[llvm-dev] Performance of JIT execution

[Haoran Xu via llvm-dev]

Thanks for the clarification Lang! I didn't know about CodeGenOpt before.
I'll give it a try to see if it fixes the issue.

Thanks again!
Haoran

Lang Hames <lhames at gmail.com> 于2020年9月6日周日 下午11:03写道：

> Hi Haoran,
>
> LLJIT uses CodeGenOpt::Default by default, whereas I suspect -O3 uses
> CodeGenOpt::Aggressive. You can configure this by setting/modifying the
> JITTargetMachineBuilder member of your LLJITBuilder before calling create.
>
> You can also try attaching a DumpObjects instance to your JIT to dump the
> JIT'd objects to disk: sometimes comparing objects can offer useful
> insights. You can find an example of this in
> llvm/examples/OrcV2Examples/LLJITDumpObjects.
>
> Regards,
> Lang.
>
> On Thu, Sep 3, 2020 at 7:01 PM Haoran Xu via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello,
>>
>> I recently noticed a performance issue of JIT execution vs native code of
>> the following simple logic which computes the Fibonacci sequence:
>>
>> uint64_t fib(int n) {
>> if (n <= 2) {
>> return 1;
>> } else {
>> return fib(n-1) + fib(n-2);
>> }
>> }
>>
>> When compiled natively using clang++ with -O3, it took 0.17s to compute
>> fib(40). However, when executing using LLJIT, fed with the IR output of
>> "clang++ -emit-llvm -O3", it took 0.26s.
>>
>> I don't know much about the internals of LLJIT, but my guess is since the
>> IR is the same, maybe LLJIT used a cheaper but lower quality instruction
>> selection pass, resulting in the slower runtime? Could someone working on
>> LLJIT clarify the difference in lowering passes between LLJIT and clang++?
>> And if I were to change this behavior, which APIs should I look at to begin
>> with?
>>
>> Thanks for your time!
>>
>> Best regards,
>> Haoran
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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