Loop Vectorization 001

Question

I have a Vectorization optimization problem.

I have a struct pDst which have 3 fields named: 'red', 'green' and 'blue'.
The type might be 'Char', 'Short' or 'Float'.This is given and can not be altered.
Theres is another array pSrc which represents an image [RGB] - Namely an array of 3 pointers which every one of them point to a layer of an image.
Each layer is built using IPP plane oriented image (Namely, Each plane is formed independently - 'ippiMalloc_32f_C1'): http://software.intel.com/sites/products/documentation/hpc/ipp/ippi/ippi_ch3/functn_Malloc.html.

We would like to copy it as described in the following code:

for(int y = 0; y < imageHeight; ++y)
{
    for(int x = 0; x < imageWidth; ++x)
    {
        pDst[x + y * pDstRowStep].red     = pSrc[0][x + y * pSrcRowStep];
        pDst[x + y * pDstRowStep].green   = pSrc[1][x + y * pSrcRowStep];
        pDst[x + y * pDstRowStep].blue    = pSrc[2][x + y * pSrcRowStep];
    }
} 

Yet, in this form the compiler can't vectorize the code.
At first it says:

"loop was not vectorized: existence of vector dependence.".

When I use the #pragma ivdep to help the compiler (Since there's no dependence) I get the following error:

"loop was not vectorized: dereference too complex.".

Anyone has an idea how to allow vectorization?
I use Intel Compiler 13.0.
Thanks.

Update:

If I edit the code as following:

Ipp32f *redChannel      = pSrc[0];
Ipp32f *greenChannel  = pSrc[1];
Ipp32f *blueChannel     = pSrc[2];
for(int y = 0; y < imageHeight; ++y)
{
    #pragma ivdep
    for(int x = 0; x < imageWidth; ++x)
    {
        pDst[x + y * pDstRowStep].red     = redChannel[x + y * pSrcRowStep];
        pDst[x + y * pDstRowStep].green   = greenChannel[x + y * pSrcRowStep];
        pDst[x + y * pDstRowStep].blue    = blueChannel[x + y * pSrcRowStep];
    }
}

For output types of 'char' and 'short' I get vecotization.
Yet for type of 'float' I don't.
Instead I get the following message:

loop was not vectorized: vectorization possible but seems inefficient.

How could that be?

Answer 1

In the following code, using pragma ivdep does surely ignore the vector dependence but the compiler heuristics/cost analysis came to a conclusion that vectorizing the loop is not efficient:

Ipp32f *redChannel      = pSrc[0];
Ipp32f *greenChannel  = pSrc[1];
Ipp32f *blueChannel     = pSrc[2];
for(int y = 0; y < imageHeight; ++y)
{
    #pragma ivdep
    for(int x = 0; x < imageWidth; ++x)
    {
        pDst[x + y * pDstRowStep].red     = redChannel[x + y * pSrcRowStep];
        pDst[x + y * pDstRowStep].green   = greenChannel[x + y * pSrcRowStep];
        pDst[x + y * pDstRowStep].blue    = blueChannel[x + y * pSrcRowStep];
    }
}

The vectorization will be inefficient since the operation involves copying contiguous block of memory from source to non-contiguous memory locations at the destination. So there is a scatter happening here. If you still want to enforce vectorization and see if there any performance improvement in comparison to non-vectorized version, please use pragma simd instead of pragma ivdep as shown below:

#include<ipp.h>
struct Dest{
float red;
float green;
float blue;
};
void foo(Dest *pDst, Ipp32f **pSrc, int imageHeight, int imageWidth, int pSrcRowStep, int pDstRowStep){
    Ipp32f *redChannel      = pSrc[0];
    Ipp32f *greenChannel  = pSrc[1];
    Ipp32f *blueChannel     = pSrc[2];
    for(int y = 0; y < imageHeight; ++y)
    {
        #pragma simd
        for(int x = 0; x < imageWidth; ++x)
        {
            pDst[x + y * pDstRowStep].red     = redChannel[x + y * pSrcRowStep];
            pDst[x + y * pDstRowStep].green   = greenChannel[x + y * pSrcRowStep];
            pDst[x + y * pDstRowStep].blue    = blueChannel[x + y * pSrcRowStep];
        }
    }
    return;
}

The corresponding vectorization report is:

$ icpc -c test.cc -vec-report2
test.cc(14): (col. 9) remark: SIMD LOOP WAS VECTORIZED
test.cc(11): (col. 5) remark: loop was not vectorized: not inner loop

More documentation on pragma simd is available at https://software.intel.com/en-us/node/514582.