Issue #16750 has been reported by Dan0042 (Daniel DeLorme).

----------------------------------------
Feature #16750: Change typedef of VALUE for better type checking
https://bugs.ruby-lang.org/issues/16750

* Author: Dan0042 (Daniel DeLorme)
* Status: Open
* Priority: Normal
----------------------------------------
VALUE is currently defined as `typedef unsigned long VALUE`, but as we all know that only creates an _alias_, not an actual _type_. Since the compiler gives no warnings when comparing with other integer values, it's easy to have bugs such as `v == 42` which should have been `v == INT2FIX(42)`. Actually not so long ago I saw nobu fixing a bug of that kind where an ID had been mixed up with a VALUE.

So in order to prevent these kinds of bugs I propose changing VALUE to a non-scalar type such as:

```c
//example for 64-bit system
typedef union VALUE {
    struct RBasic* as_basic; //easy access to obj.as_basic->klass and obj.as_basic->flags
    void* as_ptr;            //can assign ptr = obj.as_ptr without a cast
    unsigned long i;         //raw int value for bitwise operations

    int immediate : 3;   //obj.immediate != 0 if obj is immediate type such as fixnum, flonum, static symbol

    int is_fixnum : 1;   //obj.is_fixnum == 1 if obj is fixnum

    struct {
        int flag : 2;    //obj.flonum.flag == 2 if obj is flonum
        int bits : 62;
    } flonum;

    struct {
        int flag : 8;    //obj.symbol.flag == 0x0c if obj is a static symbol
        int id : 56;     //-> obj.symbol.id == STATIC_SYM2ID(obj)
    } symbol;

} VALUE;
```

This will allow proper type-checking via the compiler.

A little-known fact, structs and unions can be passed (and returned) by value to functions; this 64-bit union has the same performance as a 64-bit int. This approach also allows to simplify code like `((struct RBasic*)obj)->flags` into the much more readable `obj.as_basic->flags`, and `FIXNUM_P(obj)` can be expressed directly as `obj.is_fixnum`. The only downside is that direct comparison of union variables is not possible, so you would need to use for example `obj.i == Qfalse.i`

To summarize, stricter type-checking would eliminate an entire class of bugs, and I estimate this change would require modifications to **no more than 14%** of the codebase (62,213 lines). Very much worth it I believe.



-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>