https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100734
--- Comment #9 from Martin Sebor <msebor at gcc dot gnu.org> --- (In reply to John David Anglin from comment #5) > Breakpoint 1, attr_access::from_mode_char ( > c=<error reading variable: Cannot access memory at address 0x0>) > at ../../gcc/gcc/attribs.h:304 > 304 gcc_unreachable (); This error says that &c is null. c comes from the code below: #1 0x4000000000a8311c in init_attr_rdwr_indices (rwm=0x800003fffdff14e0, attrs=0x800003fffdf1b9b0) at ../../gcc/gcc/attribs.c:2146 mode = TREE_VALUE (mode); if (TREE_CODE (mode) != STRING_CST) continue; gcc_assert (TREE_CODE (mode) == STRING_CST); for (const char *m = TREE_STRING_POINTER (mode); *m; ) Either TREE_STRING_POINTER (mode) is null or mode is set to null after one of the calls to strtoul() later in the function.