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From Chris Olivier <cjolivie...@apache.org>
Subject Re: [DISCUSS] Use modernized C++11 range loops uniformly throughout the project
Date Fri, 28 Sep 2018 13:53:57 GMT
-1

Range loops aren’t always the most performant way. In addition, sometimes
you want the index. Or maybe you want to iterate backwards, or not start
from the first, etc. Maybe you want the iterator because you remove it from
the list at the bottom of the loop.... Seems like a rule for the sake of
having a rule.

On Fri, Sep 28, 2018 at 2:12 AM kellen sunderland <
kellen.sunderland@gmail.com> wrote:

> Hello MXNet devs,
>
> I'd like to discuss uniformly adopting C++11 range loops in the MXNet
> project.  The benefits I see are:
>
> *  Improved C++ readability (examples below).
> *  Consistency with other languages.  The range-loops are quite similar to
> loops almost all other programming languages.  Given we're a project that
> supports many languages this language consistency could be positive for our
> community.
> * Consistency within the same project.  Currently different authors have
> different loops styles which hurts codebase readability.
> *  Best available performance.  There are often multiple ways to write
> loops in C++ with subtle differences in performance and memory usage
> between loop methods.  Using range-loops ensures we get the best possible
> perf using an intuitive loop pattern.
> *  Slightly lower chance for bugs / OOB accesses when dealing with indexing
> in an array for example.
>
> If we decide to enable this uniformly throughout the project we can enable
> this policy with a simple clang-tidy configuration change.  There would be
> no need for reviewers to have to manually provide feedback when someone
> uses an older C++ loops style.
>
> -Kellen
>
> Reference PR:  https://github.com/apache/incubator-mxnet/pull/12356/
> Previous clang-tidy discussion on the list:
>
> https://lists.apache.org/thread.html/b0ae5a9df5dfe0d9074cb2ebe432264db4fa2175b89fa43a5f6e36be@%3Cdev.mxnet.apache.org%3E
>
> -------------------------
> Examples:
> for (auto axis_iter = param.axis.begin() ; axis_iter!= param.axis.end();
> ++axis_iter) {
>     CHECK_LT(*axis_iter, static_cast<int>(ishape.ndim()));
>     stride_[reverse_index] = ishape[*axis_iter];
>     ...
> -->
> for (int axis : param.axis) {
>     CHECK_LT(axis, static_cast<int>(ishape.ndim()));
>     stride_[reverse_index] = ishape[axis];
>     ...
> --------------------------
> for (size_t i = 0; i < in_array.size(); i++) {
>     auto &nd = in_array[i];
>     pre_temp_buf_.emplace_back(nd.shape(), nd.ctx(), true, nd.dtype());
> }
> -->
> for (auto & nd : in_array) {
>     pre_temp_buf_.emplace_back(nd.shape(), nd.ctx(), true, nd.dtype());
> }
>

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