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configure speedup proposal: add --assume-c99 and --assume-posix2008 flag

From: John Spencer
Subject: configure speedup proposal: add --assume-c99 and --assume-posix2008 flags
Date: Sun, 23 Mar 2014 01:24:54 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:17.0) Gecko/20130801 Thunderbird/17.0.8

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.

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

- have stdint.h
- have snprintf()
- etc

and for posix
- sizeof int == 4
- have select()
- etc

could be skipped and assumed to be always true.

only functions which are known to be non-conforming on some platforms (like the broken strerror_r() semantics of GLIBC which conflict with POSIX when certain CPPFLAGS are used), and special platform specific things like linux extensions needed to be checked separately using this mechanism.

to me, this sounds like a simple way to make configure runs at least 50% faster on most modern platforms, while not impacting legacy systems negatively at all.

having configure run much faster could also lead to a better public perception of autoconf in general.

as for how to implement, maybe these flags could just set the appropriate autoconf cache variables to true.


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