I don't know how omp simd is related to "blocking", but it can slow code by a factor of two compared with GCC optimizing the loop normally, because you get AVX(

I don't know how omp simd is related to "blocking", but it can slow code by a factor of two compared with GCC optimizing the loop normally, because you get AVX(51)2 but not FMA. (Observed with a generic C GEMM, which got about 60% of the micro-optimized one after removing such pragmas and just letting gcc do its -Ofast thing.)