Re: C++ Performance Tips

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C++ Performance Tips

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Join Date: Feb 2007

Location: Bangalore, India

Posts: 535

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Re: C++ Performance Tips

#24

Feb 23rd, 2007

@WaltP
Except if your description is true, you'd be better off using

 Help with Code Tags 
 c++ Syntax (Toggle Plain Text) 
i = 30;
while (i--)
{/*do your stuff*/}
i = 30;
while (i--)
{/*do your stuff*/}

>> Sorry but I fail to see the different between for and while loop.. Do you mean it's faster to use while instead of for?

@Salem
> //A quicker method is to simply use the value as an
Except your other two methods never risk an out of bounds memory access.
KashAI>> TRUE. So I hope that anyone able enough to understand this won't blindly copy my code.

> 1. unsigned int arithmatic is faster than signed int.
Where's your evidence?
KashAI>> I was afraid someone will ask.

. Anyway, simple answer is I don't know.
But here is what I know:
1. In VS 6.0 (on Intel H/W) a simple for loop with loop variable being unsigned is about 2 seconds faster than when loop variable is signed int. (looped some 100K and 500K times to print the value of loop variable)
2. In most cases one can see that there are seperate assemply instructions for signed and unsigned arithmetic. Which at least indicates a difference in performance.
3. Number of flags applicable (CF=carry-over-flag, SG=sign-flag, OF=overflow-flag) to signed and unsigned instructions' execution are different.
4. I'm vaguely remember an instruction called SBB (substract using borrow) which, if i'm not wrong, is only applicable to signed arithmetic.
And use of it is in case where the requested substraction of 2 signed numbers can not be completed with a single instruction due to register size.
> 2. registers are registers ! They're simply faster than memory access.
True, but any decent compiler nowadays is far more capable of deciding which variables would be best placed in registers.
KashAI>> So if I understand it right what I've written is correct, but not neccesarry.
> AND it doesn't matter which way you traverse while looping,
Well if you're using it for indexing an array, and your cache is optimised in favour of incremental access, then you lose badly.
I've only ever seen counting backwards to zero save ONE instruction on
those machines which specifically have a 'test-and-branch' instruction.
KashAI>> So in short, should one NOT optimize it this way? May be you could add some practical numbers for the benefit of readers which will help them in deciding whether to use this optimization or not?
E.g. "in 80% of cases cache is optimised in favour of incremental access" OR
"Now-a-days most machines support "'test-and-branch' instruction".

> Another way of optimizing loops is to unroll it:
Or just use the gcc flag -funroll-loops

Better yet, bone up on all the magic which a modern compiler can do,
which doesn't involve you mauling the code into an unreadable mess.
http://gcc.gnu.org/onlinedocs/gcc-4....timize-Options

KashAI>> Now that is useful info. Just checked and found that VS 6.0 and Sun Workshop 6.0 also support lopp unrolling.

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Re: C++ Performance Tips

C++ Performance Tips

Re: C++ Performance Tips