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

[Óscar Fuentes]

greened at obbligato.org (David A. Greene) writes:

> 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.

How is that? Your compiler builds in less than 3 seconds? (IIRC, the
numbers are more like 1.5 seconds with a warm cache on a Linux 4core 2.4
GHz machine) Maybe the LLVM build is starved because of your compiler's
build.

> 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.

There is no parallel loss due to recursive calls. The cmake build will
happily compile files from different libraries/executables as long as
there are enough threads available and no blocking dependencies. To
check, simply run `make -j' and you'll see how dozens of compiler
processes are started. AFAIR the same applies to the `make' build.

>> 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

LLVM/Clang uses a lot of generated headers. I think that what that page
says does not apply to our case.