Re: GNAT (GCC) Profile Guided Compilation

[Keean Schupke <keean.schupke@googlemail.com>]

On Saturday, 14 July 2012 21:43:20 UTC+1, Niklas Holsti  wrote:
> On 12-07-14 23:17 , Keean Schupke wrote:
> >
> > Okay, I think I have tracked down the performance problem, but I am
> > not sure how to fix it. It would appear that C++ code that returns a
> > boolean from a function, generates a decision tree using tests and
> > branches, whereas Ada is setting the result into a Boolean variable.
> > This has the result that C++ is bailing out of the evaluation as soon
> > as it can (IE if one side of an and is false, or one side of an or is
> > true), but Ada is always evaluating all parts of the expressions.
> >
> > Is this a difference in language semantics, and what is the best way
> > to deal with it? Do I need to rewrite all 'and' and 'or' statements
> > in conditionals as nested if statements to get the evaluate only as
> > far as necessary semantics like C/C++?
> 
> Use the short-circuit forms of "and" and "or": "and then" and "or else".
> 
> -- 
> Niklas Holsti
> Tidorum Ltd
> niklas holsti tidorum fi
>        .      @       .

Yes, this appears to be the biggest cause of the profile guided compilation not working as well for Ada as C++. With the short-circuit forms I get the following speeds (thousands of simulations per second).

C++: 40k, profile-guided: 56k
Ada: 40k, profile-guided: 54k

So thats a huge improvement (up from 46k to 54k).

Cheers,
Keean.