Chris, Thanks for the hint. Moving over the libLTO.dylib from llvm 2.5 solved all of the linkage errors. I was able to completely build xplor-nih at -O4 now. The core xplor and xplor-tcl testsuite show no regressions. I do get 7 testcases in the xplor-python testsuite failing with bus errors now. The xplor-tcl and xplor-python tests are all run by tcl and python respectively loading their xplor-nih modules. I would assume that the shared libraries created at -O4 should look like just any other shared library to MacOS X and that shared modules built at -O4 should be loadable from programs like python and tcl that aren't built with -O4, right? Jack ps Interestingly PR3376 goes latent at -O4.