Re: 64 bit->48 bit pointer hacks...

[Goswin von Brederlow <brederlo@informatik.uni-tuebingen.de>]

"Dale E. Martin" <dale@the-martins.org> writes:

>> The current generation AMD64 MMUs can only handle 64-bit pointers in
>> which the high-order 17 bits are all the same (40 bits of information).
>
> I was mulling this over the other day...  Say I've got a string class that
> looks like this:
>
> class string {
>   class shortString {
>     char flag;
>     char [7] smallString;
>   };
>
>   class {
>     union {
>       char *ptr;
>       shortString;
>     } shortStringOrPtr;
>   } data;
> };
>
> Is there some magic value I can write into "flag", that in combination with
> the understanding of how 64 bit pointers get truncated down to 48 bit
> virtual addresses that will allow me to store either a 7 byte string or a
> char * in the same space?
>
> Thanks,
>   Dale

Since any address would have leading 0 bits any flag != 0 would work,
for now, till bigger CPUs are build.

A much better way is to ensure all pointers are even and use the
lowest bit as flag, as most languages with garbage collectors do:

class shortString {
  char [7] smallString; 
  uint8_t padding:7;
  uint8_t tag:1;
}

This costs you pointers into the middle of strings though but you kind
of loose those for small strings already.

MfG
        Goswin