[Bug tree-optimization/78151] New: Fail to vectorize *min_element

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78151

            Bug ID: 78151
           Summary: Fail to vectorize *min_element
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

(from a discussion in the comments of
http://stackoverflow.com/a/40284138/1918193)

#include <vector>
#include <algorithm>

int return_min(std::vector<int> &v) {
  return *std::min_element(v.begin(), v.end());
}

Normally, min_element returns a position, which makes it hard to optimize. In
this case, we notice quite nicely that only the value is needed and not the
position. However, this only happens in PRE. We then wait until phiopt3 to
recognize a MIN_EXPR. And that's too late for the vectorizer :-(