My first guess would be an alignment problem. But that is just a guess
especially since you didn't say anything about how Elt aligns.
Also, an alignment problem is a little less likely in the code you
actually posted than it would be in the real code that you cut this out
of. Do you really have the performance problem with this code? Or have
you simplified the code to the point that you can't measure the performance?
I understand the strict-aliasing issue, but in this case it shouldn't
actually be a problem. You always access the data as Elt and never as
char, so the fact that you allocate as char should only be an alignment
issue, not an aliasing issue.
Łukasz Lew wrote:
Hi,
I have a container similar to this one.
template <typename Nat, typename Elt>
class NatMap {
public:
Elt& operator[] (Nat nat) {
return tab [nat.GetRaw()];
}
private:
Elt tab [Nat::kBound];
};
I wanted to drop the requirement for Elt to have a default constructor:
template <typename Nat, typename Elt>
class NatMap {
public:
Elt& operator[] (Nat nat) {
return ((Elt*)tab) [nat.GetRaw()];
}
private:
char tab [Nat::kBound * sizeof(Elt)];
};
I use g++-4.3 and this code works 25% slower in my application than
the previous one. Unfortunately the slowdown does not manifest in a
synthetic benchmark. I guess it is something about compiler
optimizations, aliasing, aligning, or similar stuff.
Just now I tried new g++-4.4 and it gave me a following warning for
the latter code:
dereferencing pointer '<anonymous>' does break strict-aliasing rules
What should I do to get my performance back? (while not needing the
default constructor)
Thanks for any suggestion.
Lukasz
PS
If this is lack optimizations because of aliasing info, this has to be
doable somehow as STL have the same problem, doesn'it ?
