Lost line numbers

[NightStrike]

What would cause gcc (8.1.0) to lose line numbers when printing out a warning?

$ gcc -O3 -g -c file.c -o file.o (followed by tons of -I's)
./file.h: In function 'fcn':
cc1: warning: '__builtin_memset' writing 72 bytes into a region of
size 52 overflows the destination [-Wstringop-overflow=]

Note that the function is pretty large, and there's no direct calls to
memset, so I'm guessing some optimization is converting a loop to a
memset or something.

I don't have a reduced case yet... I've got 30k lines I'm trying to
auto-shrink down.