I noticed that data.table has been warning on CRAN for r-devel-linux-x86_64-debian-gcc. That image uses GCC 12 and since GCC 12, -Waddress now warns in more cases when comparing a non-constant pointer to NULL (gcc has always optimized this out), see here. This picked up on a missing dereference in fread (line 1287):

I've looked at it a bit and I think I get what Matt was trying to do there with that optimisation but I don't have a test case that would demonstrate any speed improvement from it. For small files, I can't see any difference. So unless someone has a good test case, I'd suggest just removing this line and the associated check in end_NA_string.

(The comment in there is also currently incorrect, since the default is still 'NA' and not '\0'. Also note that atm, adding in the dereference can lead to a NULL pointer dereference a bit further down.)