Ben de Groot wrote:
-Os optimizes for size, while -O2 optimizes for speed. There is no need at all to use -Os on a modern desktop machine, and it will run comparatively slower than -O2 optimized code, which is probably not what you want.
There are a couple of schools of thought on that, and I think performance can depend a great deal on what program you're talking about.
On any machine, memory is a limited resource. Oh sure, you could just "spend a little more on decent RAM", but you could also "spend a little more on a decent CPU" or whatever. For a given amount of money you can only buy so much hardware, so any dollar spent on RAM is a dollar not spent on something else.
So, if you reduce the memory footprint of processes, then you increase the amount of memory available for buffers/cache. That cache is many orders of magnitude faster than even the fastest hard drive. You also reduce swapping which obviously greatly slows things down.
On the other hand, if you have a smaller program that does highly-CPU-intensive tasks like compression/transcoding/etc then speed optimization makes sense just about all the time (generally -O2 only - -O3 sometimes does worse due to L2 cache misses).
So, there are trade-offs. To make things even more complicated the practical results can be impacted by your CPU model - different CPUs have a different cost for looping/jumping/branching vs cache misses. And the compiler makes a big difference - many of these observations date back to the gcc 3.4 days. I've heard that newer GCC versions are more aggressive with size optimization at the expense of speed, which could tip the balance.
A while back there were some posts from folks who had done some actual benchmarking. I don't think it has been repeated. Note that I wouldn't expect -Os to have much benefit unless you compile EVERYTHING on the system that way - since the freeing up of memory is cumulative. The rather high level of pain associated with rebuilding your system with -Os vs -O2 for some benchmarking and subjective evaluation is probably the reason why everybody has an opinion but there isn't much hard data.
Right now I'm using -Os across the board, and -O2 on an exception basis. Maybe I should give that some more thought...
