[llvm-dev] Performance of JIT execution

[Lang Hames via llvm-dev]

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
>
>
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