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Comment by wycy | Hacker News Reader

> I get 3-4x speed up with Intel Fortran compiler over gfortran

Wow, even at similar optimization levels? -O3 for each?

Not quite, but at optimization levels set by default WRF configuration for each compiler (definitely not a fair comparison):

gfortran: -O2 -ftree-vectorize -funroll-loops ifort: -O3

I don't have the timing results anymore. This was in 2018 on Xeon Platinum 8168.

I recently tried replacing -O2 with -Ofast -ffast-math to gfortran settings, which gives about 18% speed up. So still far from Intel. I recently proposed it here [1].

[1] https://github.com/wrf-model/WRF/issues/1254

Have you posted about this on the gfortran mailing lists? They are generally interested in examples that show where the compiler could be improved.

I doubt they are short on inspiration for further improvements. A big part of the speed advantage of ifort is the aggressive loop unrolling, pipelining, splitting and multiversioning.

Coming up with these transformations is not the difficult part. That would be actually implementing these transformations correctly and keep the whole compiler optimization framework maintainable.

And of course there is certainly a cost for the improved runtime. Primarily in compile time and code size. For the Intel compiler it generally makes sense to sacrifice these in favor of runtime performance, because it is used on these kind of scientific codes a lot, but for gfortran the balance might different.

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