GTY and const

[Daniel Berlin]

On Tue, 2004-12-21 at 11:01 -0800, Zack Weinberg wrote:
> Geoffrey Keating <geoffk@geoffk.org> writes:
> 
> >> > Would it be better to add a cast around the assignment or to make
> >> > the GTY code handle const members?
> >> 
> >> I'd prefer that you improved gengtype.
> >
> > It's not gengtype, the problem really is that you cannot have const
> > GCed memory, for the same underlying reasons that you cannot free() a
> > const pointer.
> 
> No, that's just plain not true.  cpplib makes extensive use of 'const'
> to indicate write-once data structures (they are initialized through a
> non-const pointer, of course).  Those definitely can be GC memory.

Right. There is a significant difference between what we need to walk
for marking, and what we really need to GC allocate.

Right now we GTY/gc allocate a lot of things just so we can mark some
things they may point to.
The fact that we *have* to do this is *bad*.

Roots are roots, whether they occur in GC memory or not.
The requirement that they be in GC memory is just bogus and an artifact
of the way ggc-page marks.

--Dan