That the C code is compiled with "gcc -O7" tells me just about everything I need to know about the soundness of these experiments. "-O3" and "-Ofast" are the highest optimisation levels, where "-Ofast" includes optimisations that break some standard-compliant programs (mostly -ffast-math). Not to mention that comparing running times for a single run on an unspecified machine isn't "benchmarking".
Despite how unbelievable it looks, I would love to see contents of the final binary to see what is really happening there. gcc code looks quite alright in this case: https://godbolt.org/g/R6iBup