<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div dir="ltr"><br></div><div dir="ltr"><br><blockquote type="cite">On Nov 30, 2020, at 17:57, James Y Knight <jyknight@google.com> wrote:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr"><div dir="ltr"><div dir="ltr"></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Nov 30, 2020 at 8:44 PM Duncan P. N. Exon Smith via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><br><div><br><blockquote type="cite"><div>On 2020 Nov 27, at 20:45, Chris Lattner via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:</div><br><div><div style="overflow-wrap: break-word;"><div>Sorry for falling off the map on this thread:</div><div><br></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></body></html>