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<p class="MsoNormal">The primary reason I don’t want to provide any guarantees for what instructions are used to implement volatile memcpy is that it would forbid lowering a volatile memcpy to a library call.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">clang uses a volatile memcpy for struct assignment in C. For example, “void f(volatile struct S*p) { p[0] = p[1]; }”. It’s not really that useful, but it’s been done that way since before clang was written.<o:p></o:p></p>
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<p class="MsoNormal">-Eli<o:p></o:p></p>
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<p class="MsoNormal"><b>From:</b> Guillaume Chatelet <gchatelet@google.com> <br>
<b>Sent:</b> Thursday, June 6, 2019 2:40 AM<br>
<b>To:</b> Tim Northover <t.p.northover@gmail.com><br>
<b>Cc:</b> Eli Friedman <efriedma@quicinc.com>; llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [EXT] Re: [llvm-dev] @llvm.memcpy not honoring volatile?<o:p></o:p></p>
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<p class="MsoNormal">Thx for the explanation Eli and Tim.<o:p></o:p></p>
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<p class="MsoNormal">My understanding of volatile was that you may have a different value every time you read and as such overlapping reads may be a bug.<o:p></o:p></p>
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<p class="MsoNormal">Now, since the behaviour of volatile memcpy is not guaranteed and since
<a href="https://godbolt.org/z/CnCOLc">clang does not allow to use it anyways</a> I would like to challenge its existence.<o:p></o:p></p>
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<p class="MsoNormal">Is there a know reason for keeping the volatile argument in @llvm.memcpy?<o:p></o:p></p>
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<p class="MsoNormal">On Wed, Jun 5, 2019 at 11:28 PM Tim Northover <<a href="mailto:t.p.northover@gmail.com">t.p.northover@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Wed, 5 Jun 2019 at 13:49, Eli Friedman via llvm-dev<br>
<<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br>
> I don’t see any particular reason to guarantee that a volatile memcpy will access each byte exactly once. How is that useful?<br>
<br>
I agree it's probably not that useful, but I think the non-duplicating<br>
property of volatile is ingrained strongly enough that viewing a<br>
memcpy as a single load and store to each unit (in an unspecified<br>
order) should be legitimate; so I think this actually is a bug.<br>
<br>
As the documentation says though, it's unwise to depend on the<br>
behaviour of a volatile memcpy.<br>
<br>
Cheers.<br>
<br>
Tim.<o:p></o:p></p>
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