Re: Compiling for newer Intel CPUs with an older Intel build system?

[Mike Frysinger]

On 04 Mar 2016 16:11, Sean Byland wrote:
> I’m trying to help users get autotools-based projects to compile in our 
> somewhat unique environment. It’s fairly common for users to want to compile 
> on a Intel ivybridge system (node) with Intel broadwell-specific (a superset 
> of CPU instructions) performance optimization to run elsewhere (a compute 
> node). With default options configure won’t handle this scenario because the 
> build system can’t execute the Broadwell-specific x86 instructions. In 
> varying degrees of success configure will work by:
> 
>   1.  Setting --build to a slightly different canonical name to the --build 
> name,  indicating to configure that it should do a cross-compile
> 
>   2.  Generating a config.cache on the Ivybridge compute node, which shares 
> the majority of the file system with the Sandybridge system and can 
> successfully execute everything. Then point configure on the sandybridge 
> system at the cache generated while using the Ivybridge CPU.
> 
>   3.  Setup configure to use a test “launcher,” so configure tests will be 
> launched on the Ivybridge system.
> 
> 
> I like option one because it seems to follow a use-case that the autotools 
> were designed for but I seem to get regular failures because (for example) 
> the packager will use AC_TRY_RUN without defining a sensible 
> “[action-if-cross-compiling]”. I like that option three allows configure’s 
> main code path to function as intended but strongly dislike that it requires 
> use of an Broadwell CPU that won’t always be available for every build and 
> would probably require hacking and/or a configure user to perform packager 
> actions. Option two get’s test results from the desired execution 
> environment, allows configure to run really fast, and only requires minimal 
> use of the Ivybridge system. Is using the cache in this manner generally a 
> bad idea?
> 
> 
> I’d also appreciate any general feedback, in the past my lack of autotools 
> knowledge has led me to “fight” things and so I’d like to avoid deviating too 
> far from what these tools were designed for.

seems like the only thing you need to do is properly set CFLAGS/CXXFLAGS.
if you want a build that'll work on all x86 systems, then use something
like:
        ./configure CFLAGS='-O2 -march=x86-64 ...' CXXFLAGS='-O2 -march=x86-64 
...'

no need to mess with --build or --host, or config.cache.  i'm assuming
the configure script doesn't attempt to detect CPU extensions via some
RUN tests ... if it does, then the script should be fixed to do define
probing and/or add a configure flag to control them (like --disable-mmx).
-mike

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