[Mono-dev] Faster

[Steve Bjorg]

Is the cost of the if-null check greater than setting up an exception catch handler?

- Steve

---------------------------------
Steve G. Bjorg
MindTouch
San Diego, CA
 
619.795.8459 office
425.891.5913 mobile
http://twitter.com/bjorg

On Mar 24, 2011, at 9:00 AM, Miguel de Icaza wrote:

> Hello guys,
> 
> Today in the shower I had an idea that I believe we could use to
> improve the performance of our class library code.
> 
> Plenty of our class library code has code like this:
> 
> void Foo (Something x)
> {
>    if (x == null)
>        throw new ArgumentNullException ("x");
>    x.DoSomething ();
>    x.AndThenMore ();
> }
> 
> Arguably, if this could be inlined, and the JIT could prove that x is
> not null, we would skip the first test, for example:
> 
> Foo (new Something ());
> 
> But this is the exception, in general, the JIT would not be able to
> know this kind of information for even trivial uses like:
> 
> Foo (Bar.GetSomething ());
> 
> Rendering the optimization not very effective.
> 
> But what if we changed our code in Foo across our class libraries to
> do this instead:
> 
> void Foo (Something x)
> {
>    try {
>        x.DoSomething ();
>    } catch (NullReferenceException e){
>        if (x == null)
>             throw new ArgumentNullException ("x");
>        else
>              throw;
>    }
>    x.AndThenMore ();
> }
> 
> This has the advantage that the test for the value of "x" being null
> is delayed until we actually need it.    The downside of course is
> that DoSomething could actually take other forms and might end up
> running code that we do not need before it touches x, for example,
> this would be a problem:
> 
> // We should never add null values.
> void AddToCache (Something x)
> {
>    cache.Add (x);
> }
> 
> void Foo (Something x)
> {
>      if (x == null)
>          throw new ArgumentNullException ("x");
>      AddToCache (x);
> }
> 
> If we rewrite the above code, we would end up with a bug like the one
> I described.
> 
> Miguel
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