Re: PDP-10 backend for gcc

[lars brinkhoff <lars@nocrew.org>]

Michael Meissner <meissner@cygnus.com> writes:
> On Thu, Aug 24, 2000 at 07:50:09AM +0200, lars brinkhoff wrote:
> > I may be writing a PDP-10 backend for gcc during the next year.  It's
> > a word-adressed 36-bit machine with a 30-bit virtual address space.
> > Is there anything I should know before I start?
> Expect lots of problems.  GCC really, really does not understand
> pointers that have different formats, most of which are part of the
> original design decisions (ie, very hard to root out after 13+
> years).

Ouch.  What is the nature of the problems?  When are pointers to
different objects mixed?

Pointers to objects at least one word in size (int, long, long long,
float, double, maybe short) all have the address of the word in the
lower 30 bits.  Pointers to objects less than one word in size (char,
maybe short) uses the 6 upper bits to specify the location of the
object within a word.  The lower 30 bits specify the address of the
word.  Thus, only the 6 upper bits would have to be modified when,
say, casting from one pointer type to another.

Does it seem like it would be possible to write a back end using these
pointer formats?