more radical change?

[Henry Cejtin]

In  the  following, by a reference to an object, I mean what ever the mutator
and other objects have when the have a pointer to another object.

Consider what the point of all the header and the like words are:

    The only thing that the mutator cares about is being able to get  to  the
        length  of an array given a reference to the array and be able to get
        to the data associated with an object given a reference to it.

    The GC must be able to recognize an object given a pointer to  the  first
        word occupied by the object.  This is required since it walks through
        a semi-space and has to understand objects as it comes to them.

    In addition, if an object contains a reference to another object, the  GC
        must be able to understand the referred to object from the reference.
        This is so that it can forward it.

The difficulties we have currently are all caused by the fact that references
to  objects  are NOT the same as pointers to the first word used by an object
in the heap.  This could be changed.  The only disadvantages  I  see  to  the
change are:

    Getting  to  the  data associated with an object would involve a slightly
        larger offset from the reference.  For array  subscript  calculations
        it would mean that the address of X[i] would be
            X + offset + i * size-of-element
        where  offset is the number of bytes from the first word of the array
        object to the first first word of data.

        I don't think that this is important.

    The mutator code would now be more intimately involved with heap  layout.
        Currently  it  only  knows  when it is creating an object, but now it
        would also have to know when it is accessing  an  object.   It  would
        have to know about header words and the like.

        This could be a serious objection if we go to some fancy generational
        GC where one might want to use some kind of BBOP for small objects.

What do people think of this (more radical?) change?