<div dir="ltr"><div>Correct - NaN is not undef in IR. But we don't have a NaN in this example. We have its moral equivalent in LLVM - an uninitialized value, undef. <br><br>So we're not introducing any extra uncertainty by propagating the undef. The backend can choose whatever encoding of undef makes sense when lowering?<br><br></div>And yes, I don't know why FP-div-by-zero would ever be UB. I think that text in the LangRef should be removed regardless of any other outcome here.<br><div><div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Feb 28, 2018 at 1:18 PM, Kaylor, Andrew <span dir="ltr"><<a href="mailto:andrew.kaylor@intel.com" target="_blank">andrew.kaylor@intel.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div link="blue" vlink="purple" lang="EN-US">
<div class="m_-4668788422626485428WordSection1">
<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.<u></u><u></u></span></p>
<p class="MsoNormal"><a name="m_-4668788422626485428__MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></a></p>
<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:<a href="mailto:spatel@rotateright.com" target="_blank">spatel@rotateright.com</a><wbr>]
<br>
<b>Sent:</b> Wednesday, February 28, 2018 12:08 PM<br>
<b>To:</b> Kaylor, Andrew <<a href="mailto:andrew.kaylor@intel.com" target="_blank">andrew.kaylor@intel.com</a>><br>
<b>Cc:</b> llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>>; Nuno Lopes <<a href="mailto:nunoplopes@sapo.pt" target="_blank">nunoplopes@sapo.pt</a>>; Stephen Canon <<a href="mailto:scanon@apple.com" target="_blank">scanon@apple.com</a>>; David Majnemer <<a href="mailto:david.majnemer@gmail.com" target="_blank">david.majnemer@gmail.com</a>>; John Regehr <<a href="mailto:regehr@cs.utah.edu" target="_blank">regehr@cs.utah.edu</a>>; Sanjoy Das <<a href="mailto:sanjoy@playingwithpointers.com" target="_blank">sanjoy@playingwithpointers.<wbr>com</a>>; Friedman, Eli <<a href="mailto:efriedma@codeaurora.org" target="_blank">efriedma@codeaurora.org</a>>;
Matt Arsenault <<a href="mailto:arsenm2@gmail.com" target="_blank">arsenm2@gmail.com</a>><br>
<b>Subject:</b> Re: how to simplify FP ops with an undef operand?<u></u><u></u></span></p><div><div class="h5">
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<div>
<div>
<div>
<div>
<p class="MsoNormal">Yes, if %x is a NaN, we should expect that NaN is propagated.
<br>
<br>
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. :)<u></u><u></u></p>
</div>
<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.<wbr>html#undefined-values</a> (UB for fdiv by 0?)<u></u><u></u></p>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt">...and both of those are inconsistent with undef handling in SDAG.<u></u><u></u></p>
</div>
<p class="MsoNormal">Let me propose an alternate interpretation: <br>
<br>
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).
<br>
<br>
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.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<div>
<div>
<div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<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:<u></u><u></u></p>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<div>
<div>
<p class="MsoNormal"><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><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">-Andy</span><u></u><u></u></p>
<p class="MsoNormal"><a name="m_-4668788422626485428_m_-4918008932178653133_m_464286896021534"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></a><u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<div>
<div>
<div>
<div>
<p class="MsoNormal">%y = fadd float %x, undef<u></u><u></u></p>
</div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">Can we simplify this?
<br>
<br>
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"<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">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?<u></u><u></u></p>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div></div></div>
</div>
</blockquote></div><br></div></div></div></div>