RPath compiler wrapper breaking precompiled header builds

[paulmelis]

This issue was found when building OpenCV-4.0.1-foss-2018b-Python-3.6.6.eb with EB 4.1.1. The build has multiple errors undefined reference to main. This is caused by the rpath wrappers adding -Wl,-rpath=... options to the g++ call, which turns otherwise compile-only commands to generate precompiled headers (-x c++-header) into commands that link.

See easybuild-OpenCV-4.0.1-20200623.143513.mcEaZ.log.gz for the full logs. But it can also be shown using only the generated rpath wrapper to compile a simple file:

paulm@tcn180 14:43 ~$ m list
Currently Loaded Modulefiles:
 1) surfsara   2) 2019   3) EasyBuild/4.1.1   4) compilerwrappers   5) eb/4.1.1(default)   6) GCCcore/8.2.0   7) zlib/1.2.11-GCCcore-8.2.0   8) binutils/2.31.1-GCCcore-8.2.0   9) GCC/8.2.0-2.31.1  

paulm@tcn180:~$ cat t.cc
#include <cstdio>

# Straight g++ call, no wrappers, succeeds
paulm@tcn180 14:45 ~$ /sw/arch/RedHatEnterpriseServer7/EB_production/2019/software/GCCcore/8.2.0/bin/g++ -x c++-header t.cc
paulm@tcn180 14:45 ~$ 

# Wrapped call, fails
paulm@tcn180 14:45 ~$ /scratch-shared/paulm/eb-3OiLzK/tmpvR7vdm/rpath_wrappers/gxx_wrapper/g++ -x c++-header t.cc 
/lib/../lib64/crt1.o(.text+0x20): error: undefined reference to 'main'
collect2: error: ld returned 1 exit status

# Straight g++ call with -Wl,-rpath addition, no wrappers, fails
paulm@tcn180 14:46 ~$ /sw/arch/RedHatEnterpriseServer7/EB_production/2019/software/GCCcore/8.2.0/bin/g++ -x c++-header -Wl,-rpath=/tmp/doh t.cc
/lib/../lib64/crt1.o(.text+0x20): error: undefined reference to 'main'
collect2: error: ld returned 1 exit status

We (me and @casparvl) suggest that the rpath wrappers check for the -x c++-header (and similar) options to make sure an otherwise correct call of g++ is not turned into one that fails. Note that there is a bit of a mist as to the expected behaviour of GCC with various options, as the docs aren't entirely clear:

• For example, calling g++ t.cc fails if t.cc does not contain a main function as it does linking of the generated object code. But calling g++ -x c++-header t.cc apparently turns on compile-only mode and succeeds (but the gcc docs only say: "To create a precompiled header file, simply compile it as you would any other file, if necessary using the -x option to make the driver treat it as a C or C++ header file")
• Adding -Wl,-rpath=... makes GCC link (although the docs do not specify this), unless -c is used, in which case linking is never done.
• The examples in the GCC docs of using precompiled headers do not use -c when using -x c++-header. But upstream package could be more explicit by using -c -x c++-header, which would still work when the RPATH wrappers would add -Wl,-rpath=..., but it can't really be expected of those packages to handle that case of downstream building.

[boegel]

Ugh, what a mess, thanks for the detailed analysis @paulmelis!

I agree that letting the rpath wrapper check for -x c++header and similar options makes sense...

[boegel]

This should be fixed with the changes in #3424...