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<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">If we don’t have constrained intrinsics for some of the fp math instructions then aren’t we risking non-strict optimizations?<o:p></o:p></span></p>
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</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>
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<p class="MsoNormal"><b>From:</b> Sanjay Patel <spatel@rotateright.com> <br>
<b>Sent:</b> Monday, October 01, 2018 5:41 PM<br>
<b>To:</b> cameron.mcinally@nyu.edu<br>
<b>Cc:</b> Kevin Neal <Kevin.Neal@sas.com>; llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] [FPEnv] FNEG instruction<o:p></o:p></p>
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<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">I don't see any controversy for the preliminary requirement of removing
<span class="gmail-im">BinaryOperator::isFNeg() and friends, so start with that?</span><o:p></o:p></p>
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<p class="MsoNormal"><span class="gmail-im">That work may reveal other potential regressions that we can patch in advance too.</span><o:p></o:p></p>
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<p class="MsoNormal"><span class="gmail-im">Other than that, I think there's really only a question of do we want 1 or both of fneg and fneg_constrained (and if we choose both, then I assume we'd also add fabs_constrained and copysign_constrained).
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<p class="MsoNormal"><span class="gmail-im">I don't see why we want those redundancies. As discussed here, we already can't canonicalize to the FP bitops in IR because of non-IEEE targets (D19391). In IR and the backend, you don't want to over-constrain allowable
optimizations. Fneg folds shouldn't be disabled just because we changed the FP exception state?</span><o:p></o:p></p>
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<p class="MsoNormal">On Mon, Oct 1, 2018 at 12:20 PM Cameron McInally <<a href="mailto:cameron.mcinally@nyu.edu">cameron.mcinally@nyu.edu</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Thu, Sep 27, 2018 at 10:14 AM Sanjay Patel <<a href="mailto:spatel@rotateright.com" target="_blank">spatel@rotateright.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Regarding non-IEEE targets: yes, we definitely support those, so we do have to be careful about not breaking them. I know because I have broken them. :)
<o:p></o:p></p>
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<p class="MsoNormal">See the discussion and related links here: <a href="https://reviews.llvm.org/D19391" target="_blank">
https://reviews.llvm.org/D19391</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">But having an exactly specified fneg op makes that easier, not harder, as I see it. Unfortunately, if a target doesn't support this op (always toggle the sign bit and only the sign bit), then we can't canonicalize 'fsub -0.0, X' to 'fneg
X' because those are not identical ops (denorms, NaN).<o:p></o:p></p>
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<p class="MsoNormal">So that leads back to the m_FNeg abstraction - it will have to match both ops to not lose optimizations.<o:p></o:p></p>
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<p class="MsoNormal">I like this idea. <o:p></o:p></p>
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<p class="MsoNormal">So how do we get official approval to begin this work? <o:p></o:p></p>
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