LTO can do devirtualization and whole-program dead-code elimination(* due to the linker seeing the whole function graph and symbol visibility list). This could probably shave 1mb of code or more from libxul.so. In addition to LTO making the binary smaller, upcoming gcc -fprofile-reorder-functions option can reorder binaries based on order of PGO execution. This significantly(15-20%) reduces the size of number of pages paged in from libxul.so More info in: https://blog.mozilla.org/tglek/2010/04/12/squeezing-every-last-bit-of-performance-out-of-the-linux-toolchain/ http://hubicka.blogspot.com/2014/02/devirtualization-in-c-part-3-building.html