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<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">Yes, true.<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">This came up in the context of adding support for the constrained FP intrinsics. These intrinsics all include a metadata argument for specifying how important FP traps are. If traps
are “strict” then we can’t fold away any traps. The default FP environment would use “ignore” where there are no traps and folding is fine. I’m working on the “maytrap” case where we can eliminate traps but cannot introduce any new ones. More on constrained
intrinsics: <a href="https://llvm.org/docs/LangRef.html#constrained-floating-point-intrinsics">
https://llvm.org/docs/LangRef.html#constrained-floating-point-intrinsics</a><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">To be clear, are you saying that my interpretation of 754 is correct?<o:p></o:p></span></p>
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<p class="MsoNormal"><b>From:</b> Jacob Lifshay <programmerjake@gmail.com> <br>
<b>Sent:</b> Friday, July 09, 2021 2:32 PM<br>
<b>To:</b> Kevin Neal <Kevin.Neal@sas.com><br>
<b>Cc:</b> LLVM Developers <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] [754] Fold FP "Op SNaN" to QNaN?<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">
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<p class="MsoNormal">On Fri, Jul 9, 2021, 08:41 Kevin Neal 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"><span style="font-family:"Courier New"">I’m looking at lib/Analysis/InstructionSimplify.cpp where the function propagateNaN() has a comment asking if it should quiet a signaling NaN.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">If I understand the IEEE 754-2019 standard correctly: an SNaN shall be converted to a QNaN whenever an “operation” is done. The standard doesn’t say, or I couldn’t find it, exactly _<i>when</i>_ that
operation must be done. Which implies that the floating-point operation could be done by the compiler. In which case folding an instruction that has an SNaN operand should result in a QNaN.<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">That should work just fine, but only when you know that FP exceptions flags are ignored, since the signalling NaN causes the operation to generate the Invalid exception, and a quiet NaN doesn't generally cause exceptions.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Jacob Lifshay<o:p></o:p></p>
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