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

[Bob Friesenhahn]

On Sun, 23 Mar 2014, 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).
> often configure scripts take longer to finish than compiling the source code 
> itself on decent machines with multiple cpu cores.

Agreed.

> having an option like --assume-c99 could provide a shortcut so all checks

I don't like any approach which hard-codes assumptions in Autoconf. 
Hard-coded assumptions can easily be wrong and configure scripts could 
be used ten years later without being re-generated with the latest 
rules.

The Autoconf configuration cache mechanism (e.g. --config-cache) is 
very effective.  In my test here, using --config-cache reduced 
repeated configure run time from 20 seconds to 5 seconds.  By 
transcribing selected lines from config.cache to config.site, these 
cached values can be made the default for a system.  Modern autoconf 
searches for both $prefix/share/config.site and 
$prefix/etc/config.site and will use whichever ones are found.

E.g.

egrep '^ac_cv_func_select' config.cache >> /usr/local/share/config.cache

and in /usr/local/share/config.site

. /usr/local/share/config.cache

In fact, one could take a boilerplate configure.ac, use it to create a 
bare configure, and then use most of its config.cache to form 
config.site.

With this approach, Autoconf can be optimized for a system with known 
behavior without placing brittle assumptions within Autoconf itself.

A separate script could be used to create a reusable subset of 
config.cache based on a specification.

BTFW that some software might not configure correctly given the 
presence of a config.site file.  In my experience, a config.site file 
can cause harm when configuring GCC.

Bob
--
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/