Strange performance behavior

[Bruce Adams]

Bill Baxter Wrote:

> Bruce Adams wrote:
> > Bill Baxter Wrote:
> > 
> >> Bill Baxter wrote:
> >>> Marius Muja wrote:
> >>>> I have noticed the following strange (at least for me) performance 
> >>>> behavior with one of my programs. It is a program that does some 
> >>>> scientific computations and while trying to optimize it I noticed that 
> >>>> the code from case_B below executes faster (almost twice as fast) as 
> >>>> the code in case_A. This is a bit counterintuitive for me, since in 
> >>>> case _B there is also the cost of the function call (or should be the 
> >>>> same if the function is inlined).
> >>>> Can anybody shed some light on why it's behaving this way?
> >>>>
> >>>> case_A:
> >>>> -------------------------------
> >>>> foreach (i,index; indices) {
> >>>>    foreach (k, inout value; centers[belongs_to[i]])
> >>>>     value += vecs[index][k];
> >>>> }
> >>>> ----------------------------------
> >>>>
> >>>> case_B:
> >>>> -------------------------------
> >>>> void addTo(T,U)(T[] a, U[] b) {
> >>>>    foreach(index, inout value; a) {
> >>>>       value += b[index];
> >>>>    }
> >>>> }
> >>>> ....
> >>>> foreach (i,index; indices) {
> >>>>    addTo(centers[belongs_to[i]],vecs[index]);
> >>>> }
> >>>> _______________________________
> >>> My guess would be the compiler's failure to optimize[*] away the [index] 
> >>> indexing in A.  So you do 2 lookups per iteration rather than just one. 
> >>>  If so then this should be just as fast as case_B:
> >>>
> >>> foreach (i,index; indices) {
> >>>    auto vecs_i = vecs[index];
> >>>    foreach (k, inout value; centers[belongs_to[i]])
> >>>       value += vecs_i[k];
> >>> }
> >>>
> >> Forgot the footnote!:
> >>
> >> [*] It may not really be a failure of the optimizer -- maybe it's 
> >> unreasonably difficult to determine absolutely that vecs won't changed 
> >> by the operation '+= vecs[index][k]'.
> >>
> >> --bb
> > 
> > <shields up>
> > This is exactly the sort of thing where const would help :)
> > </shields down>
> 
> Uh oh... you should have left the shields up. Now you're unprotected...
> 
> 
> --bb

I was invariant but now I'm just const but you still can't modify me through this interface :p