With regard to compatibility between tiarmclang and GCC Arm compilers, there are three areas of concern: source code, options, and linking.  The open source Clang project built in compatibility with GCC from the start.  Thus, compatiblity with source code and options is very high.  Because tiarmclang uses the linker from the TI proprietary compiler toolchain, linker scripts must be changed.  An article published on ti.com covers all the details.