Compile nbody test from LFortran

[czgdp1807]

num = 1000000

Compiler	Time (s)	Relative
lc	1.22	1.0
clang++ -O3 -std=c++20	3.03	2.48
clang++ -std=c++20	>= 44.94	>= 36.84

[czgdp1807]

Not ready for review yet.

[certik]

If you compile using:

$ clang++ -O3 -march=native -ffast-math nbody.cpp

Then I get 2.827s on Apple M1 Max and Clang 16.0.6.

I get the following timings without optimization:

$ lc nbody.cpp
$ time ./nbody.out
...
./nbody.out  1.30s user 0.00s system 99% cpu 1.305 total

And with optimizations:

$ CPATH=$CONDA_PREFIX/include lc --show-llvm integration_tests/nbody.cpp > x.ll
$ clang++ -O3 -march=native -ffast-math x.ll src/runtime/liblc_runtime_static.a
$ time ./a.out
[...]
./a.out  0.17s user 0.00s system 98% cpu 0.170 total

As far as compilation speed goes, I get 1.019s for LC and 1.428s for Clang++, in the default (no compiler options) mode.

[certik]

I also tried the fastest C++ version from here: https://gist.github.com/certik/c7803fe6cbf4f8a5ec3070dd699ecad3

And I get

$ clang++ -O3 -march=native -ffast-math a.cpp
$ time ./a.out 1000000                       
-0.169075164
-0.169086185
./a.out 1000000  0.04s user 0.00s system 92% cpu 0.046 total

So 3.6x faster than our LC version. Overall not bad I think as a start. We'll optimize things more in the future.

This version https://benchmarksgame-team.pages.debian.net/benchmarksgame/program/nbody-gpp-3.html runs at 0.073s, so only 2.3x faster.

[czgdp1807]

Sure. I will work on these two examples next. There are excellent features which will be added through these two examples.