Re: "core C"

From	Bartc <bc@freeuk.com>
Newsgroups	comp.lang.c
Subject	Re: "core C"
Date	2015-07-01 17:01 +0100
Organization	A noiseless patient Spider
Message-ID	<mn12ql$2mi$1@dont-email.me> (permalink)
References	(7 earlier) <47f5d704-4ab6-4afc-a558-cbed67915a76@googlegroups.com> <mmugvp$mpc$1@dont-email.me> <8de0b72b-0f2c-45cc-92c7-0c3c498134e3@googlegroups.com> <mn0gap$cf3$1@dont-email.me> <lny4iz2a0i.fsf@kst-u.example.com>

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On 01/07/2015 16:05, Keith Thompson wrote:
> Bartc <bc@freeuk.com> writes:

>> I'm curious: how can a pointer and offset together be the same size as a
>> pointer?
>>
>> Or do all pointers have a spare 32-bit field on the off-chance they
>> might have to store an offset?
>
> An example I've mentioned here many times: Cray vector machines are
> word-oriented, where a word is 64 bits.  A machine-level address/pointer
> is a 64-bit quantity containing the address of a word.  Cray's C
> compiler has CHAR_BIT==8 (for Unix/POSIX compatibility), and implements
> a byte pointer as a word pointer with the byte offset stored in the
> otherwise unused high-order 3 bits.
>
> Any system with 64-bit pointers that doesn't actually support 16
> exabytes of memory is likely to be able to do something similar.

We're talking about offsets into an array which could be as big as 
available memory, and that's for a byte array. For a bit-array, the 
offset could be 8 times as big!

So it's not just a few extra bits.

With 32-bit pointers, there is nowhere to put a substantial offset. With 
64-bit ones, you might get away with it if pointers are limited to a 4GB 
range (2GB user and 2GB system), and offsets are limited to +2GB or +4GB 
(the latter being the maximum offset within a 500MB bit-array). And if 
the OS and everything else plays along.

Otherwise it gets tight. My point was that the natural way to do this 
was with a discrete pointer/offset pair that is larger (perhaps double 
the size) than a regular pointer. (With my preferred method of using bit 
pointers, then there's a better chance of storing the 0..7 bit offset in 
a spare corner of the pointer.)

-- 
Bartc

Thread

"core C" Rosario19 <Ros@invalid.invalid> - 2015-06-30 08:38 +0200
  Re: "core C" jt@toerring.de (Jens Thoms Toerring) - 2015-06-30 07:54 +0000
    Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-30 02:55 -0700
    Re: "core C" Rosario19 <Ros@invalid.invalid> - 2015-06-30 17:31 +0200
      Re: "core C" Rosario19 <Ros@invalid.invalid> - 2015-06-30 18:15 +0200
        Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 12:19 -0700
  Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 04:27 -0700
    Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 04:48 -0700
      Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 13:07 +0100
        Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 05:15 -0700
    Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 13:02 +0100
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 05:13 -0700
        Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 13:58 +0100
          Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 06:11 -0700
            Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 15:11 +0100
              Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 07:50 -0700
                Re: "core C" Rosario19 <Ros@invalid.invalid> - 2015-06-30 17:24 +0200
                Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 17:44 +0100
                Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 10:04 -0700
                Re: "core C" Bartc <bc@freeuk.com> - 2015-07-01 11:46 +0100
                Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-01 04:40 -0700
                Re: "core C" Keith Thompson <kst-u@mib.org> - 2015-07-01 08:05 -0700
                Re: "core C" Bartc <bc@freeuk.com> - 2015-07-01 17:01 +0100
                Re: "core C" Keith Thompson <kst-u@mib.org> - 2015-07-01 10:56 -0700
                Re: "core C" BGB <cr88192@hotmail.com> - 2015-07-04 12:59 -0500
                Re: "core C" Bartc <bc@freeuk.com> - 2015-07-04 20:25 +0100
                Re: "core C" BGB <cr88192@hotmail.com> - 2015-07-04 20:56 -0500
        Re: "core C" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-30 14:17 +0100
    Re: "core C" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-30 14:03 +0100
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 06:16 -0700
    Re: "core C" David Brown <david.brown@hesbynett.no> - 2015-06-30 15:46 +0200
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 07:54 -0700
      Re: "core C" Steve Thompson <stevet810@gmail.com> - 2015-07-03 12:25 +0000
    Re: "core C" jt@toerring.de (Jens Thoms Toerring) - 2015-07-01 15:04 +0000
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-01 08:12 -0700
    Re: "core C" BGB <cr88192@hotmail.com> - 2015-07-04 10:36 -0500
  Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 04:41 -0700
    Re: "core C" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-30 14:19 +0100
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 06:25 -0700
        Re: "core C" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-30 15:48 +0100
          Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 07:52 -0700
  Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 04:42 -0700
  Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 08:28 -0700
    Re: "core C" David Brown <david.brown@hesbynett.no> - 2015-06-30 22:28 +0200
      Re: "core C" Ian Collins <ian-news@hotmail.com> - 2015-07-01 08:45 +1200
        Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 13:56 -0700
          Re: "core C" David Brown <david.brown@hesbynett.no> - 2015-07-01 10:27 +0200
        Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-30 14:56 -0700
          Re: "core C" Ian Collins <ian-news@hotmail.com> - 2015-07-01 10:03 +1200
            Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-30 15:31 -0700
              Re: "core C" Ian Collins <ian-news@hotmail.com> - 2015-07-01 10:52 +1200
                Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-30 16:58 -0700
                Re: "core C" Keith Thompson <kst-u@mib.org> - 2015-06-30 17:30 -0700
                Re: "core C" David Brown <david.brown@hesbynett.no> - 2015-07-01 08:54 +0200
                Re: "core C" Keith Thompson <kst-u@mib.org> - 2015-07-01 07:54 -0700
                Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-01 09:42 -0700

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