[Mono-dev] Trivial Enumerable Optimizations

[Jonathan Pryor]

System.Core.Enumerable already has a set of "obvious" optimizations in
which the IEnumerable<T> argument is checked to see if it's another
interface and the implementation is "switched" based on the runtime
type, e.g.

        public static bool Contains<TSource> (this IEnumerable<TSource> source, TSource value)
        {
                var collection = source as ICollection<TSource>;
                if (collection != null)
                        return collection.Contains (value);
                
                return Contains<TSource> (source, value, null);
        }

Not all such optimizations were performed everywhere they could be --
I've found two, included in the attached System.Core-opt.diff patch.

The changes are noticeable (surprisingly to me); eo.cs contains my test
app.  N is the number of iterations to do in the test, S is the size of
the array:

        N,S            1.9[Long]   Opt[Long]   |  1.9[Rev]     Opt[Rev]
        ---            ----------  ----------  |  --------     --------
        1,1:           00.0127820  00.0074890  |  00.0029850  00.0013530
        1,1000:        
        10,10:         00.0103430  00.0067620  |  00.0031240  00.0013250
        1000,1000:     00.0168560  00.0070100  |  00.0100230  00.0015060
        10000000,1:    01.0322690  00.1243650  |  03.4776960  00.7064270
        10000000,1000: 98.7992270  00.1945420  |  66.5775810  01.1917510

I wasn't expecting 1 iteration of an array with 1 element to actually
provide a noticeable improvement; perhaps something else is in play.
The change is quite noticeable at larger iterations.

Permission to commit?

 - Jon

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