Cause:
This vectorization diagnostic is emitted starting from Intel(R) C++ Compiler 15.0. This diagnostic message is emitted when the outer loop vectorization is not possible using auto-vectorization. This decision is based on vectorizer heuristics and cost model. One way to override the vectorizer cost model is to use explicit vectorization tools like #pragma simd.
Examples:
void foo(float **a, float **b, int N){
int i, j;
#pragma ivdep
for (i=0;i<N;i++){
float *ap = a[i];
float *bp = b[i];
for (j=0;j<N;j++){
ap[j] = bp[j];
}
}
}
$ icpc test51.cc -c -qopt-report2 -qopt-report-phase=vec -qopt-report-file=stderr
Begin optimization report for: foo(float **, float **, int)
Report from: Vector optimizations [vec]
LOOP BEGIN at test51.cc(4,3)
remark #15541: outer loop was not auto-vectorized: consider using SIMD directive
LOOP BEGIN at test51.cc(7,5)
remark #15541: outer loop was not auto-vectorized: consider using SIMD directive
LOOP BEGIN at test51.cc(7,5)
<Multiversioned v2>
remark #15304: loop was not vectorized: non-vectorizable loop instance from multiversioning
LOOP END
LOOP BEGIN at test51.cc(7,5)
<Remainder, Multiversioned v2>
LOOP END
LOOP END
LOOP END