Re: failure of cross-compilation detection on BlueGene/L

[Christian Rössel]

Steven G. Johnson schrieb:
> Hi, I noticed a failure of the "checking whether we are cross compiling" 
> on BlueGene/L, where it erroneously decides that it is *not* 
> cross-compiling when in fact it *is*.  I'm willing to submit a patch for 
> this (my copyright assignment is on file), but first I wanted to get 
> some feedback from you guys about the problem and solution.
>
> BACKGROUND:
>
> As you may or may not know, some of the major supercomputers these days 
> (Cray XT3 and IBM BlueGene) require cross-compiling: the front-ends run 
> GNU/Linux, but the actual compute nodes run some half-baked 
> pseudo-POSIX-ish minimal OS, so every program that you want to run has 
> to be cross-compiled.  (I'm not happy with this situation, but dealing 
> with quirky, exotic system software has long been a fact of life in 
> supercomputing.) Hence, cross-compiling to quirky systems has suddenly 
> become common in scientific computing, which is unfortunate since most 
> scientific users are unfamiliar with cross-compiling in theory or in 
> practice.
>
> PROBLEM:
>
> Anyway, I noticed on BlueGene/L that when I specify a 
> --host=powerpc64-ibm flag and also give it CC manually (none of these 
> supercomputers use the GNU naming conventions for their 
> cross-compilers), it still says:
>
>     checking whether we are cross compiling... no

Hi Steven,

if I just set --host without specifying --build cross_compiling is set 
to "maybe" and I get following warning:
configure: WARNING: If you wanted to set the --build type, don't use --host.
    If a cross compiler is detected then cross compile mode will be used.

Therefore I usually specify --host *and* --build if I want to cross 
compile. If $host != $build, configure sets cross_compiling to yes and 
everything works like expected, at least on Blue Gene/P.

Note that we are working on some heuristics to detect supercomputer 
platforms like Blue Gene, Cray, NEC etc. that require cross compilation. 
This detection will be available as a macro soon and render it 
unnecessary that the user specifies --host and --build on the supported 
platforms.

Cheers,
Christian