On Wed, Oct 30, 2019 at 10:06:57AM +0100, David Brown wrote: > But this also brings up another idea. Is the OP using "-Os" > optimisation? My experience (especially with AVR, msp430, and ARM > Cortex-M targets) is that "-Os" optimisation is often quite poor > compared to "-O2". It can result in very significantly slower code for > a saving of a couple of bytes, and in some cases the code can be > significantly /bigger/ than with -O2. I don't know whether this is a > backend issue, or a general problem with "-Os", but I no longer use or > recommend "-Os" even for tiny embedded systems. I have done the same for many years now, on not so tiny systems as well: if you want compact code, you use -O2, and tune both the code and the compiler flags manually. Some of the --params are very useful. But do not forget about tuning the code itself, that is where the biggest gains are possible, always. (If a better solution is hard to come up with, try finding a better problem, instead!) But if you have no time to waste, -Os usually give you smaller code than -O2, even if some of the choices on the size <-> speed tradeoff are a bit questionable (bug reports or patches welcome, of course!) And sometimes -Os is unavoidably a bit bigger than -O2, it just sets *heuristics* after all. But if you see this often (for some specific target?), please file a PR for that as well. Segher
