Re: configure speedup proposal: add --assume-c99 and --assume-posix2008 flags

[Thomas Petazzoni]

Dear John Spencer,

On Sun, 23 Mar 2014 01:24:54 +0100, John Spencer wrote:

> there are many configure scripts out there that still check for things 
> that are standard since at least 10 years, and doing this extensively 
> and over and over (people building software themselves usually build 
> more than one package) consumes a lot of time (especially due to the 
> non-parallel nature of configure scripts).

As one of the core developer of Buildroot (http://buildroot.org), a
tool that automates the process of cross-compiling a potentially
significant number of software components to generate embedded Linux
systems, I am also very concerned about the speed of configure scripts.
On a full build, the time taken by configure scripts is very often
around 25% of the overall build time, if not more.

The suggestion of using a cache is indeed very interesting. We actually
tried it a couple of years ago, with a global cache shared by all
packages we build. However, it turned out that several packages use the
same autoconf variable name to store the result of tests that are not
exactly the same, causing a number of issues. The proposal in the
thread to use a pre-defined cache with only a selected number of
known-safe entries looks like a good idea.

However, what seems like the most important problem to me is the
non-parallel nature of autoconf tests. With CPUs going more and more
multi-core, the build part of packages (which is usually highly
parallel) is going to get faster and faster comparatively to the
configure part of packages (which is going to get stuck to a slow speed
due to the non-parallel nature of autoconf). Wouldn't it be time to
think about moving autoconf to a more parallel logic, where N tests
could run in parallel in sub-processes?

Thanks,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com