This is a backend for the PDP-10. See gcc/config/pdp10.
The PDP-10 architecture is a family of 36-bit word-addressed machines. They were manufactured primarily by DEC from 1964 to 1983, when they were cancelled in favour of the VAX family. Famous software first running on PDP-10s include: EMACS, ITS, SPELL, Collosal Cave, ADVENT, Zork, Dungeon, MacLisp. Not first: TECO.
A memory word is 36 bits long, which is also the length of the general-purpose registers. All user instructions are 36 bits and have 9 bits to specify an operation, 4 bits to specify a register, 1 bit to indicate indirect addressing, 4 bits to specify an index register, and 18 bits to specify an address or offset. The first version of the architecture was limited to a virtual address space of 256K words; a later version extended this to a maximum of 1G words.