32 bit pointers on X86_64 architecture

[John Fine]

I've given a lot of thought to doing something similar myself, but I 
haven't tried anything yet.

Maurizio Vitale wrote: 
>     is there a way to have 4 byte (data) pointers without resorting to 
> -m32?
I'm nearly certain there is not.
> I'd like to have 64 bit code, but for a specific library I'm writing 
> cache behaviour
> would improve if pointers were smaller. Pointers to code would stay at 
> 8 bytes.
I can't tell whether you are asking for more than would be practical 
without changing GCC itself.

What I had in mind was to have pointers to just a few specific data 
types be 32 bit and in all the source code accessing those data types I 
would need to declare pointers as xxx_ptr instead of xxx*

Each xxx would then be given a fixed (source code) assignment to one of 
a few pools, and each pool would be allocated 4GB of virtual address 
space at program startup (which doesn't become actual virtual memory 
until used).

Then you need appropriate override of new (I don't know those details 
myself without a little research) to get xxx object allocated into the 
pools and of course the source code must never use xxx_ptr pointing to 
and xxx objects allocated on the stack (function locals).
> If GCC dowsn't support this mode (and I couldn't find anything in the 
> documentation)
How could it?  Even if you force the heap to be contained that way, what 
about the stack and static memory?  It would be very hard to force all 
those together into one 4GB area.
> I'm thinking about using the linker scripts for forcing all data/heap 
> to be in 2Gb
Stack as well?
> and then add the top 32 bits when dereferencing "pointers", if there's 
> a way to get
> those 32 bits as a link time constant, rather than storing them in a 
> memory location.
Would that help much?  The x86_64 instruction set doesn't have much 
support for 64 bit integer constants.  I'm far from sure, but I think 
getting the top bits from a class static const variable would work 
nearly as well as making the value known at link time.

If I wanted it known at link time, I think I would declare a global 
(maybe class static would work) uninitialized array of 4G chars.  It's 
address would then be known at link time, then I'd force my override of 
new to use that as the pool.  Of course that is just for my idea of 
xxx_ptr for a few important xxx classes.

For your idea of forcing more things together at link time, so all data 
would be in one 4Gb area, if you know the linker well enough to do that, 
don't you know it well enough to make it provide the link time address?