<html>
    <head>
      <base href="https://bugs.llvm.org/">
    </head>
    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - Missed optimization: useless for-loop must be eliminated"
   href="https://bugs.llvm.org/show_bug.cgi?id=39515">39515</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Missed optimization: useless for-loop must be eliminated
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>clang
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>trunk
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>PC
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Linux
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>enhancement
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>C++
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>zamazan4ik@tut.by
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>dgregor@apple.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
          </td>
        </tr></table>
      <p>
        <div>
        <pre>clang(trunk) with '-O3 -std=c++17' for this code:

#include <vector>
#include <algorithm>

int foo(std::vector<int> v) {
    int l = v[0];
    for(const auto& x : v) {
        l = std::min(l, x);
    }

    for(const auto& x : v) {
        l = std::max(l, x);
    }

    return l;
}

gcc doesn't eliminate first loop, but gcc can, because first loop has no effect
in this function.

Same result for the version without std::vector and std::max:

int min(int a, int b)
{
    return a < b ? a : b;
}

int max(int a, int b)
{
    return a > b ? a : b;
}

int foo(int* v, int size) {
    int l = v[0];
    for(int i=0; i < size; ++i)
    {
        l = min(l, v[i]);
    }

    for(int i=0; i < size; ++i)
    {
        l = max(l, v[i]);
    }

    return l;
}</pre>
        </div>
      </p>


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