http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57380
Bug ID: 57380 Summary: GCC 4.9.0 will not vectorize std::max and similar functions Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: jewillco at osl dot iu.edu It appears that having a function that returns a const reference to one of its arguments causes vectorization of calls to that function to fail. Here is a simple test program: struct my_array { int data[4] __attribute__((aligned(16))); }; const int& my_max(const int& a, const int& b) { return a < b ? b : a; } int f(my_array a, my_array b) { int res = 0; for (int i = 0; i < 4; ++i) { res += my_max(a.data[i], b.data[i]); } return res; } The signature of my_max is a specialization of the one of std::max; std::max itself has similar problems. The loop will vectorize without trouble if my_max returns "int". The main errors from the vectorization report seem to be: vec_min_max.cpp:11: note: not vectorized: not suitable for gather load _6 = *iftmp.0_12; vec_min_max.cpp:11: note: bad data references. Other variants of the code get "control flow in loop" instead. The flags I am using are: -ftree-vectorizer-verbose=4 -Ofast -march=nocona but the code should be able to vectorize under SSE2. The GCC version I am using is "g++ (GCC) 4.9.0 20130519 (experimental)" on x86-64. GCC 4.7.2 has a similar error, while "4.4.7 20120313 (Red Hat 4.4.7-3)" can vectorize it without problems.