https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51119
--- Comment #29 from Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> --- (In reply to Thomas Koenig from comment #27) > (In reply to Joost VandeVondele from comment #22) > If the compiler turns out not to be reasonably smart, file a bug report :-) what is needed for large matrices (in my opinion) is some simple loop tiling, as can, in principle, be achieved with graphite : this is my PR14741 Good vectorization, which gcc already does well, just requires the proper compiler options for the matmul implementation, i.e. '-O3 -march=native -ffast-math'. However, this would require the Fortran runtime to be compiled with such options, or at least a way to provide specialized (avx2 etc) routines. There is however the related PR (inner loop of matmul) : PR25621, where some unusual flag combo helps (-fvariable-expansion-in-unroller -funroll-loops) I think external blas and inlining of small matmuls are good things, but I would expect the default library implementation to reach at least 50% of peak (for e.g. a 4000x4000 matrix), which is not all that hard. Actually, would be worth an experiment, but a Fortran loop nest which implements a matmul compiled with ifort would presumably reach that or higher :-). These slides show how to reach 90% of peak: http://wiki.cs.utexas.edu/rvdg/HowToOptimizeGemm/ the code actually is not too ugly, and I think there is no need for the explicit vector intrinsics with gcc. I believe I had once a bug report open for small matrices, but this might have been somewhat fixed in the meanwhile.
