While it's true that modern compilers are wondrous things, checking whether they're clever enough to optimize away a particular construct - and to do so correct

While it's true that modern compilers are wondrous things, checking whether they're clever enough to optimize away a particular construct - and to do so correctly, and to continue doing so in the next release - still takes time. If the same optimization can be done at a higher level, such that it will apply for any correct (but not necessarily clever) compiler, that's preferable. In my experience that's practically all the time. The best compiler optimizations IMO are the ones that can't be easily done at the source level.