How to vectorize code in C ++?

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How to vectorize code in C ++?

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#1 by (2 votes)

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I would like to know how to vectorize code in C ++? because the material I found on the internet is a bit fake about it.

I understand as a vectorization the use not only of vectors, but of doing in a single step a whole sequence of steps, that is, doing in one step d= (c+e)/2; instead of repeating those steps for each position of the array d[i][j] = (c[i][j]+e[i][j])/2;

For example how to vectorize the following program?

#include <iostream>

using namespace std;
int main(){

    int d[4][4],c[4][4],e[4][4];

    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++){
            c[i][j] =i+j;
            e[i][j] = 4*i;
        }
    }
    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++){
            d[i][j] = (c[i][j]+e[i][j])/2;
            if(d[i][j]<3){
                d[i][j]=3;
            } 
        }  
    }
    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++){
           cout << d[i][j] << " ";
        }
        cout << endl;
    }


    return 0;
}

When I use the vectorization flag to see how many loops are being vectored with the help of -O2 -ftree-vectorize -fopt-info-vec-optimized it responds to me "loop vectorized" ie only one loop was vectorized and if I use -all instead of -optimized it returns me that many parts of the program have not been vectorized.

c++ gcc

asked by anonymous 21.09.2017 / 20:15

1 answer

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score 2 · Accepted Answer

The problem is that the conditional if contained within the second loop does not allow it to be optimized by the compiler:

for(int i=0;i<4;i++){
    for(int j=0;j<4;j++){
        d[i][j] = (c[i][j]+e[i][j])/2;
        if(d[i][j]<3){
            d[i][j]=3;
        }
    }
}

One solution to this problem is to replace the conditional if with a conditional ternário , for example:

for(int i=0;i<4;i++){
    for(int j=0;j<4;j++){
        d[i][j] = (c[i][j]+e[i][j])/2;
        d[i][j] = ( d[i][j] < 3 ) ? 3 : d[i][j];
    }
}

Compilation Test GCC :

$ g++ -v -O2 -ftree-vectorize -fopt-info-vec-optimized vect.cpp -o vect

Output:

[...]

Analyzing loop at vect.cpp:21

Analyzing loop at vect.cpp:14

vect.cpp:14: note: vect_recog_divmod_pattern: detected: 
vect.cpp:14: note: pattern recognized: patt_3 = patt_4 >> 1;

Analyzing loop at vect.cpp:15

vect.cpp:15: note: vect_recog_divmod_pattern: detected: 
vect.cpp:15: note: pattern recognized: patt_77 = patt_1 >> 1;


Vectorizing loop at vect.cpp:15

vect.cpp:15: note: LOOP VECTORIZED.
Analyzing loop at vect.cpp:8

Analyzing loop at vect.cpp:9


Vectorizing loop at vect.cpp:9

vect.cpp:9: note: LOOP VECTORIZED.
vect.cpp:4: note: vectorized 2 loops in function.

[...]

References:

link

EDIT:

The answer applies only to the 4.8 version of GCC .

The 7.0 version is already able to vectorize loops without having to replace the if conditionals with ternary operators through the -fsplit-loops optimization option.

Reference: link