<div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Dec 1, 2020 at 2:39 PM Mehdi AMINI <<a href="mailto:joker.eph@gmail.com">joker.eph@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Dec 1, 2020 at 2:19 PM Chris Lattner via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word;line-break:after-white-space"><div><blockquote type="cite"><div><div dir="auto"><blockquote type="cite"><div dir="ltr"><div dir="ltr"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div><div><blockquote type="cite"><div><div>On Nov 17, 2020, at 1:42 PM, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>> wrote:<div><blockquote type="cite"><div><div><blockquote type="cite" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">Thoughts/suggestions:<br>- Adding the default seems very reasonable to me, and I think that 64 bytes is a good default. I think you should change the behavior so that SmallVector<LargeThing> defaults to a single inline element instead of zero though. Perhaps generate a static_assert when it is crazy large.<br></blockquote><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline">Out of curiosity: Why a single rather than zero?</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"></div></div></blockquote></div><br><div>My rationale for this is basically that SmallVector is typically used for the case when you want to avoid an out-of-line allocation for a small number of elements, this was the reason it was created. While there is some performance benefits of SmallVector<T,0> over std::vector<> they are almost trivial.</div></div></div></blockquote><div><br></div><div>The performance benefits aren't trivial.</div><div><br></div><div>std::vector grow operations will refuse to use std::move for some T, a pessimization required by its exception guarantees, even if you're building with `-fno-exceptions`. We had a massive compile-time problem in 2016 related to this that I fixed with 3c406c2da52302eb5cced431373f240b9c037841 by switching to SmallVector<T,0>. You can see the history in r338071 / 0f81faed05c3c7c1fbaf6af402411c99d715cf56.</div></div></div></blockquote><div><br></div><div>That issue, at least, is fixable without switching from std::vector just by adding noexcept to the appropriate user-defined move constructors.</div></div></div>
</div></blockquote><br><div>Sure, once we’ve added noexcept to all types in LLVM/Clang/etc. That’s a pretty long tail though; a lot of work for relatively little gain given that we don’t care about exceptions anyway and we have an optimized vector implementation in tree. </div></div></div></blockquote></div><br><div>Can you spell this out for me? Why do we need noexcept if we’re building with -fno-exceptions? What is going on here?</div></div></blockquote><div><br></div><div>It isn't clear to me that -fno-exceptions can get the same benefit as noexcept for code that is written with exceptions in mind (I think <a href="https://stackoverflow.com/questions/61417534/can-stdvectort-use-ts-move-constructor-if-exceptions-are-disabled" target="_blank">https://stackoverflow.com/questions/61417534/can-stdvectort-use-ts-move-constructor-if-exceptions-are-disabled</a> explains some of it).</div><div><br></div><div>While it is possible to write a library that optimizes for -fno-exceptions, I am not sure it would be standard-compliant for the STL to do so (and it'd be anyway another code path that the one written in term of `std::move_if_noexcept`).</div><div><br></div><div>Does it make sense or did you see it differently with your question?</div><div><br></div></div></div></div></div></div></blockquote><div><br></div><div>After digging a bit, I found that libcxx was changed last year to optimize with moving in std::vector when -fno-exceptions is present : <a href="https://reviews.llvm.org/D62228">https://reviews.llvm.org/D62228</a></div><div><br></div><div>That means that a copy-only class can't be used in vector anymore in clang-10 but it was possible in clang-9 (but only with -fno-exceptions): <a href="https://godbolt.org/z/M54bqn">https://godbolt.org/z/M54bqn</a></div><div><br></div><div>-- </div><div>Mehdi</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">
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