<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type content="text/html; charset=utf-8"><meta name=Generator content="Microsoft Word 15 (filtered medium)"><style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:"Book Antiqua";
panose-1:2 4 6 2 5 3 5 3 3 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-reply;
font-family:"Book Antiqua",serif;
color:#943634;
font-weight:normal;
font-style:normal;
text-decoration:none none;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;
mso-fareast-language:EN-US;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 72.0pt 72.0pt 72.0pt;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]--></head><body lang=EN-IE link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'>Hi Eric,<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'>In the email I sent on the 27<sup>th</sup> April (“[LibC++] Bug in implementation of 'std::shared_ptr'”), the problem I was seeing was:<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'> o LibC++ library built with -DLIBCXX_ENABLE_RTTI=ON and -D_LIBCPP_BUILD_STATIC.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'> o User code compiled with ‘-fno-rtti -D_LIBCPP_BUILD_STATIC’.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'>Having ‘_LIBCPP_BUILD_STATIC’ defined by both the library builder and the library user was where it broke. I removed ‘-D_LIBCPP_BUILD_STATIC’ from the user and it worked. But I wasn’t building the static library with RTTI disabled because it is stated as an unsupported configuration.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'>So are you advising that with the proposed fix, that I should revert to defining ‘_LIBCPP_BUILD_STATIC’ as the user?<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'>Thanks,<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'> MartinO<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'> Eric Fiselier [mailto:eric@efcs.ca] <br><b>Sent:</b> 04 May 2017 00:43<br><b>To:</b> clang developer list <cfe-dev@lists.llvm.org><br><b>Cc:</b> Marshall Clow <mclow.lists@gmail.com>; Duncan P. N. Exon Smith <dexonsmith@apple.com>; Ed Maste <emaste@freebsd.org>; Martin J. O'Riordan <martin.oriordan@movidius.com>; Anton Korobeynikov <anton@korobeynikov.info><br><b>Subject:</b> [RFC] Possible ABI break for libc++ built with -fno-rtti and -D_LIBCPP_BUILD_STATIC<o:p></o:p></span></p><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>Hi All,<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Libc++ currently has an ABI incompatibility when built as a static library with no RTTI. Specifically some std::shared_ptr internals have different vtable layouts in this configuration.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I would like to fix this by making the vtable definition consistent across all configurations, but that requires breaking the ABI of a very rare configuration of libc++. I suspect all users of this configuration can take the ABI break immediately, but I would like to confirm this before proceeding.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>The configuration in question is a static libc++ library built with -DLIBCXX_ENABLE_RTTI=OFF and -D_LIBCPP_BUILD_STATIC,<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Is anybody using this configuration and can't take an ABI break?<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Note that `_LIBCPP_BUILD_STATIC` must be manually defined by the user at both when building and using libc++, and it is not supported by the CMake build system. This should mean that almost nobody is defining it.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>The proposed changes can be found here: <a href="https://reviews.llvm.org/D32838">https://reviews.llvm.org/D32838</a><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>/Eric<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div></div></div></body></html>