array traversal

[Dan]

Stewart Gordon Wrote:

> "BCS" <ao at pathlink.com> wrote in message 
> news:ce0a3343870c8c939107e7e1a1e at news.digitalmars.com...
> <snip>
> > I think that options he is taking about are these
> > for(T* ptr = &start; ptr !is &stop; ptr++)
> > {
> >  T value = *ptr
> > }
> >
> > vs.
> >
> > for(int i = 0; i< length; i++)
> > {
> >   T value = ptr[i];
> > }
> >
> > only the second ever uses a multiplication
> 
> And even then not necessarily - some compilers may optimise one form to the 
> other.
> 
> Stewart. 
> 

Hmm... let me think of how that looks in ASM.  The first one is wrong, because you're only ptr++, it should be ptr += OFFSET.  Apart from that, you're right the first code looks mildly better in ASM.

Looping through by adding to the pointer instead of adding to the array index and performing a 'lea'.

The good news is that D *should be* optimizing this out, and it ultimately only costs a half cycle which may either align or throw off the u/v pipes - something more important than 1/2 a cycle.

I wonder if Walter has it aligning the pipes as appropriate?