<html><head><style type='text/css'>p { margin: 0; }</style></head><body><div style='font-family: arial,helvetica,sans-serif; font-size: 10pt; color: #000000'><br><hr id="zwchr"><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><b>From: </b>"Hal Finkel via cfe-dev" <cfe-dev@lists.llvm.org><br><b>To: </b>"Richard Smith" <richard@metafoo.co.uk><br><b>Cc: </b>"Clang Dev" <cfe-dev@lists.llvm.org>, "cfe-commits" <cfe-commits@lists.llvm.org><br><b>Sent: </b>Wednesday, June 29, 2016 4:17:18 PM<br><b>Subject: </b>Re: [cfe-dev] RFC: Default language standard mode policy<br><br>
<style>p { margin: 0; }</style><div id="DWT15851" style="font-family: arial,helvetica,sans-serif; font-size: 10pt; color: rgb(0, 0, 0);"><br><hr id="zwchr"><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><b>From: </b>"Richard Smith" <richard@metafoo.co.uk><br><b>To: </b>"Hal Finkel" <hfinkel@anl.gov><br><b>Cc: </b>"Clang Dev" <cfe-dev@lists.llvm.org>, "cfe-commits" <cfe-commits@lists.llvm.org><br><b>Sent: </b>Wednesday, June 29, 2016 3:01:46 PM<br><b>Subject: </b>Re: [cfe-dev] RFC: Default language standard mode policy<br><br><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Wed, Jun 29, 2016 at 12:55 PM, Hal Finkel via cfe-dev <span dir="ltr"><<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div><div style="font-family: arial,helvetica,sans-serif; font-size: 10pt; color: rgb(0, 0, 0);"><br><hr><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><b>From: </b>"Richard Smith via cfe-commits" <<a href="mailto:cfe-commits@lists.llvm.org" target="_blank">cfe-commits@lists.llvm.org</a>><br><b>To: </b>"cfe-commits" <<a href="mailto:cfe-commits@lists.llvm.org" target="_blank">cfe-commits@lists.llvm.org</a>>, "Clang Dev" <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>><br><b>Sent: </b>Wednesday, June 29, 2016 2:09:37 PM<br><b>Subject: </b>RFC: Default language standard mode policy<span class=""><br><br><div dir="ltr"><div>Hi all!<br></div><div><br></div><div>I'd like to establish a policy for Clang's default language standard (if none is specified with -std), as follows:</div><div><br></div><div> Clang defaults to the most recent published standard for the selected language that it fully implements.</div><div><br></div><div>The practical impact of this is that clang++ will default to C++14 for C++ compilations (for version 3.9 onwards) and will default to C++17 once our implementation support is complete and the standard is published (whichever happens later).</div></div></span></blockquote>I think that we need to include libc++ in this criteria as well. I think we'll also need some CMake flags to adjust the default for builds for systems on which this won't work.</div></div></blockquote><div><br></div><div>Right, it doesn't make sense to change our default in a way that breaks use of the same version of libc++, or a supported version of libstdc++ (and we should establish how old a version of libstdc++ we support here).<br></div><div><br></div><div id="DWT13803">However, I don't immediately see that we need to wait for libc++ to be feature-complete before we enable the new standard in Clang. If that's what you're suggesting, can you expand on why? We already have the SD-6 feature test macros to test for implementation of specific features.</div></div></div></div></blockquote>I think it is completely reasonable for an application to test __cplusplus for both language and library features. There are plenty of reasons why this won't always work on all systems, but I think we should endeavor to make this possible. I think that the SD-6 macros are useful is many circumstances, but I think it is unreasonable to expect users to detect all of the features they're using separately using SD-6 macros.<br><br>We can't control what libstdc++ and others do, but if we can't even provide a complete implementation (including the library), I don't think we should switch the default.<br><br><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div></div><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div><div style="font-family: arial,helvetica,sans-serif; font-size: 10pt; color: rgb(0, 0, 0);"><span class=""><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><div dir="ltr"><div>I'd suggest that we apply the same policy for clang-cl, but if it's important that we enable a not-yet-fully-implemented standard for cl compatibility, that seems reasonable.</div><div><br></div><div>The question of whether the default mode for the GCC-compatible driver should be -std=gnuXXX or -std=cXXX is separate, but also likely worth discussing. Enabling GNU keywords by default is a very odd choice, and if we believe we can change our defaults without breaking the world then this seems like a good time to do so.</div></div></blockquote></span>Unfortunately, on many systems, some standard system headers won't even parse without GNU extensions enabled. I think we'll need to leave the GNU extensions on by default (at least for parsing system headers).</div></div></blockquote><div><br></div><div id="DWT14633">Can you give an example? -std=c++11 works fine on a broad range of systems. Note that this is not about GNU *extensions*, which are enabled in both modes; it's about GNU *keywords* (and a small number of non-conforming extensions) -- in particular, the 'typeof' GNU keyword, and support for the asm keyword in C and the inline keyword in C89 (without __decoration__).<br></div></div></div></div></blockquote>I recall having problems with this with fenv.h on various systems. They might just be older systems, however. We should check the current state of the world.<br></div></blockquote>A quick data point:<br><br>$ rpm -q --whatprovides /usr/include/aio.h<br>glibc-headers-2.12-1.166.el6_7.7.ppc64<br>$ clang -fsyntax-only -std=c11 -w /usr/include/aio.h<br>/usr/include/aio.h:42:19: error: field has incomplete type 'struct sigevent'<br> struct sigevent aio_sigevent; /* Signal number and value. */<br> ^<br>/usr/include/aio.h:42:10: note: forward declaration of 'struct sigevent'<br> struct sigevent aio_sigevent; /* Signal number and value. */<br> ^<br>1 error generated.<br>$ clang -fsyntax-only -std=gnu11 -w /usr/include/aio.h<br>(works)<br><br>$ rpm -q --whatprovides /usr/include/aio.h <br>glibc-headers-2.17-106.el7_2.4.x86_64<br>$ clang -fsyntax-only -std=c11 -w /usr/include/aio.h<br>
(works)<br>$ clang -fsyntax-only -std=gnu11 -w /usr/include/aio.h<br>
(works)<br><br>So I might have problems with a c11 default on my older systems, but hopefully not on the newer ones.<br><br> -Hal<br><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><div style="font-family: arial,helvetica,sans-serif; font-size: 10pt; color: rgb(0, 0, 0);"><br>Thanks again,<br>Hal<br><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div></div><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div><div style="font-family: arial,helvetica,sans-serif; font-size: 10pt; color: rgb(0, 0, 0);"><span class=""><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><div dir="ltr"><div>I also intend to make explicit in our documentation that our -std=XXX flag enables the selected standard, *plus all relevant issues in Defect Report status from the relevant language committee* (it doesn't make sense to support a language without its bugfixes).</div></div></blockquote></span>+1<br><br> -Hal<span class=""><br><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px; color: rgb(0, 0, 0); font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><div dir="ltr"><div></div><div><br></div><div>Thoughts?</div></div>
<br>_______________________________________________<br>cfe-commits mailing list<br><a href="mailto:cfe-commits@lists.llvm.org" target="_blank">cfe-commits@lists.llvm.org</a><br><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits</a><br></blockquote><br><br><br></span><span class="HOEnZb"><font color="#888888">-- <br><div><span></span>Hal Finkel<br>Assistant Computational Scientist<br>Leadership Computing Facility<br>Argonne National Laboratory<span></span><br></div></font></span></div></div><br>_______________________________________________<br>
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</blockquote><br><br><br>-- <br><div><span></span>Hal Finkel<br>Assistant Computational Scientist<br>Leadership Computing Facility<br>Argonne National Laboratory<span></span><br></div></div><br>_______________________________________________<br>cfe-dev mailing list<br>cfe-dev@lists.llvm.org<br>http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev<br></blockquote><br><br><br>-- <br><div><span name="x"></span>Hal Finkel<br>Assistant Computational Scientist<br>Leadership Computing Facility<br>Argonne National Laboratory<span name="x"></span><br></div></div></body></html>