Peter Maydell <peter.mayd...@linaro.org> writes: > On 15 March 2016 at 09:29, Markus Armbruster <arm...@redhat.com> wrote: >> = What to do about it = >> >> The immediately obvious thing to do is reduce "recompile the world" >> headers that change frequently. I've started to do that. > > This is obviously worthwhile. > >> Another one is attacking widely included bulky files (see "Top >> scorers"). Some can simply be included less. Others need to be split, >> in particular the generated tracers. > > But how important is this? Yes, the headers may make up a large > %-of-total-source-lines, but how much extra actual compile time > do they add? My off-the-top-of-my-head guess is that most of the > time will be spent on optimization passes of actual code, and that > things like structure definitions, typedefs, etc that you find in > headers aren't going to make a major contribution.
I readily concede that we shouldn't speculate about compile time without measurements. We need to do something about the generated tracers, though. Every time you fiddle with a tracepoint, you get to recompile ~900 out of ~4000 files. I suspect it's only 900 mostly because people avoid tracepoints for that exact reason. I certainly do. If only the fiddled-with file got recompiled, I'd happily use them for debugging, and when done post any new tracepoints I found useful for others to enjoy. >> Yet another one is reviewing the way we include system and GLib headers. > > We must include glib.h from osdep.h because it has compatibility > fixups (in glib-compat.h) and we need to avoid the risk of a .c > file including the system header directly without the fixups. I understand the problem. Perhaps we can find a less heavy-handed solution. Not a priority for me right now. >> But our root problem is our undisciplined use of #include. Can we agree >> on a sane set of rules? Here's my proposal: >> >> 1. Have a carefully curated header that's included everywhere first. We >> got that already thanks to Peter: osdep.h. >> >> 2. Headers should normally include everything they need beyond osdep.h. >> If exceptions are needed for some reason, they must be documented in >> the header. If all that's needed from a header is typedefs, put >> those into qemu/typedefs.h instead of including the header. >> >> 3. Cyclic inclusion is forbidden. > > I would be happy with these rules; we're a fair way away from them > though. (Part of my aim with the osdep cleanup was to get to a point > where it was easier to make header files self-contained without that > implying every header needing a lot of system #includes.) Okay, I can try to take us a few more steps in this direction.
