On SPARC, leaf function optimization omits the register window sliding (and the associated register name changes). This might result in miscompilation of procedures containing inline assembly, as some of the register constraints used may interfere with the register usage of optimized functions, so we disable leaf procedure optimization on those procedures to prevent it from happening.
Leaf functions allow the compiler to omit the setup and teardown of a frame pointer, therefore avoiding the exchange of the in/out register. According to the SPARC architecture manual every reference to %i0-%i5 should be replaced with %o0-o5, if the target register is already in use a further remapping step to %g1-%g7 is required to free the output register.
Add a simple check to make sure not to stomp on any output register that's already in use.