[Bug middle-end/58927] New: Despite loop->safelen=INT_MAX / GCC ivdep: loop versioned for vectorization because of possible aliasing

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58927

            Bug ID: 58927
           Summary: Despite loop->safelen=INT_MAX / GCC ivdep: loop
                    versioned for vectorization because of possible
                    aliasing
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org

Compiling the testsuite's pr33426-ivdep-4.cc
  g++ -std=c++11 -O3 -fopt-info-vec-optimized g++.dg/vect/pr33426-ivdep-4.cc

shows that the C++11 range-based loop has loop versioning, even though it
shouldn't due to GCC ivdep:

g++.dg/vect/pr33426-ivdep-4.cc:13:19: note: loop vectorized
g++.dg/vect/pr33426-ivdep-4.cc:13:19: note: loop versioned for vectorization
because of possible aliasing
g++.dg/vect/pr33426-ivdep-4.cc:13:19: note: loop peeled for vectorization to
enhance alignment


The function is simply:

#include <vector>

template<class T, class T2>
void Loop(T *b, T2 c) {
#pragma GCC ivdep
  for (auto &i : *b) {
    i *= *c;
  }
}

void foo(std::vector<int> *ar, int *b) {
 Loop<std::vector<int>, int*>(ar, b);
}