If you locate +, you see the following source code from the file prim:
This code is written in a variant of the vmgen language.
The first line provides the primitive Forth name, stack effect
(including stack item names), wordset name, and pronounciation (which
also serves as the C name of the primitive), while the remaining lines
are C code that refer to the stack items by name.
Our vmgen variant extracts a lot of information from the
stack effect notations: The number of items popped from and pushed on
the stack, their type, and by what name they are referred to in the C
code. It generates a C code from this, incorporating the C code given
explicitly. For more details See Vmgen in Vmgen.
When you see +, you see the variant of the primitive that’s
used in threaded code: starting in the canonical state and ending in
the canonical state, with NEXT (including IP update):
see + Code + $55DE56E88427: add $0x8,%rbx $55DE56E8842B: add $0x8,%r10 $55DE56E8842F: add (%r10),%r13 $55DE56E88432: mov (%rbx),%rax $55DE56E88435: jmp *%rax end-code
You can see the source and disassembled code of other primitives in the same way.