[Rivet] --no-as-needed only exists for ELF linkers

[Andy Buckley]

Here's a useful description/illustration of what the GNU ld --as-needed
flag does, along with how it can fail to get properly applied with
libtool (I don't think this issue affects us because we're not using
pkg-config to detect system libs):

http://sigquit.wordpress.com/2011/02/16/why-asneeded-doesnt-work-as-expected-for-your-libraries-on-your-autotools-project/

A bit of digging indicates that the OS X equivalent is
-dead_strip_dylibs, described at
https://developer.apple.com/library/mac/documentation/darwin/reference/manpages/man1/ld.1.html
as:

  Remove dylibs that are unreachable by the entry point or exported
  symbols. That is, suppresses the generation of load command commands
  for dylibs which supplied no symbols during the link. This option
  should not be used when linking against a dylib which is required at
  runtime for some indirect reason such as the dylib has an important
  initializer.

Anyway, enough background (i.e. this email is a reference for when *I*
forget this in future)... I decided that we perhaps don't want to use
--no-as-needed on the whole libRivet build, so I've added a check to the
rivet-buildplugin script to try and first work out whether the compiler
will understand it:

## Detect whether the linker accepts the --no-as-needed flag and prepend
the linker flag with it if possible
if (cd /tmp && echo -e 'int main() { return 0; }' > $$.cc; $mycxx
-Wl,--no-as-needed $$.cc -o $$ 2> /dev/null); then
  myldflags="-Wl,--no-as-needed $myldflags"
fi

Ok?

Andy


On 22/07/13 22:39, Hendrik Hoeth wrote:
> Thus spake Andy Buckley (andy.buckley at cern.ch):
> 
>> the -as-needed flag (implicitly switched on?)
> 
> At least on Ubuntu boxes, yes.
> 
>> allows the linker to be selective about what symbols it imports from a
>> linked library, which presumably was overzealous at doing so when
>> encountering the fastjettools plugin...
> 
> Yes it was.   :-/
> 


-- 
Dr Andy Buckley, Royal Society University Research Fellow
Particle Physics Expt Group, University of Edinburgh / PH Dept, CERN