But also with a very small number of restrictions, you can make C have non-conservative GC.
ANSI C forbids aliasing as any type but a character. Furthermore, unions are to be interpreted as the last type they are accessed as.
The three places you run into issues are:
1) You can treat two structs that begin with the same values interchangeably so long as you only access the common beginning of the structures.
2) You can freely convert pointers to integers and back.
3) Storage returned by malloc() can be used heterogeneously (however, with strict aliasing rules, you can't switch types of the data once you've accessed it)
If you forbid those three things, then you can have a strict gc by adding runtime overhead to accessing data (once malloced data is accessed by a non-character type, you can treat it like that type forever).
However, lots and lots of code violates these rules, (the most famous code that does is probably the fast inverse square-root)[1]
But also with a very small number of restrictions, you can make C have non-conservative GC.
ANSI C forbids aliasing as any type but a character. Furthermore, unions are to be interpreted as the last type they are accessed as.
The three places you run into issues are:
1) You can treat two structs that begin with the same values interchangeably so long as you only access the common beginning of the structures.
2) You can freely convert pointers to integers and back.
3) Storage returned by malloc() can be used heterogeneously (however, with strict aliasing rules, you can't switch types of the data once you've accessed it)
If you forbid those three things, then you can have a strict gc by adding runtime overhead to accessing data (once malloced data is accessed by a non-character type, you can treat it like that type forever).
However, lots and lots of code violates these rules, (the most famous code that does is probably the fast inverse square-root)[1]
http://betterexplained.com/articles/understanding-quakes-fas...