Re: New LLVM JIT Features

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.
> 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.
>
> Greetings,
>
> Andres Freund

--
Have a great day!
PreeJackie