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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.
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<div>Thanks,</div>
<div>Schwieb</div>
<div>Microsoft Mac Team<br>
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<div>On Jul 23, 2014, at 9:48 AM, jahanian <<a href="mailto:fjahanian@apple.com">fjahanian@apple.com</a>> wrote:</div>
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On Jul 23, 2014, at 9:45 AM, jahanian <<a href="mailto:fjahanian@apple.com">fjahanian@apple.com</a>> wrote:</div>
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<div>On Jul 22, 2014, at 5:26 PM, Reid Kleckner <<a href="mailto:rnk@google.com">rnk@google.com</a>> wrote:</div>
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<div>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.</div>
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<div>Also, Clang does respect alignas in this case when targeting the Microsoft C++ ABI.</div>
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<div>I tried the test case with these options for MS ABI:</div>
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<div style="margin: 0px; font-size: 11px; font-family: Menlo;">clang -cc1 -triple x86_64-windows-msvc -std=c++11 -S t.cpp</div>
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<div style="margin: 0px; font-size: 11px; font-family: Menlo;">Assembly indicates that it does not seem to respect alignas. Should I be using other options?</div>
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<div style="margin: 0px;">main:</div>
<div style="margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"></span>movl<span class="Apple-tab-span" style="white-space: pre;">
</span>$28, %eax</div>
<div style="margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"></span>movl<span class="Apple-tab-span" style="white-space: pre;">
</span>$0, -4(%rsp)</div>
<div style="margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"></span>retq</div>
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Oops. I was using an old clang. Sorry for the noise.</div>
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<div style="margin: 0px; font-size: 11px; font-family: Menlo;">main:</div>
<div style="margin: 0px; font-size: 11px; font-family: Menlo;"><span class="Apple-tab-span" style="white-space: pre;"></span>movl<span class="Apple-tab-span" style="white-space: pre;">
</span>$64, %eax</div>
<div style="margin: 0px; font-size: 11px; font-family: Menlo;"><span class="Apple-tab-span" style="white-space: pre;"></span>retq</div>
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<div>- Fariborz</div>
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