Code optimization about array iteration

[zaeminkr@gmail.com]

Hello.

I believed below two functions would have same performance.

However, I realized the second form of array iteration are found more
in many performance critical code like Microsoft CRT source code.

Really the second form of array iteration(count _neg2) is faster?


Thanks in advance.

*************** *************** *************** *************** **************
unsigned int count_neg1(int* int_array, unsigned int n)
{
unsigned int c = 0;
unsigned int i = 0;

while(i < n)
{
if(int_array[i++] < 0)
{
++c;
}
}

return c;
}
*************** *************** *************** *************** **************
unsigned int count_neg2(int* int_array, unsigned int n)
{
unsigned int c = 0;
int* end = int_array + n;

while(int_array < end)
{
if(*int_array++ < 0)
{
++c;
}
}

return c;
}

[Keith Thompson]

Re: Code optimization about array iteration

zaeminkr@gmail. com writes:

I believed below two functions would have same performance.
>
However, I realized the second form of array iteration are found more
in many performance critical code like Microsoft CRT source code.
>
Really the second form of array iteration(count _neg2) is faster?
>
>
Thanks in advance.
>
*************** *************** *************** *************** **************
unsigned int count_neg1(int* int_array, unsigned int n)
{
unsigned int c = 0;
unsigned int i = 0;
>
while(i < n)
{
if(int_array[i++] < 0)
{
++c;
}
}
>
return c;
}
*************** *************** *************** *************** **************
unsigned int count_neg2(int* int_array, unsigned int n)
{
unsigned int c = 0;
int* end = int_array + n;
>
while(int_array < end)
{
if(*int_array++ < 0)
{
++c;
}
}
>
return c;
}

The comp.lang.c FAQ is at <http://www.c-faq.com/>. You've asked
question 20.14.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"