X[]==Y[] is 7X slower than it should be -- why?

[Sean Kelly]

== Quote from Sean Kelly (sean at invisibleduck.org)'s article
> == Quote from Jarrett Billingsley (kb3ctd2 at yahoo.com)'s article
> > "Don" <nospam at nospam.com.au> wrote in message
> > news:g3ggq5$ui5$1 at digitalmars.com...
> > > What's it doing? Is it doing a byte-by-byte comparison? (it shouldn't do
> > > that even for strings). Is it doing bounds checking for every array
> > > element?? (even with -release -O)
> > See dmd/src/phobos/internal/adi.d  The important function here is:
> > extern (C) int _adEq(Array a1, Array a2, TypeInfo ti)
> > {
> >     if (a1.length != a2.length)
> >         return 0;
> >     auto sz = ti.tsize();
> >     auto p1 = a1.ptr;
> >     auto p2 = a2.ptr;
> >     if (sz == 1)
> >         // We should really have a ti.isPOD() check for this
> >         return (memcmp(p1, p2, a1.length) == 0);
> >     for (size_t i = 0; i < a1.length; i++)
> >     {
> >         if (!ti.equals(p1 + i * sz, p2 + i * sz))
> >             return 0;
> >     }
> >     return 1;
> > }
> > If the size of an element is 1 (bytes, chars, bools), it just does memcmp.
> > There's even a comment in there that memcmp should be used if the type is
> > POD (plain old data, which uints are).  But otherwise, it uses the RTTI
> > .equals function to compare the elements, and it doesn't even use parallel
> > pointers, it does manual indexing for each element.  .equals will, for
> > uints, just return true if they are equal.
> > What flabbergasts me is that in the uint[] typeinfo class, there is an
> > equals method that uses memcmp, but it's not used unless you explicitly call
> > it (or if you have a uint[][]).  Try doing
> > bool b = typeid(uint[]).equals(&X[], &Y[]);
> > and see what kind of performance you get.
> My mistake for assuming the compiler would generate the appropriate code for
> this.  I'm fixing this in Tango today, assuming no problems crop up.

Done :p

http://dsource.org/projects/tango/changeset/3651


Sean