Wednesday, May 25, 2011


Aliasing describes a situation in which a data location in memory can be accessed through different symbolic names in a program. I use aliasing a lot in the garbage collector, it was a wrong assumption on my part on the semantics of the C language. Turns out, aliasing is not supported by default to make some optimizations work.

The problem is that I need aliasing. Thunks are supposed to consist of memory cells which mostly hold integers or pointers, but sometimes they hold boxed character lists, floats, doubles, or whatever what one would like to store in a constant.

I am not really writing on the compiler at the moment, but -as I am dropping the dependency on libffi- I would like to drop the compiler switch which turns strict aliasing off, thereby making aliasing possible...

I am not sure there's even a compatible manner in which to support this.

Update: Strange, the standard makes an exception for char pointers, so I guess that I'ld need to support that. Can't I use this in a reverse manner using a char pointer as an intermediate?

Update: It may be impossible if I read this rant.

Update: Sometimes a question and answer isn't defined well; the guys at comp.lang.c helped me figure it out. If I want to compile with strict aliasing, I just need to make sure that pointers of different type refer to different values. (Or, stated differently, it's a rather silly idea wanting to alias stuff where strict aliasing tells one you cannot.) So, wherever I cast/alias stuff now, I'll need memcpy. Hope that is sufficient.