How do I find out why g++ takes a very long time on a particular file?

Question

I am building a lot of auto-generated code, including one particularly large file (~15K lines), using a mingw32 cross compiler on linux. Most files are extremely quick, but thi

Answer 1

It most probably includes TONNES of includes. I believe -MD will list out all the include files in a given CPP file (That includes includes of includes and so forth).

Answer 2

What the compiler sees is the output of the pre-processor, so the size of the individual source is not a good measure, you have to consider the source and all the files it includes, and the files they include etc. Instantiation of templates for multiple types generates code for each separate type used, so that could end up being a lot of code. If you have made extensive used of STL containers for many classes for example.

15K lines in one source is rather a lot, but even if split up, all that code still needs to be compiled; however using an incremental build may mean that it does not all need compiling all the time. There really is no need for a file that large; its just poor practice/design. I start thinking about better modularisation when a file gets to 500 lines (although I am not dogmatic about it)