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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">So you don’t think sNaNs can just be treated as if they were qNaNs? I understand why we would want to ignore the signaling part of things, but the rules for operating
on NaNs are pretty clear and reasonable to implement. The signaling aspect can, I think, be safely ignored when we are in the mode of assuming the default FP environment.<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">As for the distinction between IEEE and LLVM IR, I would think we would want to define LLVM IR in such a way that it is possible to create and IEEE-compliant
compiler. I know we’re not there yet, but we’re working toward it.<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Chris Lattner [mailto:clattner@nondot.org]
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<b>Sent:</b> Wednesday, February 28, 2018 8:42 PM<br>
<b>To:</b> Friedman, Eli <efriedma@codeaurora.org><br>
<b>Cc:</b> Kaylor, Andrew <andrew.kaylor@intel.com>; Sanjay Patel <spatel@rotateright.com>; Matt Arsenault <arsenm2@gmail.com>; llvm-dev <llvm-dev@lists.llvm.org>; John Regehr <regehr@cs.utah.edu><br>
<b>Subject:</b> Re: [llvm-dev] how to simplify FP ops with an undef operand?<o:p></o:p></span></p>
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<p class="MsoNormal">On Feb 28, 2018, at 6:33 PM, Friedman, Eli <<a href="mailto:efriedma@codeaurora.org">efriedma@codeaurora.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On 2/28/2018 5:46 PM, Chris Lattner wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Feb 28, 2018, at 3:29 PM, Kaylor, Andrew 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" style="background:white"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">For the first part of Sanjay’s question, I think the answer is, “Yes, we can fold all of these to NaN in the general case.”
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<p class="MsoNormal">Agreed. Those IR instructions are undefined on SNAN, and that undef could take on an SNAN value. Folding these instructions to undef seems reasonable, and it is arguable that you could even fold it to an ‘unreachable'.<o:p></o:p></p>
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fdiv snan, snan is undefined? As opposed to producing a qnan, as specified by IEEE-754?<o:p></o:p></p>
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<p class="MsoNormal">You’re talking about IEEE, I’m talking about LLVM IR. LLVM IR is undefined on SNaNs. It looks like LangRef isn’t clear about this, the only mention of SNaNs is in this statement:<o:p></o:p></p>
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<p class="MsoNormal"><span class="apple-tab-span"> </span>"fdiv is not (currently) defined on SNaN’s.”<o:p></o:p></p>
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<p class="MsoNormal">However, fdiv/fmul/etc are pervasively treated as not having side effects. The intention, and the only sensible definition for them, is that they are undefined on SNaNs. <o:p></o:p></p>
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<p class="MsoNormal">-Chris<o:p></o:p></p>
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