[m-dev.] Why do strings in Mercury need to be word aligned?

[Paul Bone]

On Wed, May 06, 2009 at 10:46:09PM +1000, Peter Ross wrote:
> On Wed, May 6, 2009 at 6:57 PM, Paul Bone <pbone at csse.unimelb.edu.au> wrote:
> > On Wed, May 06, 2009 at 06:31:26PM +1000, Peter Ross wrote:
> >> Hi,
> >>
> >> Why does a MR_String need to be word-aligned?  What code will ever
> >> look at the tag bits of a string and hence do the wrong thing?  I'm
> >> sure that there is code, but I just can't think of what it is.
> >
> > I think that if you have:
> >
> > :- type maybe(T)
> >    --->    yes(T)
> >    ;       no.
> >
> > And then use the value yes("fish") the word-aligned string is packed with the
> > tag bits for maybe so that it uses only two memory cells one for the actual
> > string and one for maybe.
> >
> >  +-----+---------+        +--------+
> >  | tag | pointer |------> | String |
> >  +-----+---------+        +--------+
> >
> > If we couldn't use the lower two bits of the string pointer this structure
> > would have to be:
> >
> >  +-----+---------------------------+     +----------------+     +--------+
> >  | tag | pointer to string pointer |---> | String Pointer |---> | String |
> >  +-----+---------------------------+     +----------------+     +--------+
> >
> > Other examples with other structures will also do similar things.  It also
> > doesn't require use of the ploymorphic type as above.
> >
> Sounds like a reasonable explanation to me, only one slight concern is
> that the compiler doesn't seem to do this optimization today at least
> for the following function.
> 
> :- func f(string) = maybe(string).
> f(A) = yes(A).
> 
> Is the reason above correct, or is there another more fundamental
> reason than not allowing an optimization?
> 

I'm supprised by that and can't think of a reason why it wouldn't do the above.

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