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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Sounds good to me. What’s the best way to track this? Should I file a bug somewhere? (Usually I work directly with Fariborz and Argyrios and other Apple
folks in Radar, but this isn’t for an Apple-specific issue so it should go towards the open clang community instead…)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Schwieb<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> metafoo@gmail.com [mailto:metafoo@gmail.com]
<b>On Behalf Of </b>Richard Smith<br>
<b>Sent:</b> Wednesday, July 23, 2014 12:49 PM<br>
<b>To:</b> Erik Schwiebert<br>
<b>Cc:</b> jahanian; Reid Kleckner; cfe-dev Developers; Argyrios Kyrtzidis<br>
<b>Subject:</b> Re: [cfe-dev] c++11's alignas() ignored inside #pragma pack<o:p></o:p></span></p>
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<p class="MsoNormal">On Wed, Jul 23, 2014 at 10:18 AM, Erik Schwiebert <<a href="mailto:eriksc@microsoft.com" target="_blank">eriksc@microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Would it be possible to have clang support the alignas inside a pragma pack directive, if the structure in question is also inside a pragma ms_struct directive? There are some teams at Microsoft using clang to target non-Windows platforms
but sharing code that expects MSVC structure layout. Using the Microsoft C++ ABI isn’t feasible as the platform in question is not a Microsoft Windows platform. <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Our documentation says:<o:p></o:p></p>
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<p class="MsoNormal">"clang supports the Microsoft #pragma pack feature for controlling record layout. GCC also contains support for this feature, however where MSVC and GCC are incompatible clang follows the MSVC definition."<o:p></o:p></p>
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<p class="MsoNormal">... so I think we should do the obviously sane thing here in all cases (that is, alignas on a field should beat #pragma pack).<o:p></o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Schwieb<o:p></o:p></p>
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<p class="MsoNormal">Microsoft Mac Team<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Jul 23, 2014, at 9:48 AM, jahanian <<a href="mailto:fjahanian@apple.com" target="_blank">fjahanian@apple.com</a>> wrote:<o:p></o:p></p>
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On Jul 23, 2014, at 9:45 AM, jahanian <<a href="mailto:fjahanian@apple.com" target="_blank">fjahanian@apple.com</a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif"">On Jul 22, 2014, at 5:26 PM, Reid Kleckner <<a href="mailto:rnk@google.com" target="_blank">rnk@google.com</a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif"">Presumably they handle alignas the same way they handle __declspec(align(N)), which overrides packing. MSVC appears to have two notions of alignment, required and advisory,
while the C++ standard, Clang, and GCC only have one. One way to observe this is to see that they will align an int64 to 8 bytes in a struct, but they will not realign the stack to 8 bytes when that struct is stack allocated.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif"">Also, Clang does respect alignas in this case when targeting the Microsoft C++ ABI.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif"">I tried the test case with these options for MS ABI:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:8.5pt;font-family:"Menlo","serif"">clang -cc1 -triple x86_64-windows-msvc -std=c++11 -S t.cpp<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:8.5pt;font-family:"Menlo","serif"">Assembly indicates that it does not seem to respect alignas. Should I be using other options?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:8.5pt;font-family:"Menlo","serif"">main:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:8.5pt;font-family:"Menlo","serif"">movl $28, %eax<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:8.5pt;font-family:"Menlo","serif"">movl $0, -4(%rsp)<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:8.5pt;font-family:"Menlo","serif"">retq<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif"">Oops. I was using an old clang. Sorry for the noise.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:8.5pt;font-family:"Menlo","serif"">movl $64, %eax<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:8.5pt;font-family:"Menlo","serif"">retq<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:8.5pt;font-family:"Menlo","serif"">- Fariborz<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif"">_______________________________________________<br>
cfe-dev mailing list<br>
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</span><a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev" target="_blank"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif"">http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev</span></a><o:p></o:p></p>
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