Path: utzoo!utgpu!water!watmath!clyde!bellcore!decvax!decwrl!labrea!agate!ucbvax!bloom-beacon!athena.mit.edu!jfc
From: jfc@athena.mit.edu (John F Carr)
Newsgroups: comp.arch
Subject: Re: RISC data alignment
Message-ID: <3001@bloom-beacon.MIT.EDU>
Date: 17 Feb 88 22:15:44 GMT
References: <2635@calmasd.GE.COM> <28200092@ccvaxa> <496@ecrcvax.UUCP>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: jfc@athena.mit.edu (John F Carr)
Organization: Massachusetts Institute of Technology
Lines: 31

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.
}
}	Within a structure, the objects declared have addresses
}	which increase as their declarations are read left-to-right.


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)