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From: Miles Bader (miles.bader_at_[hidden])
Date: 2007-10-04 22:22:56

"Phil Endecott" <spam_from_boost_dev_at_[hidden]> writes:
> Yes, a very good page showing several solutions:
> 1. A vector of member pointers (may have some overhead).
> 2. Anonymous structs and unions (may be non-standard).
> 3. x, y and z implemented as references (makes object bigger).
> 4. Relying on (&x)[1] pointing to y.

Regarding (2), note that using a union can cause severe performance
degradation with some compilers (recent versions of gcc, at least). I
guess that the compiler makes pessimistic assumptions about aliasing
when it sees a union.

As an example: I have some code that offers both ".x/.y/.z" and
array-style access to the members of 3d vectors.

The code originally only used ".x/.y/.z" fields. When I first added the
"array-style" feature, I used a construct like:

   union {
     struct { float x, y, z; };
     float els[3];
   float &operator[] (unsigned i) { return els[i]; }
   const float &operator[] (unsigned i) const { return els[i]; }

The resulting object code was _much_ worse than the original.

I subsequently changed the code to use your option (4):

   T &operator[] (unsigned i) { return (&x)[i]; }
   const T &operator[] (unsigned i) const { return (&x)[i]; }

And the resulting object code was much better -- basically the same as
the original .x/.y/.z fields-only code.

[I guess that doing this could potentially cause the compiler to miss
some aliasing that it shouldn't, but in practice this isn't a problem,
as all my code uses either the field-style access or array-style access,
and never mixes the two.]

Option (3) doesn't really seem practical to me given that such small
vectors are used very heavily, and often in great numbers.

I dunno about option (1): on first glance, it looks even more hacky
than the other options, and the the use of a global variable seems like
it might cause pessimal assumptions by the compiler (though at least,
hopefully, that would only affect the array-style access, not all access
like option (2) does).


"Suppose He doesn't give a shit?  Suppose there is a God but He
just doesn't give a shit?"  [George Carlin]

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