Re: -Os versus -O2

[Segher Boessenkool <segher@kernel.crashing.org>]

>> 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).
>
> actually, what you are saying is that the compiler can't know enough 
> to figure out how to optimize for speed. it will just do what you tell 
> it to, either unroll loops or not.

It bases its optimisation decisions on the options you give
it, the profile feedback information you either or not gave
it, and a whole bunch of heuristics.

> this argues that both O2 and Os are incorrect for a project to use and 
> instead the project needs to make it's own decisions on this.

For optimal performance, you need to fine-tune options yes,
per file (or per function even!)

> if this is the true feeling of the gcc team I'm very disappointed, it 
> feels like a huge step backwards.

I speak only for myself.  However this is the only way it _can_
be, the compiler isn't clairvoyant.  Some of the heuristics sure
could use some tuning, but they stay heuristics.


Segher