Where is the splitting of MIPS %hi and %lo relocations handled?

[David Daney]

I am going to try to fix:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29721
Which is a problem where a %lo relocation gets separated from its 
corresponding %hi.

What is the mechanism that tries to prevent this from happening?  And 
where is it implemented?

Thanks,
David Daney