Bug ID: 81719
           Summary: Range-based for loop on short fixed size array
                    generates long unrolled loop
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot
          Reporter: jzwinck at gmail dot com
  Target Milestone: ---

C++11 range-based for loops over arrays of size known at compile time result in
 bloated, branchy, and unreachable code with -O3 optimization.  For example:

    typedef int Items[2];

    struct ItemArray
        Items items;
        int sum_x2() const;

    int ItemArray::sum_x2() const
        int total = 0;
        for (int item : items)
            total += item;
        return total;

Clang compiles the above to [mov, add, ret].  GCC with -O2 compiles it to a few
more than that, and with -O3, a whopping 81 instructions.  Add -march=haswell
and behold about 130 instructions to add two ints.

GCC (all versions, 4 to 7) generates code to handle a variable-sized array up
to about 6 to 14 elements, depending on -march.  The number of elements is
known at compile time to be 2 (other small values also elicit the bug).  GCC
should generate three instructions in both -O2 and -O3.  It actually does, if
sum_x2() is a free function instead of a member function.  The problem also
goes away if you use a C-style loop.

There are lots of permutations of this, including using a range-based for loop
to assign a common value to every element of an array whose size is known at
compile time (120 instructions to assign a single int:

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