Comments on: finally rebuilding

By: Steve

Steve — Fri, 25 Jul 2008 16:05:02 +0000

For a modern desktop, building with -Os you won’t notice much differences, yes … but in working with small systems with an incredible small amount of space and memory, your compiler settings will make a huge difference.

By: Mart Raudsepp

Mart Raudsepp — Fri, 25 Jul 2008 15:34:55 +0000

I basically also wanted to say what Tamas already said.
I guess I can add that if you have big CPU caches, which a Pentium D has I believe, then I don’t see how -Os can be good in any way. I generally don’t think twice about using -O2 if I have 1MB of L2 cache at least. If it’s some old thing with some 256KB CPU cache, then I do ponder for a while before choosing though

By: Tamas

Tamas — Fri, 25 Jul 2008 09:28:13 +0000

The -Os in GCC does not help with memory constrained systems, as it reduces only the code size, which makes up only a negligible proportion of an average desktop applications memory usage.

Also generally it’s not slower than other optimization levels in GCC. It uses almost the same optimizations as -O2, except for missing a couple ones that increase the code size. The result should be a 0-5% performance hit, however the smaller code helps with cache locality, thus it sometimes improves performance over -O2.

Furthermore, compiler optimization barely makes any difference for typical desktop applications, they are hardly CPU bound.