`pri` is clobbered in `OP_CMPS` with custom `_R`.

[Y-Less]

If you set a custom _R macro the CMPS opcode becomes:

        pri=0;
        for (i=0; i+4<offs && pri==0; i+=4)
          pri=_R32(data,alt+i)-_R32(data,pri+i);

This code is simultaneously using pri as the second comparison address and the comparision result. And since the comparison result in pri must remain 0 to continue the loop the second address is just always 0 instead of whatever was given.

Fixed code using val instead:

        val=0;
        for (i=0; i+4<offs && val==0; i+=4)
          val=_R32(data,alt+i)-_R32(data,pri+i);
        for ( ; i<offs && val==0; i++)
          val=_R8(data,alt+i)-_R8(data,pri+i);
        pri=val;

Edit: I double-checked the similar opcodes FILL and MOVE, neither of them have the same bug.