Re: GNAT compiler switches and optimization

["Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>]

Gautier wrote:
> 
> The Sum variable was *removed* by someone at some point of the 
> discussion in order to challenge a bit more the Ada compiler's optimizer.
> If you replace a good algorithm by a bad one, don't be surprised that 
> the program is slow. At some point of bad coding the best code optimizer 
> won't be able to help you.

I did that in my 1st version. I wanted to see if the optimizer would 
result in equivalent code. No such luck.

> Eventually the optimizer will transform this:
>          for R in A'range (2) loop
>             C (I, J) := C (I, J) + A (I, R) * B (R, J);
>          end loop;
> into something like:
>          Sum:= C (I, J); -- hopefully a Sum is mapped to a register
>          for R in A'range (2) loop
>             Sum := Sum + A (I, R) * B (R, J);
>          end loop;
>          C (I, J):= Sum;
> but in that case it probably won't be able to guess that the C(I,J) was 
> zeroed before and replace the first line by:
>          Sum:= 0.0;

Thanks for the discussion.

The initialization of C is static, so a good optimizer could. They're 
hard to find, though.

> sparing the reading of C(I,J) (must cost much time...).
> If you are luckier, the optimizer will do
>          Sum:= 0.0;
>          for R in A'range (2) loop
>             Sum := Sum + A (I, R) * B (R, J);
>          end loop;
>          C (I, J):= C (I, J) + Sum;
> But still, it won't spare the time lost to fill the C matrix with zeros.

I didn't include that in the timing.

> If you want to do a benchmark with Fortran, it's really not a good idea 
> to begin with "pessimizing" the Ada code.

I'm more interested in seeing what makes a difference in the Ada. In 
this case, the high-level features that let you write less code.

-- 
Jeff Carter
"I unclog my nose towards you."
Monty Python & the Holy Grail
11