Re: [boost] Cutting over to MPL - any suggestions on improvingperformance?

From: Phil Richards (news_at_[hidden])
Date: 2004-01-20 13:44:32

On Sun, 18 Jan 2004 00:19:44 -0600, Aleksey Gurtovoy wrote:
[with respect to my performance problems using the MPL]
> Sounds like something is wrong. Let's figure it out.

[what I should be looking for]
> O(n*n) algorithms :).

Well, as far as I can tell there are none :-(

> The code you've posted seems pretty innocent. I'd need a compliable test
> case which reproduces the problem to be able to say more.

Ok, here's a stripped down version that demonstrates the problem.  Compile
with -DLEVEL=# for #=0..6.  The results I get with gcc 3.3.2 on my 900MHz
Athlon (yes, I know, it's time for an upgrade) are as follows (to first
approximation).

LEVEL	Time (seconds)
0	0.6
1	0.6
2	0.6
3	1.4	(1 fold)
4	2.0	(2 folds)
5	2.6	(3 folds)
6	3.0	(4 folds)

Each instantiation of a "fold" causes a depressingly large increase in
compile time - I've split it down a little, and the zip_view has a bit of
an impact, but the guilty party really looks like it is a result of what
fold is doing to the compiler.  I've tried replacing the apply result with
a "typedef base type;" with no real difference.  I flipped the vector7
over into a list and no difference.

--- snip snip ---
#include <boost/mpl/zip_view.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/vector.hpp>
                                                                                
struct base {};
                                                                                
struct op {
    template<typename ResultSoFar, typename Value>
    struct apply {
        struct type {};
    };
};
                                                                                
struct dummy1 {};
struct dummy2 {};
                                                                                
int
main(int, char **)
{
#if LEVEL > 0
    using namespace boost::mpl;
                                                                                
#if LEVEL > 1
    typedef vector7<dummy1, dummy1, dummy1, dummy1, dummy1, dummy1, dummy1>
V1;
    typedef vector7<dummy2, dummy2, dummy2, dummy2, dummy2, dummy2, dummy2>
V2;
                                                                                
    V1 v1; (void)v1;
    V2 v2; (void)v2;
                                                                                
#if LEVEL > 2
    fold< zip_view< vector2<V1, V2> >, base, op >::type f1; (void)f1;
#if LEVEL > 3
    fold< zip_view< vector2<V2, V1> >, base, op >::type f2; (void)f2;
#if LEVEL > 4
    fold< zip_view< vector2<V1, V1> >, base, op >::type f3; (void)f3;
#if LEVEL > 5
    fold< zip_view< vector2<V2, V2> >, base, op >::type f4; (void)f4;
#endif
#endif
#endif
#endif
#endif
#endif
                                                                                
    return 0;
}
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phil
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