I wrote my own firmware for some IoT devices as a personal project. At first I tried to be careful about optimizing all the things, but when I changed course and rewrote parts to be more readable instead it compiled to exactly the same binary. So now I just leave optimizations to the compiler. (Nothing I’m writing is so performance critical that I need to manually optimize further, like I’m sure is necessary for other situations.)
If your thing is too big/ slow/ ugly/ stupid, you need to be able to quantify that, so that you can tell whether you really improved it.
If you aren't measuring, any "optimisation" is just wanking, whether you're wanking before the product works or after. If it's many orders of magnitude different the "measurement" may be pretty casual. I had a script which used to take two weeks, now it runs overnight, I can't give you exact timings, however "overnight" versus "two weeks" is a measurement. But often improvements are smaller, which means your measurements need to be more careful.
Measuring also helps focus on actual goals. I'm pretty sure that script could be improved to take under an hour. But, I'm also certain nobody cares whether it takes one hour or ten, so, not a priority. Whereas when it took weeks that was causing problems.
I completely agree with you.
For work I'm writing a performance critical application, and using gprof/valgrind for profiling has been very useful. Not only for determining how time is spent, but also where it is _not_ spent. Some pieces of code can be implemented in a fast and stupid way, without impacting overall performance.
That said, inlining is precisely an area where I find profiling to be difficult. It's difficult to see where in a function time is spent if a lot of function-calls in that function are inlined. However, I'm not certain that the every part of my code is equally slower/faster depending on what optimizations I turn on/off in the compiler. Thus, reducing optimizations might give me the wrong impression about which functions take up the most runtime.
