I actually agree with you.

My point wasn't "don't optimize" it was "don't optimize the wrong thing".

Trying to replace a division with a bit shift is an example of worrying about the wrong thing, especially since that's a simple optimization the compiler can pick up on.

But as you said, it can be very worth it to optimize around things like the icache. Shrinking and aligning a hot loop can ensure your code isn't spending a bunch of time loading instructions. Cache behavior, in general, is probably the most important thing you can optimize. It's also the thing that can often make it hard to know if you actually optimized something. Changing the size of code can change cache behavior, which might give you the mistaken impression that the code change was what made things faster when in reality it was simply an effect of the code shifting.

I originally got into writing compilers because I was convinced I could write a better code generator. I succeeded for about 10 years in doing very well with code generation. But then all the complexities of the evolving C++ (and D!) took up most of my time, and I haven't been able to work much on the optimizer since.

Fortunately, D compilers gdc and ldc take advantage of the gcc and llvm optimizers to stay even with everyone else.