Re: [java] RFC/RFA: libgcj_bc.so.1

[Andrew Haley <aph at redhat dot com>]

Bryce McKinlay writes:
 > Currently, binaries built with GCJ's "Binary Compatibility" ABI link 
 > against the same SONAME as binaries built with the C++ ABI. This is 
 > problematic because changes to the Java class libraries frequently break 
 > C++ ABI compatibility: we need to bump the SONAME with each release, but 
 > changing the SONAME itself breaks BC-ABI applications which would 
 > otherwise be compatible. If the two ABIs are to continue to co-exist, we 
 > need different SONAMEs for them.
 > 
 > This solution works by creating a shared library called libgcj_bc.so 
 > which contains empty, "fake" declarations of all the symbols that BC-ABI 
 > applications are allowed to reference in libgcj. At compilation time, if 
 > -findirect-dispatch is specified, gcj will link with -lgcj_bc instead of 
 > -lgcj. The linker will find libgcj_bc.so, which has an SONAME of 
 > libgcj_bc.so.1.
 > 
 > However, libgcj_bc.so.1 itself is actually a symlink to the real 
 > libgcj.so. So at runtime, the dynamic linker loads the real library to 
 > resolve the libgcj_bc.so.1 SONAME.
 > 
 > This approach also has the side-benefit of ensuring that BC applications 
 > only link against legitimate BC-ABI symbols.
 > 
 > Currently this is only enabled for linux systems. It may make sense to 
 > enable it for other ELF platforms too.
 > 
 > OK for trunk?

It doesn't work for me.  I end up with this:

rwxrwxrwx 1 aph aph       15 Jul 14 14:29 libgcj.so -> libgcj.so.7.0.0*
lrwxrwxrwx 1 aph aph       14 Jul 14 14:34 libgcj.so.7 -> libgcj_bc.so.1*
-rwxr-xr-x 1 aph aph 83942015 Jul 14 14:29 libgcj.so.7.0.0*

Note that this means 'gij' doesn't work, becasue it's linked against
libgcj.so.7, and 'gij' needs gcj::verifyClasses().

Andrew.