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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">I'm convinced that it's best to keep it as a warning.<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">In reading over the responses, one thing that comes to mind which I think is analogous is<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">a shift by a literal amount that's too large. For example, a shift of a 4-byte 'int' by<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">the literal 32 results in a warning, not an error:<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"> unsigned int foo(unsigned int a0, unsigned int a1)<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"> return (a0 >> 32) + a1;<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"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">gets:<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"> warning: shift count >= width of type [-Wshift-count-overflow]<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">So for this, we give a warning. And at -O2, we legitimately optimize-away all expressions<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">based on the result of the undefined-behavior-shift (producing just a 'return' in the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">above case). I wouldn't consider changing the warning that comes for the above shift to<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">an error, and so by the same token, I no longer consider an incorrect (literal)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">memory-model parameter for an atomic op to be a good candidate for an error. Keeping it<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">a warning keeps us consistent in this regard.<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 all, for the feedback.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">-Warren<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">--<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">Warren Ristow<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">SN Systems - Sony Computer Entertainment Group<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>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> metafoo@gmail.com [mailto:metafoo@gmail.com]
<b>On Behalf Of </b>Richard Smith<br>
<b>Sent:</b> Monday, February 02, 2015 5:07 PM<br>
<b>To:</b> Tim Northover<br>
<b>Cc:</b> Reid Kleckner; cfe-dev@cs.uiuc.edu; Ristow, Warren<br>
<b>Subject:</b> Re: [cfe-dev] [RFC] Should an illegal C++11 memory model in an atomic op be an error?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Mon, Feb 2, 2015 at 1:50 PM, Tim Northover <<a href="mailto:tnorthover@apple.com" target="_blank">tnorthover@apple.com</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">> I'd be in favor or promoting it to an error. We recently started rejecting this code, for example:<br>
<br>
I'm not sure I would. Deciding on the diagnostics for our own intrinsics is one thing, but this would trigger on well-formed programs according to the standard.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I agree that this should not be an error. The builtin does not require a constant; we shouldn't say it's ill-formed if we happen to be able to evaluate the order parameter and find it's out-of-range.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I think this sort of thing is completely reasonable (perhaps in the implementation of a C++11-compatible atomics library):<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">#define atomic_builtin(kind, order, ...) \<o:p></o:p></p>
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<p class="MsoNormal"> ({ switch(order) { \<o:p></o:p></p>
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<p class="MsoNormal"> case memory_order_relaxed: kind(__VA_ARGS__, __ATOMIC_RELAXED); break; \<o:p></o:p></p>
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<p class="MsoNormal"> [...]<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">That's rather a bigger step to take, and we should probably be comparing it to situations with that similarity (the only one I know of is ARM64 complaining more strongly about functions without prototypes).<o:p></o:p></p>
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<p class="MsoNormal"><br>
Cheers.<br>
<br>
Tim.<br>
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<p class="MsoNormal"><o:p> </o:p></p>
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