llvm dev - Jun 2016 - MCJIT -- Poor run-time performance for Fibonacci example in LLVM 3.8.1

We have been using LLVM 3.4 and are currently migrating to LLVM 3.8.1. We have
been using the Old JIT and since it has been removed since 3.6, we have to use
MCJIT. I find that run-time performance is very poor in 3.8.1 for the Fibonacci
example in llvm/examples/fibonacci.cpp. Logs below for fib34 and fib381 for
input values of 30 and 40. I first thought it could be that MCJIT compile time
is slower, but as I increase the input value, fib381 gets slower and slower in
run-time performance (almost 200X for an input value of 40). Any hints on what
could be going wrong? TIA.

% time fib34 30
...
starting fibonacci(30) with JIT...
Result: 832040
0.008u 0.000s 0:00.01 0.0%      0+0k 0+0io 0pf+0w

% time fib34 40
...
starting fibonacci(40) with JIT...
Result: 102334155
0.427u 0.003s 0:00.44 95.4%     0+0k 0+0io 0pf+0w

% time fib381 30
...
starting fibonacci(30) with JIT...
Result: 832040
0.795u 0.001s 0:00.81 97.5%     0+0k 0+0io 0pf+0w
% time fib381 40
...
starting fibonacci(40) with JIT...
Result: 102334155
97.591u 0.006s 1:37.71 99.8%    0+0k 0+0io 0pf+0w

Compile command used for each version (Platform: Linux
2.6.32-504.16.2.el6.x86_64):

clang++ -std=c++11 fibonacci.cpp `llvm-config --cxxflags --ldflags --libs` -ldl
-lpthread -lncurses -o fib381
clang++ fibonacci.cpp `llvm-config --cxxflags --ldflags --libs` -o fib34
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On 10 June 2016 at 05:31, Riyaz Puthiyapurayil via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> We have been using LLVM 3.4 and are currently migrating to LLVM 3.8.1. We
> have been using the Old JIT and since it has been removed since 3.6, we
have
> to use MCJIT.
Hi Riyaz,

I don't know much about it, but MCJIT was also replaced, by a new JIT
called "ORC".

The Kaleidoscope example has been updated, you can look there:

http://llvm.org/docs/tutorial/LangImpl4.html

cheers,
--renato

Hi Riyaz, Renato,

Attempting to run Fibonacci from LLVM 3.8.1 on Darwin I get:

./fib381: Failed to construct ExecutionEngine: Interpreter has not been
linked in.

This suggests that the reason for the slowdown isn't in the JIT itself.
Instead, when we removed the old JIT we probably didn't update the
Fibonacci example to request a JIT at all, so now it's falling back to the
interpreter. I'm just digging in to confirm this now, and will update the
example code as needed to fix this.

Cheers,
Lang.


On Fri, Jun 10, 2016 at 5:38 AM, Renato Golin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On 10 June 2016 at 05:31, Riyaz Puthiyapurayil via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > We have been using LLVM 3.4 and are currently migrating to LLVM 3.8.1.
We
> > have been using the Old JIT and since it has been removed since 3.6,
we
> have
> > to use MCJIT.
>
> Hi Riyaz,
>
> I don't know much about it, but MCJIT was also replaced, by a new JIT
> called "ORC".
>
> The Kaleidoscope example has been updated, you can look there:
>
> http://llvm.org/docs/tutorial/LangImpl4.html
>
> cheers,
> --renato
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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