[llvm-bugs] [Bug 39515] New: Missed optimization: useless for-loop must be eliminated

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39515

            Bug ID: 39515
           Summary: Missed optimization: useless for-loop must be
                    eliminated
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zamazan4ik at tut.by
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

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;
}

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