How to make C++ running efficient?
I have a block of following C++ code to run,
if(n-i>0 && n-i<a.size()) st[n]+=p[i]*a[n-i];
where a is a double vector of the same size as st, p is a double vector of size 2500.
I found that this block takes considerable running time. Do you have any good idea to improve this block running efficiency?
Thanks in advance.
a cheap first pass at it.
const int st_size = st.size(); //lets not call a function every iteration!
const int p_size = p.size();
const int a_size = a.size();
register int i; //try to help the compiler make a smart decision
const int nmi = n-i; //this is used a lot
if(nmi>0 && nmi<a_size)
Additional thoughts -- dont use double if you dont NEED double.
Re-arranging the code carefully to access memory sequentially instead of random will reduce page faults (can consume considerable effort)
Re-arranging the code to only have one loop will help free up registers and resources. Nontrivial coding effort.
If you really, really need a boost drop the vector and use an array and manage the size yourself. There is some overhead in the vector class that can be cleared up at the cost of a nice object.
Finally, I am almost certain you can lose the conditional(busted pipeline, many bad things) AND code the for loops in a way that reduces the "do nothing" iterations and only loops over valid indexes (that do work). This should cut out a lot of wasted time and should not be a difficult fix! A better algorithm is always the first step toward speed.
Thanks a lot. I will see what I can do with these points.
The vector and double type cannot be saved since this is a block in a class where they already exisit there.
Another point: you probably can replace the double with int, execute the loop on a vector of int, and then copy the result to a vector of double. I see nothing that warrants the use of double anyway.
Another, perhaps the most curcial point: turn your compiler's optimization switches on. Modern compilers are very good at optimizing loops, so perhaps you won't need to work very hard...
Finally, I disagree with jonnin with respect to the register declaration. I doubt that Visual C++ (even in its older versions) heeds this hint since the register keyword, like auto, is completely transparent -- the compiler ignores it because it usually knows best which variables are the best candidates for being stored in the CPU's registers.
Last edited by Danny; 10-14-2005 at 09:30 AM.
I havent had much luck with register to be sure. But I stick it in anyway as part of self documentation (hey reader, this is gonna be used a lot and might be a spot to somehow optimize).
You can't see the incoming data (p and a) -- how can you know that double/floating type isnt necessary? I dont see that -- it looks for all the world like a matrix multiply to me.
I guess you're right with respect to the double/float. Anyway, since I didn't see any positive evidence warranting the usage of a floating point datatype, I didn't exclude this option...
As for register: I think I can agree with you. It doesn't matter to the compiler anyway, so at least the keyword documents that this variable is used frequently. I never use register and inline in my code, btw.
I *have* to use inline -- we have the cheap visual .net and it does not support optimization (the release is set to only inline marked functions and cannot be changed.)
duh! why can't it optimize the code?
The cheap version has hardcoded the optimizer settings to "only functions labeled inline" among other dumb settings. I didnt see this when we bought it and now im stuck for a while -- I thought the cheap version lost the profiler and some less useful things.
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