New calling convention gotham_call

[Frederick Virchanza Gotham via Gcc]

I want to write a new calling convention into the GNU g++ compiler,
specifically for the x86_64 instruction set.

On MS-Windows, the x64 calling convention puts the first argument in RCX,
and puts the return value in RAX. I wish to write a new calling convention
which uses RAX for both the first argument and the return value, to be used
as follows:

    unsigned Func(unsigned const a) __attribute__((gotham_call))
    {
        return a * a;
    }

Use of this new calling convention would shave a few CPU cycles off some
function invocations, as the above function which previously was compiled
to:

    imul rcx, rcx
    mov rax, rcx
    ret

 would now be compiled to:

    imul rax, rax
    ret

Could someone please point me to the GNU compiler source files in which I
would add a new calling convention? I would appreciate a few pointers on
how to add a new calling convention to the GNU g++ compiler.