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<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">Which exactly was the plan?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">Add a new, regular instruction?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">Add a new constrained math intrinsic?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">Both?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">Andrew Kaylor made a good point here:<o:p></o:p></span></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoPlainText" style="mso-list:l0 level1 lfo1">As I said, all LLVM IR FP instructions are //assumed// to have no side effects. I'm not sure we want an instruction that goes beyond this to be //defined// as having no side effects. It adds a complication
to the language and introduces restrictions on the code generator that aren't needed in the ordinary case of non-constrained FP. The target code generators are free to do anything they want with the other FP instructions, including things that introduce new
FP status flags being set that otherwise wouldn't be, and for the normal case the back ends should be free to do that with fneg as well.<o:p></o:p></li></ul>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">Personally, I’m not sure I like the idea of having exceptions to the rule that FP instructions also have constrained versions. So I lean towards having both a regular FNEG and a constrained
version.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">But I think I remember pushback. I can’t put my fingers on the message, though.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Courier New";color:#44546A">--</span><span style="color:#44546A">
<br>
</span><span style="font-size:10.0pt;font-family:"Courier New";color:#44546A">Kevin P. Neal</span><span style="color:#44546A"><br>
</span><span style="font-size:10.0pt;font-family:"Courier New";color:#44546A">SAS/C and SAS/C++ Compiler<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Courier New";color:#44546A">Host Research and Development<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Courier New";color:#44546A">SAS Institute, Inc.</span><span style="color:#44546A"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b>From:</b> xkrebstarx <xkrebstarx@gmail.com> <br>
<b>Sent:</b> Tuesday, September 11, 2018 2:29 PM<br>
<b>To:</b> Cameron Jordan McInally <cameron.mcinally@nyu.edu><br>
<b>Cc:</b> t.p.northover@gmail.com; llvm-dev@lists.llvm.org; ulrich.weigand@de.ibm.com; Kevin Neal <Kevin.Neal@sas.com><br>
<b>Subject:</b> Re: [llvm-dev] [FPEnv] FNEG instruction<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p><b><i><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:red">EXTERNAL</span></i></b><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:red">
</span><o:p></o:p></p>
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<p class="MsoNormal">Ping...<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'd like to hear from the major stakeholders on whether we can proceed with this change or not. Either way, it would be nice to pass this road block. If anyone watching is in close proximity to those that can approve such a change, I would
appreciate it if you would tap them on the shoulder for me.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks again,<o:p></o:p></p>
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<p class="MsoNormal">Cameron<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 Thu, Aug 30, 2018 at 11:38 AM Cameron McInally via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Thu, Aug 30, 2018 at 11:14 AM, Tim Northover <<a href="mailto:t.p.northover@gmail.com" target="_blank">t.p.northover@gmail.com</a>> wrote:<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 don't think it matters for the question at hand, but I tested<br>
AArch64 too and it exhibits the behaviour you were describing. That<br>
is, we'd have problems if an fsub -0.0 was actually CodeGened like<br>
that (it's not, of course).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Great data point. So it's not just a theoretical problem. Thanks, Tim!<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
LLVM Developers mailing list<br>
<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
<a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev<br>
</a><o:p></o:p></p>
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