Re: PR 6212

[Mark Mitchell <mark at codesourcery dot com>]

>     Using types gives you a way to provide feedback about possibly invalid
>     conversions, like converting a pointer to a byte-aligned int to a
> pointer     to a word-aligned int.  The usual type theoretic rules would
> imply     you cannot go that way, but you can go the other way.  This is
> the     whole point of a type system.
>
> I don't follow.  Objects still have types, but those types are more
> generic than the object.  In other words, if I have a record type that's

That's the point; saying that the type is more generic than the object
is what's weird here.

> 3 bytes long and byte-aligned, I can make an object that's 4-byte-aligned
> (and hence 4 bytes long).  If I make a pointer to that object, that's a
> pointer to a 3-byte type, not a 4-byte type.  I don't see how you can do

No, it's a pointer to a 4-byte type.  You may be able to implicitly
convert it to a pointer to a 3-byte type, if you like, but those two
types are different.  (The basic idea behind a type is that it tells
you what operations you can do; you can't do "copy 4 bytes" form a
3-byte object.)  The 4-byte type is a subtype of the 3-byte type;
you can do (strictly) more things with it; you can do all the 3-byte
types and then some.

> Moreover, what do you do about bitfields?  Do we now have types with
> sizes that are not multiples of bytes?

Bitfields, at least in C, are not addressable, so this doesn't matter.
The only thing you can do with a bitfield lvalue is write to it, and
then you know just where you're writing.  Reading from it yields a
promotion to the appropriate integer type.

--
Mark Mitchell                   mark@codesourcery.com
CodeSourcery, LLC               http://www.codesourcery.com