Thread: Optimization

It seems to me that if I were due to release some software, I would hand-optimize the code before running it through the compiler, where those optimizations would be purely for that release and not part of the main code repository. Once I had compiled and released the code, I would then continue to develop the main repository and only introduce the optimizations when compiling for release.

Could a compiler have some kind of mega-optimization option, or could there be a separate optimization program, that could work for minutes or hours, and substantially reorganize the code, before giving it to a compiler? Or is it always better to optimize while compiling?

No, that would be an utter waste of time.

You should not be thinking optimization (except at a high level, like algorithm selection) when writing code. That is, it's much more important to select the proper data structure for a particular operation than it is to choose what order to write a few statements in.

Modern compilers can do what is called link-time optimization, which optimizes the entire program, regardless of how many translation units it spans across. At least gcc (via -flto), Intel's compiler (-ipo), Solaris Studio (-xipo), Open64 (-ipa), and clang (-O4 with a suitable linker) support it. You might consider this to be a mega-optimization. LTO can have a great effect on the speed of your code, depending on how it's written. On a project of mine, I saw gains around 5% to 30% with gcc's LTO, depending on what operations were being done, mainly because the compiler was able to inline a lot of small functions across translation units.

If you do want to optimize your code by hand (which is sometimes a useful endeavor), you should profile it first, and find where it's actually slow. Valgrind includes great profiling tools called called cachegrind and callgrind; you can view the latter's reports with kcachegrind, which is an amazingly useful visualization tool.

Originally Posted by Richardcavell

It seems to me that if I were due to release some software, I would hand-optimize the code before running it through the compiler, where those optimizations would be purely for that release and not part of the main code repository.

Did you know this idea has a name?

It's called .... "Introducing untraceable errors".