[cfe-dev] [LLVMdev] RFC: Upcoming Build System Changes

[David A. Greene]

Óscar Fuentes <ofv at wanadoo.es> writes:

> A good measure of how fast a set of Makefile are is to run the build
> with all targets up-to-date. Both builds takes a few seconds (3 or so)
> on my Linux quad core box. Whatever improvement can be achieved on this
> seems pretty insignifant.

Oh, it's significant.  When I build the Cray compiler with only non-LLVM
stuff changed, the actual compiles (the Cray stuff) finish before the
LLVM figures out nothing has changed.  This is a sginificant
productivity loss.  The Cray compiler uses a non-recursive make and so
gets tons of parallelism the LLVM build simply can't see because it's
recursive.

> Furthermore, recursive make is necessary for automatic generation of
> header dependencies, among other things. The makefiles generated by
> cmake are "partially" recursive for that reason:

Eh?  This is not true.  See for example:

http://mad-scientist.net/make/autodep.html

                           -Dave