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List:       kde-core-devel
Subject:    Re: Krazy: prefix vs. postfix ++ and -- operators
From:       Matthew Dawson <matthew () mjdsystems ! ca>
Date:       2011-01-11 15:44:28
Message-ID: 201101111044.38606.matthew () mjdsystems ! ca
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On Tuesday 11 January 2011 10:16:23 Christoph Feck wrote:
> Krazy says:
> 
> "You should use ++ and -- as prefix whenever possible as these are more 
> efficient than postfix operators. Prefix increments first and then uses the 
> variable, postfix uses the actual; the variable is incremented as well. 
> Because of this, the prefix operators are inherently more efficient."
> 
> Considering that today's CPUs have multiple pipelines, using a value directly 
> seems faster to me than waiting for it to be incremented or decremented.
> 
> Is this krazy warning still correct?
> 
> Christoph Feck (kdepepo)
> 

(This all applies equally to the -- operator, it is omitted for clarity.)

I'm assuming this Krazy warning is for incrementing a variable in a for loop as \
opposed to incrementing and assigning the variable.  Prefix is generally considered \
faster as it avoids an unnecessary copy of the data.  Effectively, the prefix \
increment operator looks like: int &operator++(){ //Prefix
	number = number+1;
	return number;
}

Verus postfix looks like:
int operator++(int){
	int temp = number;
	number = number+1;
	return temp;
}

When executing a for loop, using the postfix operator incurs that copy and thus can \
slow down the code.  This would be especially more relevant with iterators or the \
like as the compiler would have a harder time optimizing out the copy.

However when using the ++ operator in assignment, like:
int num2 = num1++;
int num3 = ++num1;

Both lines will have different results in num2 and num3 (num2 != num3) and the \
appropriate operator should be used.  So postfix doesn't use the value directly.  In \
fact the prefix uses the value directly.

Matthew


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