How to overcome "existence of vector dependence" in icc

Question

I want to vectorize following loop in C:

for(k = 0; k < SysData->numOfClaGen; k++)
            A[k] = B[k] * cos(x1[2 * k] - x1[ind0 + k]);

where, there is no alias between variables and ind0 is a constant. None of the other pointers (A or B) point to ind0 and therefore, ind0 remains constant throughout the loop.

When I compile the code with icc, it says that this loop cannot be vectorized due to possible vector dependence. Here is the message:

loop was not vectorized: existence of vector dependence.

I narrowed the problem down and found out that replacing ind0 with a constant number solves the problem. So, I assume that icc thinks A may point to ind0 and therefore, ind0 may change.

I would like to know how I can help the compiler to know that it is safe to vectorized such loop.

Thanks in advance for your help.

Answer 1

Add #pragma ivdep in front of the for loop, it instructs the compiler to ignore assumed vector dependencies.

#pragma ivdep
for(k = 0; k < SysData->numOfClaGen; k++)
            A[k] = B[k] * cos(x1[2 * k] - x1[ind0 + k]);

for more info about ivdep, see icc doc

Answer 2

Use of the restrict modifier for pointers asserts to the compiler that there is no aliasing. This keyword was introduced in C99. C++ does not support it, but many C++ compilers support __restrict as an equivalent proprietary extension. With the Intel compiler, one has to enable use of restrict by adding the command line flag -restrict (Linux) or /Qrestrict (Windows). In the following version of your code the loop is vectorized as desired when using Intel compiler version 13.1.3.198:

#include <math.h>

struct bar {
    int numOfClaGen;
};

void foo (double * restrict A, 
          const double * restrict B,
          const double * restrict x1,
          const struct bar * restrict SysData,
          const int ind0)
{
    int k;
    for (k = 0; k < SysData->numOfClaGen; k++) {
        A[k] = B[k] * cos(x1[2 * k] - x1[ind0 + k]);
    }
}

Invoking the compiler as follows (on a 64-bit Windows system)

icl /c /Ox /QxHost /Qrestrict /Qvec-report2 vectorize.c

the compiler reported

vectorize.c(14): (col. 5) remark: LOOP WAS VECTORIZED.

Answer 3

icc was changed a year ago to set -ansi-alias as a default for linux and Mac. For Windows, this default can't be counted on, as it conflicts with Microsoft usage. This option is equivalent to gcc -fstrict-aliasing, which has been a default since gcc 3.0. I think it's much better to set this option than to set ivdep restrict or simd for such a limited issue. Although it's not well documented, icc treats __restrict the same as gcc and doesn't require the restrict or C99 option to accept it. In principle, it should come into play only for the objects being modified (A[] in the example above). Strangely, __restrict has a slightly different meaning for MSVC++. It permits non-vector optimizations which might otherwise be prevented by possible dependencies, but doesn't enable vectorization (but it might apply to the present case).