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From Naveen Swamy <mnnav...@gmail.com>
Subject Re: [DISCUSS] Use modernized C++11 range loops uniformly throughout the project
Date Sun, 30 Sep 2018 02:02:23 GMT
Thanks Kellen & Anton, for your detailed explanation and links to
advantages, appreciate it.
changing my vote to *-0*, I suggest to show as warnings.

On Sat, Sep 29, 2018 at 8:06 PM Anton Chernov <mechernov@gmail.com> wrote:

> And if you want a more authoritative opinion on that check out what the C++
> core guidelines are saying [1]:
>
> > ES.71: Prefer a range-for-statement to a for-statement when there is a
> choice
> > Reason
> > Readability. Error prevention. Efficiency.
>
> Best regards
> Anton
>
> [1]
>
> https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Res-for-range
>
>
> сб, 29 сент. 2018 г. в 16:13, Anton Chernov <mechernov@gmail.com>:
>
> > +1
> >
> > Maybe it's not necessary to enforce usage of range-based for, but I would
> > highly encourage to to it due to already named advantages. If code would
> be
> > introduced using the old-style there could be a comment suggesting the
> new
> > way. But why do the manual work and not leave that to the automated tool?
> >
> > And since it's already automated - wouldn't it be better to keep a
> unified
> > modern style?
> >
> > Just to make this a trend - C++ evolves quickly and this will not be only
> > upgrade that would needed to be made. And the easier such upgrades get
> > accepted the easier in general is to upgrade the codebase.
> >
> > Soon the standard will get ranges and concepts and this will change the
> > way C++ applications get written significantly. It is a good habit to be
> > open for changes and keep up with the trends. By using the new
> > possibilities the language can offer you prepare yourself for further
> > changes and are more likely to accept them, evolving your programming
> style.
> >
> > Take a look at a new examples on modern usages (taken from [1]):
> >
> > // since C++17
> > for (auto&& [first,second] : mymap) {
> >     // use first and second
> > }
> >
> > // since C++20
> > for (auto& x : foo().items()) { /* .. */ } // undefined behavior if foo()
> > returns by value
> > for (T thing = foo(); auto& x : thing.items()) { /* ... */ } // OK
> >
> > // since C++11
> > struct cow_string { /* ... */ };
> > // a copy-on-write string cow_string str = /* ... */;
> > // for(auto x : str) { /* ... */ } // may cause deep copy
> > for(auto x : std::as_const(str)) { /* ... */ }
> >
> > Regarding performance: it's really easy to prove that generated assembly
> > is not changing at all. There is a really handy tool for that [2]. You
> can
> > check online the assembly for different language constructs and different
> > compilers.
> >
> > Best regards,
> > Anton
> >
> > [1] https://en.cppreference.com/w/cpp/language/range-for
> > [2] https://gcc.godbolt.org
> >
> > сб, 29 сент. 2018 г. в 13:15, kellen sunderland <
> > kellen.sunderland@gmail.com>:
> >
> >> It's more readable because it's concise and it's consistent for many
> types
> >> you're looping over (i.e. primitive arrays, stl iterators, etc all work
> >> the
> >> same way).  It's also useful because it's consistent with other
> >> programming
> >> languages, making C++ codebases much easier to read for novice and
> >> intermediate developers.  IMO it also leads to better naming in loop
> >> bodies
> >> as the concise style means you're less likely to have important 1 letter
> >> variable names describing loop elements (e.g. no int i =0 or it ...).
> >> More
> >> motivation can be found in the cpp standards proposals for C++11
> >> http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1868.html and
> >> http://open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3853.htm.
> >>
> >>
> >>
> >> On Sat, Sep 29, 2018 at 6:38 PM Naveen Swamy <mnnaveen@gmail.com>
> wrote:
> >>
> >> > Kellen,
> >> >
> >> > Could you please explain why you think range loops are better and how
> it
> >> > improves readability?  this is a relatively new feature, many of them
> >> are
> >> > used to the old syntax, shouldn't we leave it for the developers to
> >> choose
> >> > the one that best suits the need and their familiarity.
> >> > In general I support the notion of standardizing where necessary,
> >> enforcing
> >> > rules on loops seems little bit like micro-managing how you should
> write
> >> > C++ code for MXNet.
> >> >
> >> > -1(open to change based on new information)
> >> >
> >> >
> >> >
> >> > On Fri, Sep 28, 2018 at 5:20 PM Chris Olivier <cjolivier01@gmail.com>
> >> > wrote:
> >> >
> >> > > ok then, my vote is still -1, however, because it’s just adding
> >> needless
> >> > > friction for developers imho.
> >> > >
> >> > > On Fri, Sep 28, 2018 at 7:42 AM kellen sunderland <
> >> > > kellen.sunderland@gmail.com> wrote:
> >> > >
> >> > > > "Range loops aren’t always the most performant way" Do you
have an
> >> > > example
> >> > > > where there's a perf difference?
> >> > > >
> >> > > > "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."
> >> > > >
> >> > > > I should have been more clear about this point.  If you're using
> the
> >> > > index
> >> > > > in the loop, doing reverse iteration, or not iterating from
> >> > start-to-end
> >> > > > this inspection is smart enough to realize it and will not suggest
> >> > > > optimizing that type of loop.  The loops that would be changes
are
> >> > _only_
> >> > > > the loops which are detected as equivalent to range-loops.
> Examples
> >> > can
> >> > > be
> >> > > > found here:
> >> > > >
> >> > >
> >> >
> >>
> https://clang.llvm.org/extra/clang-tidy/checks/modernize-loop-convert.html
> >> > > > or you can look at what's been changed in the ref PR.  I've
> >> initially
> >> > set
> >> > > > our confidence level at 'reasonable' but we could also set to
> 'safe'
> >> > > which
> >> > > > would further reduce the number of loops the check would apply
to.
> >> > > >
> >> > > > -Kellen
> >> > > >
> >> > > > On Fri, Sep 28, 2018 at 3:54 PM Chris Olivier <
> >> cjolivier01@apache.org>
> >> > > > wrote:
> >> > > >
> >> > > > > -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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