​I'm not sure if this is the last.   This is a pointer to the V7 Ritchie Compiler:  https://minnie.tuhs.org//cgi-bin/utree.pl?file=V7/usr/src/cmd/c  - The sources and the makefile build the three passes /lib/c[012]  

This should be a good starting point/base line.

Be careful because the Johnson Compiler (pcc) was also included with V7 and is a different technology.  

This is important because their are modifications to both the Ritchie and Johnson compilers 'in-the-wild' for other back-ends and new optimizations.    I for instance, re-targeted the Ritchie compiler to what would become the 68000 (it was not yet numbered, it was an experimental chip when we had access to it in the late 1970s in Tek Labs - mine was a 16 bit 'int' as I was coming primarily from the PDP-11 at the time and the chip was a 16 bit chip internally - so the code was tight and clean and I basically substituted PDP-11 instruction sequences for 68000 sequences).   IIRC, Jack Test's 68000 compiler from MIT which was about 18 mons later was based on the Johnson compiler but he used a 32 bit 'int' which proved easier for porting programs from the Vax, as the chip supported 32 bit words even though it took 2 ticks to do anything [so Jack's compiler generated slower code for many simple ops].

I recommend, that google for the old USENIX tapes and see what you turn up and compare.