[Webkit-unassigned] [Bug 128644] New: Remove some unintended copies due in ranged for loops

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=128644

           Summary: Remove some unintended copies due in ranged for loops
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKit Misc.
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: bfulgham at webkit.org


Once I noticed the handful of errors in Bug 128578, I did some more searching and turned up a few more.

Per http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2049.pdf, the ranged iterator syntax:

    for( type-specifier-seq simple-declarator : expression )
        statement

is syntactically equivalent to

    typedef decltype(expression) C;
    auto&& rng(expression);
    for (auto begin(std::For<C>::begin(rng)), end(std::For<C>::end(rng)); begin != end; ++ begin) {
        type-specifier-seq simple-declarator(∗begin);
        statement
    }

The issue here is that the type of auto (instead of 'auto&' or 'const auto&') causes us to make a copy of each element as we pass through the loop.

When the container is just a  set of pointers, it's not much of an issue. But when we have iterate over reference-counted types, strings, or other larger objects we incur unnecessary costs.

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