[llvm-dev] MCJit Runtine Performance

[Morten Brodersen via llvm-dev]

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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