I suspect the line

long p = (1L<<32)-5; // prime: 2ˆ32 - 5

causes overflow. long on my machine is 32-bit (printf("%ld\n", LONG_MAX); prints 2147483647), and compiler would give a warning -Wshift-count-overflow on that line. Indeed long type cannot hold that prime number.

I suspect it could be corrected as

unsigned long p = (1L<<32) - 5;

with printf("%lu\n", p); prints 4294967291, which is desired.

However the -Wshift-count-overflow warning: left shift count >= width of type still persists.

With

unsigned long p = (~0L) - 4;

(*)
the warning vanishes.

In summary, I think that line should be (*).