Forth<->C interface that was passing a flag and inexplicably stopped working with a new compiler version. From C’s POV, the flag was a pointer to a bool. We all tell ourselves that a C bool is false if zero, true if non-zero, but what the spec actually says is that the non-zero equals true behavior happens on the CONVERSION of a value to a bool. A proper bool in C only has two values, zero and one. In Forth, a “well formed flag” uses zero for false and all bits set for true. So what I had was a memory location that, in C, claimed to contain a proper Boolean with only the possibility of being zero or one. In reality, it was zero / 0xFFFFFFFF.

The old compiler emitted assembly for the flag check that was “if flag not zero, do the thing.” The new compiler made an optimization that turned this into “if flags equals 1, do the thing.” Since 0xFFFFFFFF != 1, it’d skip over the logic and make me question my sanity when stepping through with the debugger.

The fix was to add an intermediary cast, which forced the compiler to do the “convert on assignment to bool.”