Path: utzoo!mnetor!uunet!mcvax!unido!ecrcvax!johng
From: johng@ecrcvax.UUCP (John Gregor)
Newsgroups: comp.arch
Subject: Re: RISC data alignment
Message-ID: <504@ecrcvax.UUCP>
Date: 22 Feb 88 11:31:50 GMT
References: <2635@calmasd.GE.COM> <28200092@ccvaxa> <496@ecrcvax.UUCP> <3001@bloom-beacon.MIT.EDU>
Reply-To: johng@ecrcvax.UUCP (John Gregor)
Organization: ECRC, Munich 81, West Germany
Lines: 54

In article <3001@bloom-beacon.MIT.EDU> jfc@athena.mit.edu (John F Carr) writes:
>In article <496@ecrcvax.UUCP> johng@ecrcvax.UUCP (John Gregor) writes:
>>In article <28200092@ccvaxa> aglew@ccvaxa.UUCP writes:
>}}I have often wanted an optimizing compiler that could take
>}}struct { long a; int b; long c; char d; }
>}}which is usually mapped into bytes as AAAABBxxCCCCDxxx
>}}(on a machine where misaligned accesses are penalized)
>}}and make it into AAAACCCCBBDx.
>}I would like that also, but don't call it a struct.  Rearranging the order
>}breaks lots of code and violates K&R p. 196.
>Isn't code which depends on allignment and spacing risky anyway?  
>If a program assumes that for
>struct { int a;
>         int b;
>       } s;
> (&(s.a) - &(s.b)) == 1 it is making an unsupported assumption.  Can 
>someone post an example of a program which is portable and depends on
>the K&R rule quoted above?
>  --John Carr   (jfc@ATHENA.MIT.EDU)

That's not a good example of the type of code that would break.  I use
the ordering property of structs to hide data.  If I have a struct which
has some house keeping at the end of the struct, I don't have to tell
the rest of the modules about it.  e.g.

struct private_foo_t { 
    int   a;
    short b;    /* would be reordered to AAAACCCCBB */
    int   c;
} fooba;

struct public_foo_t { 
    int   a;
    short b;    
} foobr;

By K&R I am guaranteed that: 

       (&(fooba.b) - &(fooba.a)) == (&(foobr.b) - &(foobr.a))

       /* i.e. the offsets are identical as long as the types 
       ** are the same.
       */

This would break if structs were reordered.  I believe that C++ uses this
feature of C to implement its data hiding also.  I do like the idea of an 
unordered aggregate type however.  I've had to reorder structs for memory
usage too many times and it's very machine dependent.

-- 
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John Gregor                                     johng%ecrcvax.UUCP@germany.CSNET