21.9 Different Behaviour

The intent is that compiled byte-code and the corresponding code executed by the Lisp interpreter produce identical results. However, there are some circumstances where the results will differ.

• Arithmetic operations may be rearranged for efficiency or compile-time evaluation. When floating point numbers are involved, this may produce different values or an overflow.
• Some arithmetic operations may be optimized away. For example, the expression (+ x) may be optimized to simply x. If the value of x is a marker, then the value will be a marker instead of an integer. If the value of ‘x’ is a cons cell, then the interpreter will issue an error, while the bytecode will not.

  If you’re trying to use ‘(+ object 0)’ to convert object to integer, consider using an explicit conversion function, which is clearer and guaranteed to work. Instead of ‘(+ marker 0)’, use ‘(marker-position marker)’. Instead of ‘(+ char 0)’, use ‘(char-int char)’.

For maximal equivalence between interpreted and compiled code, the variables byte-compile-delete-errors and byte-compile-optimize can be set to nil, but this is not recommended.