I have the pleasure of working on a system with a compiler not as complete as those you find with C/C++/C# or in fact, as complete as almost all other compilers.
What it does not do, is any form of optimisation.
Although I derive a pleasure of sort in working out new ways to optimise code and have even wrote a pre-processor to handle some common cases, I dearly wish it would just to it for me.
Primary reason being, we have large batch processes that run every night, as well as even larger day long jobs at every end of month. My hand done optimisations generally result in anywhere between 10-200% improvements. Factor that in to process that take more then 1 day to run and you have meaningful improvements. With a code base > 1M LoC spread over 3000+ programs; it's a task I cannot do in an effective manner.
Give me a supported compiler that does it however and we can simply recompile everything!
And life would be good.