[llvm-dev] MCJit Runtine Performance

[Morten Brodersen via llvm-dev]

Hi Rafael,

Not easily (llc).

Is there a way to make MCJit not use the large code model when JIT'ing?

Cheers
Morten

On 05/02/16 14:28, Rafael Espíndola wrote:
> Can you build the code with llc? Try with the large code model. I
> think that is the default for MCJIT and can be less efficient.
>
> Cheers,
> Rafael
>
>
> On 4 February 2016 at 22:26, Morten Brodersen via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Hi Hal,
>>
>> We are using the default register allocator. I assume the greedy one is
>> default?
>>
>> As for other target machine optimizations:
>>
>> I have tried:
>>
>> llvm::TargetMachine* tm = ...;
>>
>> tm->setOptLevel(llvm::CodeGenOpt::Aggressive);
>>
>> And it doesn't make much of a difference.
>>
>> And also:
>>
>> tm->setFastISel(true);
>>
>> (previous email).
>>
>> Is there anything else I can try?
>>
>>
>> On 05/02/16 11:16, Hal Finkel wrote:
>>> ----- Original Message -----
>>>> From: "Keno Fischer via llvm-dev" <llvm-dev at lists.llvm.org>
>>>> To: "Morten Brodersen" <Morten.Brodersen at constrainttec.com>
>>>> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
>>>> Sent: Thursday, February 4, 2016 6:05:29 PM
>>>> Subject: Re: [llvm-dev] MCJit Runtine Performance
>>>>
>>>>
>>>>
>>>> Yes, unfortunately, this is very much known. Over in the julia
>>>> project, we've recently gone through this and taken the hit (after
>>>> doing some work to fix the very extreme corner cases that we were
>>>> hitting). We're not entirely sure why the slowdown is this
>>>> noticable, but at least in our case, profiling didn't reveal any
>>>> remaining low hanging fruits that are responsible. One thing you can
>>>> potentially try if you haven't yet is to enable fast ISel and see if
>>>> that brings you closer to the old runtimes.
>>> And maybe the register allocator? Are you using the greedy one or the
>>> linear one? Are there any other MI-level optimizations running?
>>>
>>>    -Hal
>>>
>>>> On Thu, Feb 4, 2016 at 7:00 PM, Morten Brodersen via llvm-dev <
>>>> llvm-dev at lists.llvm.org > wrote:
>>>>
>>>>
>>>> Hi All,
>>>>
>>>> We recently upgraded a number of applications from LLVM 3.5.2 (old
>>>> JIT) to LLVM 3.7.1 (MCJit).
>>>>
>>>> We made the minimum changes needed for the switch (no changes to the
>>>> IR generated or the IR optimizations applied).
>>>>
>>>> The resulting code pass all tests (8000+).
>>>>
>>>> However the runtime performance dropped significantly: 30% to 40% for
>>>> all applications.
>>>>
>>>> The applications I am talking about optimize airline rosters and
>>>> pairings. LLVM is used for compiling high level business rules to
>>>> efficient machine code.
>>>>
>>>> A typical optimization run takes 6 to 8 hours. So a 30% to 40%
>>>> reduction in speed has real impact (=> we can't upgrade from 3.5.2).
>>>>
>>>> We have triple checked and reviewed the changes we made from old JIT
>>>> to MCJIt. We also tried different ways to optimize the IR.
>>>>
>>>> However all results indicate that the performance drop happens in the
>>>> (black box) IR to machine code stage.
>>>>
>>>> So my question is if the runtime performance reduction is
>>>> known/expected for MCJit vs. old JIT? Or if we might be doing
>>>> something wrong?
>>>>
>>>> If you need more information, in order to understand the issue,
>>>> please tell us so that we can provide you with more details.
>>>>
>>>> Thanks
>>>> Morten
>>>>
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>>>>
>>>>
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