RISC Machine Data Structure Word Alignment Problems?

[Terry Ingoldsby]

In article <1648 at skye.ed.ac.uk>, richard at aiai.ed.ac.uk (Richard Tobin) writes:
> In article <LJ81OX3ggpc2 at ficc.uu.net> peter at ficc.uu.net (Peter da Silva) writes:
> >Worse, since non-word-aligned values do cost extra cycles to access, any
> >68020 C compiler that didn't pad that structure is broken. 
> 
> This is nonsense.  Which you want depends whether speed or size is more
> important.  A valid criticism would be that too many C compilers don't let
> you specify which kind of optimisation you want.
> 

This discussion, IMHO, is pointless.  The C compilers work just fine the way
are (or at least the ones I am familiar with).  I don't think some of the
people discussing this realize the implications of what they propose.

I work on an Intergraph Clipper based workstation.  Unless I am mistaken,
floating point values can only be aligned on 8 byte boundaries if the
processor is to be able to access them in a single instruction.  If you
try to access a floating point value that is not 8 byte aligned, it
actually grabs the value at the next lowest 8 byte boundary.  It doesn't
even give a bus error trap!  In theory, the compiler could place it on
arbitrary boundaries by generating a sequence of instructions that would
read adjacent values and AND and OR the values into memory.  It sounds to
me that we are talking about 4 or 5 instructions to do this, so your
access speed would be the pits!

The reason people seem to want to be able to store values at arbitrary
locations seems to have to do with the need to write out contiguous
regions of memory to a binary file.  They then complain that reading
that file back into the memory of another machine doesn't work.  No
one ever said it would.  If you want portable code, don't write it
that way.  It is almost always possible to sacrifice portability for
speed.

I don't know why this is so astonishing; you can't write out binary
values for integers between machines, what would lead anyone to believe
that structures should be any different.

C is a low level language.  If you want greater data abstraction, move
to a higher level language that guarantees that data will appear to
be in the same format across systems.  That guarantee is not in the
C definition; doing so would probably limit C's ability to blast bits.
The only format that C guarantees to understand is ascii represented
numeric values.

The only thread of this discussion that might relate to comp.arch is
why processors (such as Clipper) do not give a trap if you try to
access memory on illegal boundaries.  Surely that would not require
much silicon?
-- 
  Terry Ingoldsby                ctycal!ingoldsb at calgary.UUCP
  Land Information Systems                 or
  The City of Calgary       ...{alberta,ubc-cs,utai}!calgary!ctycal!ingoldsb