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If you don't put in ANY optimization flags, it puts in full debugging info, which slows things down a lot.

I don't know anything about Java optimization.

Java doesn't have any optimization flags and the debugging info slowdown makes sense I suppose. That is the exact difference between them, GCC with -O3 is almost 100x faster in another test I just did: 10s for Java, 0.1 for C. When you turn off the optimization flags C jumps right at the same 10s figure though.

Really cool interesting stuff!

That's what I figured - a lot of people who say "Java is as fast as C" aren't doing proper compiler optimizations on their C. I suspected that was true, but I didn't spend the time on a proper test.

-O2 or -O3 does some neat stuff like unrolling loops and inlining functions

--fast_math is another one I use

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