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13.7.5 Registers That Form a Stack
There are special features to handle computers where some of the “registers” form a stack. Stack registers are normally written by pushing onto the stack, and are numbered relative to the top of the stack.
Currently, GCC can only handle one group of stack-like registers, and they must be consecutively numbered. Furthermore, the existing support for stack-like registers is specific to the 80387 floating point coprocessor. If you have a new architecture that uses stack-like registers, you will need to do substantial work on reg-stack.c and write your machine description to cooperate with it, as well as defining these macros.