[Patch, fortran] PR24518 and PR24520 - Improvements to MOD and DOT_PRODUCT

[Paul Thomas]

Janne,

>BTW, does BLAS do anything fancy for dot product that might help for
>big vectors? I mean, is it worth thinking about inlining only for
>small vectors? What happpens when the vectors won't fit into cache?
>I'm not saying this as a criticism of your patch, just idle
>wondering..
>  
>
I'll check to see what happens with big vectors.  Of BLAS, I can only 
claim ignorance.

>For some further idle speculation, for big vectors would it pay off to
>insert prefetch hints for archs that support it?
>  
>
How do I do that?  Does the backend not do it for me?

>Seems like pathscale is even more demanding. I had to go to O1 to get
>it to execute the loops. Same for dot_demo.f90.
>  
>
It goes with the performance, I think.

>I'm not entirely comfortable with this. This might give the user a
>false sense of security. At least a huge value is obviously wrong.
>  
>
I was wondering about this. How about NaN instead?

Another way to circumvent this would be to use the builtin fmod in the 
case where precision is falling by the wayside.

> WOW!!! That's great!
>
I cannot recall if I remarked anywhere that the code that this 
DOT_PRODUCT produces, is the same as that produced by z(:) = SUM (x(:), 
y(:)).  This is similarly nifty.

Best regards

Paul