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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Why is NaN “just ‘undef’ in IR”? NaN is a specific value with well-defined behavior. I would think that unless the no-NaNs flag is used we need to preserve the
behavior of NaNs.<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"> Sanjay Patel [mailto:spatel@rotateright.com]
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<b>Sent:</b> Wednesday, February 28, 2018 12:08 PM<br>
<b>To:</b> Kaylor, Andrew <andrew.kaylor@intel.com><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org>; Nuno Lopes <nunoplopes@sapo.pt>; Stephen Canon <scanon@apple.com>; David Majnemer <david.majnemer@gmail.com>; John Regehr <regehr@cs.utah.edu>; Sanjoy Das <sanjoy@playingwithpointers.com>; Friedman, Eli <efriedma@codeaurora.org>;
Matt Arsenault <arsenm2@gmail.com><br>
<b>Subject:</b> Re: how to simplify FP ops with an undef operand?<o:p></o:p></span></p>
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<p class="MsoNormal">Yes, if %x is a NaN, we should expect that NaN is propagated.
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I'm still not sure what to do here. We can take comfort in knowing that whatever we do is likely an improvement over the current situation though. :)<o:p></o:p></p>
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<p class="MsoNormal">That's because the code in InstSimplify is inconsistent with the LangRef:<br>
<a href="http://llvm.org/docs/LangRef.html#undefined-values" target="_blank">http://llvm.org/docs/LangRef.html#undefined-values</a> (UB for fdiv by 0?)<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">...and both of those are inconsistent with undef handling in SDAG.<o:p></o:p></p>
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<p class="MsoNormal">Let me propose an alternate interpretation: <br>
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1. The meaning of snan as written in IEEE754-2008 is: "Signaling NaNs afford representations for uninitialized variables..."<br>
2. That matches our intent with 'undef' here in IR as written in the LangRef: "unspecified bit-pattern".<br>
3. The current fdiv transform is actually correct (any SNaN UB/trapping commentary is irrelevant because we assume exceptions are off by default).
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The undef operand represents an uninitialized variable, and the result of any FP op with that uninitialized variable is well-defined: it's another NaN which is just 'undef' in IR.<o:p></o:p></p>
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<p class="MsoNormal">On Wed, Feb 28, 2018 at 11:43 AM, Kaylor, Andrew <<a href="mailto:andrew.kaylor@intel.com" target="_blank">andrew.kaylor@intel.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">I’m not sure the transformation happening with fdiv is correct. If we have “%y = fdiv float %x, undef”
and %x is a NaN then the result will be NaN for any value of the undef, right? So if I understand the undef rules correctly (never a certainty) then we can’t safely replace the expression with undef. We could, I think, replace it with “%y = %x” though. I think
the same is true for fadd, fsub, fmul, and frem.</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">-Andy</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">%y = fadd float %x, undef<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Can we simplify this?
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Currently in IR, we do nothing for fadd/fsub/fmul. For fdiv/frem, we propagate undef. The code comment for fdiv/frem says:<br>
"the undef could be a snan"<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">If that's correct, then shouldn't it be the same for fadd/fsub/fmul? But this can't be correct because we support targets that don't raise exceptions...and even targets that raise
exceptions do not trap by default on snan?<o:p></o:p></p>
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