From nios2-elf-gcc 4.9.2 onwards the Nios® II GCC compiler supports new options:
-march, -mbmx, -mno-bmx, -mcdx, -mno-cdx
These options cannot be used with the publicly released versions of the Nios II processor and are set to off by default. These options are currently intended only for internal Altera use.
Altera has developed a version of the Nios II processor with a modified instruction set encoding (known as R2) that is currently only intended for internal Altera use. Both the R1 (original) and R2 (new) instruction set encodings are supported by the same Nios II GCC compiler in order to streamline compiler development and maintenance. The Nios II R2 instruction set is largely assembly-language upward compatible with the Nios II R1 instruction set, but supports additional mandatory and optional instructions.