2013/6/2 Etienne Sandré-Chardonnal <etienne.san...@m4x.org>
> Dear all,
>
> I'm using Mingw gcc 4.8.0 rubenvb seh build for x64 (native compiler)
>
> In my program, I am using large binary trees, each node being a Node class
> which is either a true node (two children pointers) or a leaf (pointer on
> some data), never both. It also has an integer member which allows
> recognizing if this is a true node or leaf.
>
> As the trees are huge (millions of nodes) and memory use large (typically
> 8GB), I'm trying to fine tune the data structures memory footprints.
>
> So I tried using a union for the Node class, unionning the true node and
> leaf pointers. This by doing a union of anonymous structs.
>
> The new code works well, and this spared about 5% memory allocation (which
> is significant here), but it also runs about 3% slower (which is also
> significant), the tree traversal representing about 30% of CPU use, this
> means its code is about 10% slower.
>
> I also tried, instead of a union, using a void* ptr instead of two
> pointers (one children for the true node, and leaf data pointer) to achieve
> a similar thing, using reinterpret_cast in member functions for getting the
> proper pointer types. This was even slower than the union.
>
> The Node class has no inheritance and nothing virtual.
>
> Do someone has an explanation for this? I was expecting a slight speed
> increase due to better cache efficiency of the tree traversal, so I'm
> disappointed!
>
I suggest asking this (with a tad more code if possible) on
StackOverflow.com. There are some incredibly knowledgeable people there
that might give you some hints on what exactly is biting you. I would guess
it's pointer aliasing that's preventing some optimization.
At least you've approached this from a "measure, measure, measure"
perspective, which is a good thing ;-)
Sorry for the delayed answer,
Ruben
> Thanks,
>
> Etienne
>
>
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