<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On 2020 Dec 1, at 14:52, Mehdi AMINI <<a href="mailto:joker.eph@gmail.com" class="">joker.eph@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><br class="Apple-interchange-newline"><br style="caret-color: rgb(0, 0, 0); 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; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><div class="gmail_quote" style="caret-color: rgb(0, 0, 0); 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; -webkit-text-stroke-width: 0px; text-decoration: none;"><div dir="ltr" class="gmail_attr">On Tue, Dec 1, 2020 at 2:39 PM Mehdi AMINI <<a href="mailto:joker.eph@gmail.com" class="">joker.eph@gmail.com</a>> wrote:<br class=""></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" class=""><div dir="ltr" class=""><div dir="ltr" class=""><div dir="ltr" class=""><div dir="ltr" class=""><br class=""></div><br class=""><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" class="">llvm-dev@lists.llvm.org</a>> wrote:<br class=""></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;" class=""><div class=""><blockquote type="cite" class=""><div class=""><div dir="auto" class=""><blockquote type="cite" class=""><div dir="ltr" class=""><div dir="ltr" class=""><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 class=""><div class=""><blockquote type="cite" class=""><div class=""><div class="">On Nov 17, 2020, at 1:42 PM, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank" class="">dblaikie@gmail.com</a>> wrote:<div class=""><blockquote type="cite" class=""><div class=""><div class=""><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;" class="">Thoughts/suggestions:<br class="">- 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 class=""></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;" class=""><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;" class="">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;" class=""></div></div></blockquote></div><br class=""><div class="">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 class=""><br class=""></div><div class="">The performance benefits aren't trivial.</div><div class=""><br class=""></div><div class="">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 class=""><br class=""></div><div class="">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 class=""><div class="">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 class=""><div class="">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 class=""><br class=""></div><div class="">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<span class="Apple-converted-space"> </span><a href="https://stackoverflow.com/questions/61417534/can-stdvectort-use-ts-move-constructor-if-exceptions-are-disabled" target="_blank" class="">https://stackoverflow.com/questions/61417534/can-stdvectort-use-ts-move-constructor-if-exceptions-are-disabled</a><span class="Apple-converted-space"> </span>explains some of it).</div><div class=""><br class=""></div><div class="">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 class=""><br class=""></div><div class="">Does it make sense or did you see it differently with your question?</div><div class=""><br class=""></div></div></div></div></div></div></blockquote><div class=""><br class=""></div><div class="">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" class="">https://reviews.llvm.org/D62228</a></div><div class=""><br class=""></div><div class="">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" class="">https://godbolt.org/z/M54bqn</a></div></div></div></blockquote><br class=""></div><div>Oh great, I didn't think this landed!</div><div><br class=""></div><div>IMO, once all host toolchains we support have the same behaviour it would be safe to start using std::vector again (doing it sooner could give wildly divergent performance characteristics depending on what LLVM was built with).</div></body></html>