Auto-vectorizing: Convincing the compiler that alias check is not necessary

Question

I am doing some image processing, for which I benefit from vectorization. I have a function that vectorizes ok, but for which I am not able to convince the compiler that the input and output buffer have no overlap, and so no alias checking is necessary. I should be able to do so using __restrict__, but if the buffers are not defined as __restrict__ when arriving as function argument, there is no way to convince the compiler that I am absolutely sure that 2 buffers will never overlap.

This is the function:

__attribute__((optimize("tree-vectorize","tree-vectorizer-verbose=6")))
void threshold(const cv::Mat& inputRoi, cv::Mat& outputRoi, const unsigned char th) {

    const int height = inputRoi.rows;
    const int width = inputRoi.cols;

    for (int j = 0; j < height; j++) {
        const uint8_t* __restrict in = (const uint8_t* __restrict) inputRoi.ptr(j);
        uint8_t* __restrict out = (uint8_t* __restrict) outputRoi.ptr(j);
        for (int i = 0; i < width; i++) {
           out[i] = (in[i] < valueTh) ? 255 : 0;
        }
    }
}

The only way I can convince the compiler to not perform the alias checking is if I put the inner loop in a separate function, in which the pointers are defined as __restrict__ arguments. If I declare this inner function as inlined, again the alias checking is activated.

You can see the effect also with this example, which I think is consistent: http://goo.gl/7HK5p7

(Note: I know there might be better ways of writing the same function, but in this case I am just trying to understand how to avoid alias check)

Edit:
Problem is solved!! (See answer below)
Using gcc 4.9.2, here is the complete example. Note the use of the compiler flag -fopt-info-vec-optimized in place of the superseded -ftree-vectorizer-verbose=N.
So, for gcc, use #pragma GCC ivdep and enjoy! :)

Answer 1

if you are using Intel compiler, you can try to include the line:

#pragma ivdep 

The following paragraph is quoted from Intel compiler user manual:

The ivdep pragma instructs the compiler to ignore assumed vector dependencies. To ensure correct code, the compiler treats an assumed dependence as a proven dependence, which prevents vectorization. This pragma overrides that decision. Use this pragma only when you know that the assumed loop dependencies are safe to ignore.

In gcc, one should add the line:

#pragma GCC ivdep

inside the function and right before the loop you want to vectorize (see documentation). This is only supported starting from gcc 4.9 and, by the way, makes the use of __restrict__ redundant.

Answer 2

Another approach for this specific issue that is standardised and fully portable across (reasonably modern) compiler is to use the OpenMP simd directive, which is part of the standard since version 4.0. The code then becomes:

void threshold(const unsigned char* inputRoi, const unsigned char valueTh,
               unsigned char* outputRoi, const int width,
               const int stride, const int height) {
    #pragma omp simd
    for (int i = 0; i < width; i++) {
        outputRoi[i] = (inputRoi[i] < valueTh) ? 255 : 0;
    }
}

And when compiled with OpenMP support enabled (with either full support or only partial one for simd only, like with -qopenmp-simd for the Intel compiler), then the code is fully vectorised.

In addition, this gives you the opportunity to indicate possible alignment of vectors, which can come handy in some circumstances. For example, had your input and output arrays been allocated with an alignment-aware memory allocator, such a posix_memalign() with an alignment requirement of 256b, then the code could become:

void threshold(const unsigned char* inputRoi, const unsigned char valueTh,
               unsigned char* outputRoi, const int width,
               const int stride, const int height) {
    #pragma omp simd aligned(inputRoi, outputRoi : 32)
    for (int i = 0; i < width; i++) {
        outputRoi[i] = (inputRoi[i] < valueTh) ? 255 : 0;
    }
}

This should then permit to generate an even faster binary. And this feature isn't readily available using the ivdep directives. All the more reasons to use the OpenMP simd directive.

Answer 3

The Intel compiler at least as of version 14 does not generate aliasing checks for threshold2 in the code you linked indicating that your approach should work. However, the gcc auto-vectorizer misses this opportunity for optimization but does generate vectorized code, tests for proper alignment, tests for aliasing and non-vectorized fall-back/clean-up code.