Reading from array is faster than writing ???

[Oleg Kornilov]

Hello !
Who can explain why readind from array[][] is fast, but
writing (same place) might take a lot of time (I need many loops)
How to solve it (declare as static or force some compiler (VC 6.0)
options) ??? Thanks in advance
Here is example (but critical !!!) code...

void main(int argc, char* argv[])
{
unsigned char r[360][180], dat2, i,j,k;
int p;

for (p=0; p<650; p++) {
for (i=0; i<200; i++) {
for (j=0; j<200; j++) {
for (k=0; k<100; k++) {

dat2=r[j][i]; dat2=r[j][i]; dat2=r[j][i];

// r[j][i]=dat2; r[j][i]=dat2; r[j][i]=dat2;
// critical point: without it almost no execution time
// with it - 6 seconds on P3-1000 ???

}
}
}
}
}

[Claudio Puviani]

"Oleg Kornilov" <ol***********@mail.ru> wrote

Hello !
Who can explain why readind from array[][] is fast, but
writing (same place) might take a lot of time (I need many loops)
How to solve it (declare as static or force some compiler (VC 6.0)
options) ??? Thanks in advance
Here is example (but critical !!!) code...

void main(int argc, char* argv[])
{
unsigned char r[360][180], dat2, i,j,k;
int p;

for (p=0; p<650; p++) {
for (i=0; i<200; i++) {
for (j=0; j<200; j++) {
for (k=0; k<100; k++) {

dat2=r[j][i]; dat2=r[j][i]; dat2=r[j][i];

// r[j][i]=dat2; r[j][i]=dat2; r[j][i]=dat2;
// critical point: without it almost no execution time
// with it - 6 seconds on P3-1000 ???

}
}
}
}
}

My first guess is that the compiler is (correctly) optimizing out your reads
since the resulting value is never used for anything.

Claudio Puviani

[John Ericson]

"Oleg Kornilov" <ol***********@mail.ru> wrote in message
news:9c**************************@posting.google.c om...

Hello !
Who can explain why readind from array[][] is fast, but
writing (same place) might take a lot of time (I need many loops) How to solve it (declare as static or force some compiler (VC 6.0) options) ??? Thanks in advance
Here is example (but critical !!!) code...

void main(int argc, char* argv[])
{
unsigned char r[360][180], dat2, i,j,k;
int p;

for (p=0; p<650; p++) {
for (i=0; i<200; i++) {
for (j=0; j<200; j++) {
for (k=0; k<100; k++) {

dat2=r[j][i]; dat2=r[j][i]; dat2=r[j][i];

// r[j][i]=dat2; r[j][i]=dat2; r[j][i]=dat2;
// critical point: without it almost no execution time
// with it - 6 seconds on P3-1000 ???

}
}
}
}
}

Aren't you accessing outside your array bounds?

[Ruediger Knoerig]

Oleg Kornilov wrote:

Hello !
Who can explain why readind from array[][] is fast, but
writing (same place) might take a lot of time (I need many loops)
How to solve it (declare as static or force some compiler (VC 6.0)
options) ??? Thanks in advance
Here is example (but critical !!!) code...

One possible answer could be the cache. Another answer could be the
optimization: if you don't use the values you read in the compiler may omit
the loop completely.
Some nice performance tips:
* use unsigned int / size_t - variables for loop indicies.
* use Blitz++ arrays or 1D-valarrays for storing the data:
valarray<unsigned char> r(360*180);
r[180*row+column]=value;
* make usage of the superscalar architecture / SIMD calls
for(size_t i=0;i < 100;i++) value += data[i];
to
sum1=sum2=sum3=sum4=0.0;
for(size_t i=0;i < 100; i+=4)
{
sum1 += data[i];
sum2 += data[i+1];
sum3 += data[i+2];
sum4 += data[i+3];
}
sum1 += (sum2 + sum3 + sum4);

Although its for Athlons some nice tips are still usuable on
other modern machines:
http://www.amd.com/us-en/assets/cont...te_papers_and_
tech_docs/22007.pdf

[Victor Bazarov]

"John Ericson" <je************@pacbell.net> wrote...

"Oleg Kornilov" <ol***********@mail.ru> wrote in message
news:9c**************************@posting.google.c om...

Hello !
Who can explain why readind from array[][] is fast, but
writing (same place) might take a lot of time (I need many

loops)

How to solve it (declare as static or force some compiler

(VC 6.0)

options) ??? Thanks in advance
Here is example (but critical !!!) code...

void main(int argc, char* argv[])
{
unsigned char r[360][180], dat2, i,j,k;
int p;

for (p=0; p<650; p++) {
for (i=0; i<200; i++) {
for (j=0; j<200; j++) {
for (k=0; k<100; k++) {

dat2=r[j][i]; dat2=r[j][i]; dat2=r[j][i];

// r[j][i]=dat2; r[j][i]=dat2; r[j][i]=dat2;
// critical point: without it almost no execution time
// with it - 6 seconds on P3-1000 ???

}
}
}
}
}

Aren't you accessing outside your array bounds?

Nah, 'j' never goes above 199, which is well within 0..359 space,
so when 'i' goes above 179 (for only 20), he's not stepping on any
other memory, only the next 20 elements of the 'j+1' array or the
'r' superarray.

V

[John Ericson]

"Victor Bazarov" <v.********@comAcast.net> wrote in message
news:r26Xb.27653$uV3.49996@attbi_s51...

"John Ericson" <je************@pacbell.net> wrote...

"Oleg Kornilov" <ol***********@mail.ru> wrote in message
news:9c**************************@posting.google.c om...

Hello !
Who can explain why readind from array[][] is fast, but writing (same place) might take a lot of time (I need many

loops)

How to solve it (declare as static or force some

compiler (VC 6.0)

options) ??? Thanks in advance
Here is example (but critical !!!) code...

void main(int argc, char* argv[])
{
unsigned char r[360][180], dat2, i,j,k;
int p;

for (p=0; p<650; p++) {
for (i=0; i<200; i++) {
for (j=0; j<200; j++) {
for (k=0; k<100; k++) {

dat2=r[j][i]; dat2=r[j][i]; dat2=r[j][i];

// r[j][i]=dat2; r[j][i]=dat2; r[j][i]=dat2;
// critical point: without it almost no execution

time // with it - 6 seconds on P3-1000 ???

}
}
}
}
}

Aren't you accessing outside your array bounds?

Nah, 'j' never goes above 199, which is well within 0..359

space, so when 'i' goes above 179 (for only 20), he's not stepping on any other memory, only the next 20 elements of the 'j+1' array or the 'r' superarray.

V

:) You've got a point <ROTFL> - Victor, I sure enjoy your
posts!

Best regards, JE

[alexhong2001]

Ruediger Knoerig <ru******@knoerig.de> wrote in message
news:c0**********@mamenchi.zrz.TU-Berlin.DE...

Oleg Kornilov wrote:

Hello !
Who can explain why readind from array[][] is fast, but
writing (same place) might take a lot of time (I need many loops)
How to solve it (declare as static or force some compiler (VC 6.0)
options) ??? Thanks in advance
Here is example (but critical !!!) code... One possible answer could be the cache. Another answer could be the
optimization: if you don't use the values you read in the compiler may

omit the loop completely.
Some nice performance tips:
* use unsigned int / size_t - variables for loop indicies.
Would you elabrate on this for us please?
* use Blitz++ arrays or 1D-valarrays for storing the data:

What is Blitz++ arrays?