I Found this during testing:

printf.c: In function ‘printf’:
printf.c:239: internal compiler error: in df_ref_record, at df-scan.c:2803

This was the faulting expression:
(mem/c:HI (plus:HI (reg/f:HI 10 r10)
(const_int 18 [0x12])) [6 %sfp+10 S2 A16])

Since this expression doesn't show up in the debug output, I need to backtrace to find out what happened here

df_ref_record (cl=DF_REF_REGULAR)
df_uses_record (in the POST_INC case)

RTL of offending instruction:
(set (reg/v:QI 2 r2 [orig:49 c ] [49])
(mem:QI (post_inc:HI (mem/c:HI (plus:HI (reg/f:HI 10 r10)
(const_int 18 [0x12])) [6 %sfp+10 S2 A16])) [0 S1 A8]))

It looks like this came from this instruction, seen in 168r.asmcons

(insn 39 168 41 5 printf.c:152 (set (reg/v:QI 49 [ c ])
(mem:QI (post_inc:HI (reg/v/f:HI 46 [ p.44 ])) [0 S1 A8])) 71 {movqi} (expr_list:REG_INC (reg/v/f:HI 46 [ p.44 ])
(nil)))

In a more readable form:
movb 18(r10+), r2

It took me a while to figure this out, but GCC is not trying to increment R10, but the expression (R10+18), which rightly fails.

The pseudoregisgter R46 is "p" (a pointer into the current string) which is used later, as is R10, which is the stack pointer. This means that the post-increment expression needs to be removed somehow.

I imagine if I used the "register" keyword, that would probably fix this problem, but this would just show up again sometime later.

I need to take a closer look at the PDP11 config to see how they dealt with the problem.