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Subject: [Bug c++/94695] Implement -Wrange-loop-analysis
Date: Tue, 29 Sep 2020 23:03:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94695

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:969baf03acd8124345617cea125b148568c7370a

commit r11-3539-g969baf03acd8124345617cea125b148568c7370a
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Sep 24 14:30:50 2020 -0400

    c++: Implement -Wrange-loop-construct [PR94695]

    This new warning can be used to prevent expensive copies inside range-b=
ased
    for-loops, for instance:

      struct S { char arr[128]; };
      void fn () {
        S arr[5];
        for (const auto x : arr) {  }
      }

    where auto deduces to S and then we copy the big S in every iteration.
    Using "const auto &x" would not incur such a copy.  With this patch the
    compiler will warn:

    q.C:4:19: warning: loop variable 'x' creates a copy from type 'const S'
[-Wrange-loop-construct]
        4 |   for (const auto x : arr) {  }
          |                   ^
    q.C:4:19: note: use reference type 'const S&' to prevent copying
        4 |   for (const auto x : arr) {  }
          |                   ^
          |                   &

    As per Clang, this warning is suppressed for trivially copyable types
    whose size does not exceed 64B.  The tricky part of the patch was how
    to figure out if using a reference would have prevented a copy.  To
    that point, I'm using the new function called ref_conv_binds_directly_p.

    This warning is enabled by -Wall.  Further warnings of similar nature
    should follow soon.

    gcc/c-family/ChangeLog:

            PR c++/94695
            * c.opt (Wrange-loop-construct): New option.

    gcc/cp/ChangeLog:

            PR c++/94695
            * call.c (ref_conv_binds_directly_p): New function.
            * cp-tree.h (ref_conv_binds_directly_p): Declare.
            * parser.c (warn_for_range_copy): New function.
            (cp_convert_range_for): Call it.

    gcc/ChangeLog:

            PR c++/94695
            * doc/invoke.texi: Document -Wrange-loop-construct.

    gcc/testsuite/ChangeLog:

            PR c++/94695
            * g++.dg/warn/Wrange-loop-construct.C: New test.=