tnum_sub
struct tnum tnum_sub(struct tnum a, struct tnum b);
return tnum_sub(TNUM(0, 0), a);
dst_reg->var_off = tnum_sub(ptr_reg->var_off, off_reg->var_off);
dst_reg->var_off = tnum_sub(dst_reg->var_off, src_reg.var_off);