Well, one of the platforms where I'm having issues is sparcv9 (aka sparc64) on Solaris. This appears to be because of issues I'm still investigating in the rust unwinding libraries / dwarf handling. gcc's equivalent functionality has no issue on the same platform.
Likewise, there are various math functions that have been hand-optimised in gcc for sparcv9 (and other platforms) that rust has no equivalent of. rust instead is relying on the optimisation and code generation capabilities of either LLVM or gcc, which are not always as optimal.
I'm currently resyncing my existing sparcv9 port work and getting it into the appropriate rust-related repositories. Once I've done that, I intend to revisit the unwinding issue to see if recent alignment fixes have resolved it, or if there's still more work to be done.