He specifically asked gcc for optimisation for code size (-Os). For speed, he should be using -O3 only. He used "-Os -O3". This invalidates the benchmark.
The only justification for using -Os in a speed benchmark is "I tried it both with and without the flag, and it was faster this way". I don't see any such assertion.
Really? It seems to me that this is quite enough:
> I’ve just re-run the C benchmark without -Os (only -O3) but the results are the same.
Incidentally, this is probably the largest reason why so many people still use -O3 -- it wins in exactly the kind of programs that are used as simple and common benchmarks. It solidly loses on almost everything else.
Do you have any data on that? Most CPU bound programs should have pretty good instruction locality, negating the effects of smaller code. But without some numbers this is pointless guesswork.
As we know, however, benchmarking can often come down to tuning. If this most basic of compiler options has not been set to the obvious choice for speed, how can we have any confidence that the C code as written is written in an efficient way?
Are we comparing language against language here, or somebody's implementation in one language against somebody's implementation in another?
I note that there appear to be hand optimisations in the C code. Were these done well, or would the compiler have done a better job?
$ gcc -c -Q -Os -O3 --help=optimizers > Os-O3-opts
$ gcc -c -Q -O3 --help=optimizers > O3-opts
$ diff Os-O3-opts O3-opts
$