<div dir="ltr"><div dir="ltr"><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Aug 27, 2020 at 8:29 AM Robinson, Paul <<a href="mailto:paul.robinson@sony.com">paul.robinson@sony.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal" style="margin-left:0.5in">Once upon a time, C++ said that POD types could be memcpy'd, even if they were used as base classes. That rule got changed retroactively by core language defect reports, and you're no longer allowed to memcpy base
class subobjects (nor [[no_unique_address]] members), but too late for the ABI, so as a consequence of a rule that we're now pretending never existed, the Itanium ABI is careful not to reuse the tail padding of a base class for members of the derived class
if the base class is "POD for the purpose of layout" (which means POD according to some specific old C++ standard version).<u></u><u></u></p>
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<p class="MsoNormal">As an aside, I believe PS4 got caught by something along these lines; some combination of (the rule changed) and (Clang had a bug) was in an unfortunate state right at the time we had to nail down our ABI. We’ve never gotten up the gumption
to post a patch upstream, though.<u></u><u></u></p>
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<p class="MsoNormal">IIUC (which is not super likely but I’ll give it a shot), pair is never trivially copyable, even with trivially copyable members. </p></div></div></blockquote><div><br>Yes, but with nuance: It's true that "is_trivially_copyable" will be false, but that requires both copy ctor and move ctor to be trivially copyable. It's specifically the copy assignment operator that is non-trivial. Pair is trivially copy /constructible/ when its members are.<br> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-US"><div class="gmail-m_-405106319533475460WordSection1"><p class="MsoNormal">Which seems sad. Both pair and tuple have rules that say the destructor is trivial if the members are trivially
destructible; seems like an oversight not to make triviality just as transitive for construct/move/copy. </p></div></div></blockquote><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-US"><div class="gmail-m_-405106319533475460WordSection1"><p class="MsoNormal">Unless that ABI/tail-padding thing is what gets in the way.</p></div></div></blockquote><div><br>If I understand correctly its two layered:<br><br>1) making the copy assignment operator pass through triviality would be an ABI break today (but, hey, we have the libc++ unstable ABI option for this sort of reason, so if that were the only reason - it might be feasible (not sure what the C++ spec would have to say about this - perhaps it would need words to sanction this behavior))<br>2) but if we did that, it would make some objects sizeof larger, which would be bad/not-so-good, unless we had some extra attribute saying "it's POD, but not POD for the purposes of layout", or rather than an attribute, just a whole mode of "I'm building all C++ with this compiler - use the best ABI you can, with no concerns for compatibility with past/future compiler versions, or other compiler vendors", which would also allow the reuse of tail padding & thus the triviality to be pure goodness, rather than an awkward tradeoff<br> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-US"><div class="gmail-m_-405106319533475460WordSection1"><p class="MsoNormal"><u></u><u></u></p>
<p class="MsoNormal">--paulr<u></u><u></u></p>
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<p class="MsoNormal"><b>From:</b> cfe-dev <<a href="mailto:cfe-dev-bounces@lists.llvm.org" target="_blank">cfe-dev-bounces@lists.llvm.org</a>> <b>On Behalf Of
</b>Richard Smith via cfe-dev<br>
<b>Sent:</b> Wednesday, August 26, 2020 9:24 PM<br>
<b>To:</b> David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>><br>
<b>Cc:</b> <a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a><br>
<b>Subject:</b> Re: [cfe-dev] std::pair not trivially copyable?<u></u><u></u></p>
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<p class="MsoNormal">On Wed, 26 Aug 2020 at 11:42, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">On Wed, Aug 26, 2020 at 11:34 AM Richard Smith via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">On Wed, 26 Aug 2020 at 10:18, Nevin Liber via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">On Tue, Aug 25, 2020 at 4:37 PM David Blaikie via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">From what I can tell, reading this ( <a href="https://urldefense.com/v3/__https:/stackoverflow.com/questions/58283694/why-is-pair-of-const-trivially-copyable-but-pair-is-not__;!!JmoZiZGBv3RvKRSx!rMuXOBoosPAUEb_L7l9qA3KHSuisIvxQH9YC3nw5t0zEC7c5K6WEpAqMrB47qTd29A$" target="_blank">https://stackoverflow.com/questions/58283694/why-is-pair-of-const-trivially-copyable-but-pair-is-not</a> )
and the C++17 spec, it just doesn't specify the copy and move assignment operators as defaulted, or say anything about their triviality, so they aren't trivial even for trivial types. Perhaps an oversight when thinking about the other complexities of when
they shuold be deleted. <u></u><u></u></p>
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<p class="MsoNormal">In general, move/copy assignment cannot be defaulted for pair, because assignment of reference types has meaning:<u></u><u></u></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">std::pair<int&, int> p1(i, 5); </span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">p2 = p1; // j now has the value 2</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">A a1(i, 5);</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">a2 = a1; // Compile time error - deleted copy assignment operator</span><u></u><u></u></p>
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<p class="MsoNormal">Now, this doesn't that pair couldn't have trivial copy/move assignment operators when it holds trivially copy/move assignable types, but I don't know how much of an ABI break this would be.<u></u><u></u></p>
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<p class="MsoNormal">Making std::pair be trivially-copyable whenever possible would affect whether std::pair is POD for the purpose of layout, which could result in an ABI break for at least any code that derives from std::pair or that contains a [[no_unique_address]]
std::pair member: <a href="https://urldefense.com/v3/__https:/godbolt.org/z/GTvo4r__;!!JmoZiZGBv3RvKRSx!rMuXOBoosPAUEb_L7l9qA3KHSuisIvxQH9YC3nw5t0zEC7c5K6WEpAqMrB6_XykF_w$" target="_blank">
https://godbolt.org/z/GTvo4r</a><u></u><u></u></p>
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<p class="MsoNormal">This is certainly not something we could do for the libc++ stable ABI. Maybe for the unstable ABI, but given the above change only makes layout worse, it's not clear that there would be a strong motivation. We already give std::pair trivial
copy/move construction and trivial destruction whenever possible, so we can pass and return it efficiently.<u></u><u></u></p>
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Interesting - so POD for the purpose of layout is... not good? or more restrictive on the layout than if not. (probably an impractical idea, but would it be reasonable then to have an attribute that says "not POD for the purposes of layout" but I guess that'd
have to be cross-compiler/etc/etc... probably too much work)<u></u><u></u></p>
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<p class="MsoNormal">Once upon a time, C++ said that POD types could be memcpy'd, even if they were used as base classes. That rule got changed retroactively by core language defect reports, and you're no longer allowed to memcpy base class subobjects (nor
[[no_unique_address]] members), but too late for the ABI, so as a consequence of a rule that we're now pretending never existed, the Itanium ABI is careful not to reuse the tail padding of a base class for members of the derived class if the base class is
"POD for the purpose of layout" (which means POD according to some specific old C++ standard version).<u></u><u></u></p>
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<p class="MsoNormal">An attribute to turn this off might make sense, for classes that really care. But so would an ABI-affecting flag to request that we always permit tail padding reuse. I do wonder if Clang should have a flag for "give me the best ABI you
can, I don't care if it's non-standard; I'm building the whole world this way with this version of this compiler", just like libc++ does.</p></div></div></div></div></div></div></blockquote><div><br></div><div>Certainly crossed my (& you & other folks) mind, for sure. Would we be able to use that at Google? (do we have a strong C++ ABI boundary, or just a strong C++ standard library ABI boundary?) (apologies if this is a bit too internal to discuss here, but seems relevant enough & non-proprietary) <br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-US"><div class="gmail-m_-405106319533475460WordSection1"><div style="border-top:none;border-right:none;border-bottom:none;border-left:1.5pt solid blue;padding:0in 0in 0in 4pt"><div><p class="MsoNormal"><u></u></p>
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<p class="MsoNormal">What's the cost of pair having non-trivial copy assignment? More expensive vector of pair resizing (though I guess the copy ctor calls would be readily optimized away & LLVM could get that back to a memcpy anyway?)?<u></u><u></u></p>
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<p class="MsoNormal">Yes, this is probably only significant for code that's detecting the triviality and taking different action based on it. Clang has code in IR generation that tries to emit the actual copy assignment operator as a single memcpy regardless.</p></div></div></div></div></blockquote><div><br>Oh, nice! Didn't know that triggered even when technically non-trivial.<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-US"><div class="gmail-m_-405106319533475460WordSection1"><div style="border-top:none;border-right:none;border-bottom:none;border-left:1.5pt solid blue;padding:0in 0in 0in 4pt">
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<p class="MsoNormal">(the original issue with the warning could be fixed by narrowing the warning to only worry about trivial copy construction, not trivial copyability in general)<u></u><u></u></p>
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<p class="MsoNormal">That sounds right to me. Is it worth also considering the triviality of the destructor?</p></div></div></div></div></blockquote><div><br>Maybe? I'm OK either way on that. Not sure who wrote the original/would be interested in fixing it in some way. +Sam McCall in case he's interested/can point the right people at this<br><br>- Dave</div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-US"><div class="gmail-m_-405106319533475460WordSection1"><div style="border-top:none;border-right:none;border-bottom:none;border-left:1.5pt solid blue;padding:0in 0in 0in 4pt"><div><div><blockquote style="border-top:none;border-right:none;border-bottom:none;border-left:1pt solid rgb(204,204,204);padding:0in 0in 0in 6pt;margin-left:4.8pt;margin-right:0in"><div><div><div>
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