Messages in this thread | | | From | Segher Boessenkool <> | Subject | Re: -Os versus -O2 | Date | Mon, 25 Jun 2007 09:03:35 +0200 |
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> then do we need a new option 'optimize for best overall performance' > that goes for size (and the corresponding wins there) most of the > time, but is ignored where it makes a huge difference?
That's -Os mostly. Some awful CPUs really need higher loop/label/function alignment though to get any performance; you could add -falign-xxx options for those.
> in reality this was a flaw in gcc that on modern CPU's with the larger > difference between CPU speed and memory speed it still preferred to > unroll loops (eating more memory and blowing out the cpu cache) when > it shouldn't have.
You told it to unroll loops, so it did. No flaw. If you feel the optimisations enabled by -O2 should depend on the CPU tuning selected, please file a PR.
Also note that whether or not it is profitable to unroll a particular loop depends largely on how "hot" that loop is, and GCC doesn't know much about that if you don't feed it profiling information (it can guess a bit, sure, but it can guess wrong too).
Segher
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