM symbols have
> arbitrary stub associated which resolve to function address at function
> address.

I was assuming that you'd want to improve latency by not compiling all
the functions at the start of the executor (like we currently do), but
have sub-functions compiled in the background. That'd require
patchpoints to be able to initially redirect to a function to wait for
compilation, which then can be changed to directly jump to the function.
Because we already just compile all the functions reachable at the start
of execution in one go, so it's not a one-by-one function affair.


> > do is to, if we decide it's worthwhile at plan time, generate the LLVM
> > IR time at the beginning of execution, but continue to use interpreted
> > execution initially. The generated IR would then be handed over to a
> > background [process|thread|whatnot] for optimization of code
> > generation. Then, when finished, I'd switch over from interpreted to JIT
> > compiled execution.  That approach will, in my view, yield better
> > latency behaviour because we can actually evaluate quals etc for which
> > we've not yet finished code generation.
> > 
> > 
> > > And also I'm working on designing a ORC in-place dynamic profiling support, by
> > > this JIT will automatically able to identify the hot functions, and compile
> > > it in higher optimization level to achieve good performance.
> > I think that's a nice concept, but at the moment the generated code is
> > so bad that it's much more likely to get big benefits by improving the
> > generated IR, compared to giving more hints to the optimizer.
> By improving the generated IR, you mean by turning pgsql queries into LLVM
> IR? If it is the case, this design doesn't handles that, it works only when
> the given program representation is in LLVM IR.

My point is that we generate IR that's hard for LLVM to optimize. And
that fixing that is going to give you way bigger wins than profile
guided optimization.

Greetings,

Andres Freund