I forgot to CC the list. Sorry.
------- Forwarded Message
From: tom fogal <tfogal@xxxxxxxxxxxxxx>
To: Å?ukasz Lew <lukasz.lew@xxxxxxxxx>
Subject: Re: Aliasing performance problem.
In-Reply-To: Your message of "Sun, 01 Nov 2009 19:35:37 +0100."
<c55009e70911011035o1f206eadq36ade953b48543a@xxxxxxxxxxxxxx>
References: <c55009e70911011035o1f206eadq36ade953b48543a@xxxxxxxxxxxxxx>
Date: Sun, 01 Nov 2009 12:17:08 -0700
Å?ukasz Lew <lukasz.lew@xxxxxxxxx> writes:
> 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()];
> }
use reinterpret_cast here.
How big is a `Nat'? Could you instead pass it by reference?
> 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)
first, compile with -fno-strict-aliasing to get your correctness back.
Then -- does it help if you force inlining of the method? You might
also consider playing with the attributes on the function:
http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Function-Attributes.html
e.g. `hot', `pure' or `const' if applicable, `nothrow' though I bet
it won't help and I'm not sure if that's equivalent to `throw()', the
latter generally being a bad idea.
If that doesn't work, I would say compile it both ways and look at the
generated asm.
> If this is lack optimizations because of aliasing info, this has to
> be doable somehow as STL have the same problem, doesn'it ?
The STL `Container' concept requires the contained type to be
`DefaultConstructible', which has the semantics you would think. I'm
going off the SGI STL docs, so that knowledge is a bit dated, though I
would imagine still generally accurate.
HTH,
- -tom
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