Re: short pointers (32 bit) in 64 bit apps

[Andrew Haley <aph at redhat dot com>]

On 05/19/2010 09:54 AM, Reza Roboubi wrote:
> Nicholas Sherlock wrote:
>> On 19/05/2010 3:58 p.m., Reza Roboubi wrote:
>>> Do you know what it takes to do it? How much work for a newcomer to get
>>> up to speed with gcc? (Maybe this should go to the developer list?)
>>
>> You would need to write a memory allocator which divides the address
>> space into 4GB (32-bit pages). Then you could create a new malloc
>> routine which has an extra argument to allow you to allocate memory
>> from within a specified 4GB page. That way you can make all of your
>> allocations for a given data structure lie within the same region.
> 
> Since we are trying to _compress_ these structs in the first place
> malloc'ing them individually is pointless anyway.  We should probably
> have some sort of custom (fixed size) allocator.
> 
> <SIDE-NOTE>
> Quote from git source code: "The standard malloc/free wastes too much
> space for objects, [...]"
> </SIDE-NOTE>
> 
> Really, this whole thing is for people who care about scaling something
> specific and don't mind doing sbrk and mmap when they need to.  So the
> main change is a C language extension (nothing glibc/library related.)
> 
> Essentially, you would want a pointer type modifier:
> 
> struct list {
>   struct list short * next;
> };
> 
>>
>> Then you can add the top 32-bits to pointer addresses within your data
>> structure before you attempt to dereference them.
> 
> That's the point:  not having to do this manually.

I don't see the point of changing the language.  Define a custom allocator
and store the pointers in uint32_t variables.  C++ makes this very easy:
see below.

Andrew.





#include <cstdint>
#include <cstdio>

template<class T> class shortptr
{
  uint32_t the_ptr;

public:

  T& operator*()
  {
    return *(T*)the_ptr;
  }

  shortptr(T *p)
  {
    the_ptr = (uint32_t)(uintptr_t)p;
  }
};

int foo (shortptr<int> p)
{
  return *p;
}

int a = 99;

int
main ()
{
  shortptr<int> p = &a;

  int m = foo (p);
  printf ("%d %d\n", m, sizeof p);
}