On Sun, May 24, 2015 at 2:15 PM, Robert Mijakovic <mijakovi@xxxxxxxxx> wrote: > Dear all, > > I have done some experiments with optimization flags enabled by different O > flag levels. > I took optimizations enabled by O[1..3] and tried to enable them all at once > without calling > O flag levels. I have found out that improvements I got by using all flags > used by the > particular O flag level are not even close to what I would get by explicitly > using O flag levels. > What is the reason for not reaching the same level of performance of O flag > levels? > Related, -Os usually gives me the best performance. I believe -Os often gives me the best performance because it keeps the caches hot. Its the same observation Richter made with respect to Windows performance. Richter is a Windows author and guru. He wrote the Advanced Windows Programming series (http://www.amazon.com/dp/1572315482). -Os is also useful on mobile, where you have to watch for code bloat. Jeff
