All,
Recently, one of the computing centers I run on updated their OS. And in that
update, the model went from "working with GNU" to "crashing with GNU". No code
change on our side, just OS.
Some experimenting later and I found that the code did run with debugging
options, and it still ran with our "aggressive" options (much of which is due
to Jerry DeLisle from here). Only our release flags failed. Surprising since
the Aggressive options seem more likely to have issues as they are speed for
speed's sake (different MPI layouts lead to different answers).
But, one of the main differences are the aggressive flags use -O2 and our
release flags are -O3. So I test our release flags with -O2 and boom, works
again! Bad news: much slower.
Our release flags are (essentially):
-O3 -march=haswell -mtune=generic -funroll-loops -g -fPIC -fopenmp
so we aren't doing anything fancy (portability at the cost of speed).
Staring at the man page I saw this:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
and when I did that I saw:
$ diff /tmp/O2-opts /tmp/O3-opts | grep enabled
> -fgcse-after-reload [enabled]
> -fipa-cp-clone [enabled]
> -floop-interchange [enabled]
> -floop-unroll-and-jam [enabled]
> -fpeel-loops [enabled]
> -fpredictive-commoning [enabled]
> -fsplit-loops [enabled]
> -fsplit-paths [enabled]
> -ftree-loop-distribution [enabled]
> -ftree-partial-pre [enabled]
> -funroll-completely-grow-size [enabled]
> -funswitch-loops [enabled]
> -fversion-loops-for-strides [enabled]
Now, I'll be doing some experiments, but...that's a lot of tests and rebuilds.
I was hoping maybe someone here can point me to "this flag is useful for
Fortran" vs "this doesn't matter".
And maybe which one might be triggered by an OS update? ¯\_(ツ)_/¯
Thanks,
Matt
--
Matt Thompson, SSAI, Ld Scientific Programmer/Analyst
NASA GSFC, Global Modeling and Assimilation Office
Code 610.1, 8800 Greenbelt Rd, Greenbelt, MD 20771
Phone: 301-614-6712 Fax: 301-614-6246
http://science.gsfc.nasa.gov/sed/bio/matthew.thompson
