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<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A">Do we really want to have fneg be the only instruction with guaranteed no side effects? That just sounds like a gotcha waiting to happen. Or it could result in horrible code depending
on the architecture.<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">I’m still leaning towards having both an intrinsic and an instruction, and if they happen to have the same behavior then that’s fine. If fneg is to be a special instruction with extra
promises (vs the other FP instructions) then this fact needs to be loudly present in the documentation.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Courier New";color:#44546A">--</span><span style="color:#44546A">
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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>
<p class="MsoNormal"><span style="font-family:"Courier New";color:#44546A"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>Sanjay Patel via llvm-dev<br>
<b>Sent:</b> Wednesday, September 26, 2018 1:25 PM<br>
<b>To:</b> cameron.mcinally@nyu.edu<br>
<b>Cc:</b> 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 class="MsoNormal"><b><i><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#44546A"><o:p> </o:p></span></i></b></p>
<p class="MsoNormal">To bring it back to the question of fneg, let me know if this is an accurate summary:
<o:p></o:p></p>
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<p class="MsoNormal">3. We want fneg as a 1st class instruction even though the related fabs/copysign bitstring ops are intrinsics (because fneg is more common than the others?).<o:p></o:p></p>
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<p class="MsoNormal">4. Adding fneg to IR means we do not need to add a constrained intrinsic for fneg (likewise, there's no need for constrained fabs/copysign because those intrinsics already exist).<o:p></o:p></p>
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