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