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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi James,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">thanks for your answer.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Your suggestion looks sensible to me, then I understand that a combiner like the following one (DAGCombiner::visitSUBCARRY)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">
<a href="http://www.llvm.org/doxygen/DAGCombiner_8cpp_source.html#l02498">http://www.llvm.org/doxygen/DAGCombiner_8cpp_source.html#l02498</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">that assumes that “subcarry x, y, 0” can be simplified as “usubo x, y” should be parameterized using TargetLowering, as this assumes that the third input of
subcarry is a borrow (e.g. x86) rather than a “carry” (e.g. ARM).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Does this make sense? Maybe I’m mixing things here.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">( For context: I’m trying to establish if I can avoid adding ARM specific combiners in
<a href="https://reviews.llvm.org/D34515">https://reviews.llvm.org/D34515</a> and use the generic ones instead. )<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Kind regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Roger<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> James Y Knight [mailto:jyknight@google.com]
<br>
<b>Sent:</b> 27 June 2017 17:00<br>
<b>To:</b> Roger Ferrer Ibanez<br>
<b>Cc:</b> llvm-dev; nd<br>
<b>Subject:</b> Re: [llvm-dev] Question about ISD::SUBCARRY<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's an excellent question. :)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I'd say it should _probably_ be defined as having the same boolean value as the hardware's carry-flag, and there should be a TargetLowering function which describes the semantics upon subtract.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Tue, Jun 27, 2017 at 4:53 AM, Roger Ferrer Ibanez via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">Dear all,<br>
<br>
a couple of new generic DAG nodes ISD::ADCARRY and ISD::SUBCARRY were recently introduced in
<a href="https://reviews.llvm.org/D29872" target="_blank">https://reviews.llvm.org/D29872</a><br>
<br>
These nodes have three inputs and two outputs, the second output being the "carry". I understand that carry is well defined for ADDCARRY but my question is about SUBCARRY.<br>
<br>
Some architectures set the "carry" of a "x - y" subtraction is set when x < y (e.g x86, "borrow") and some others set it when x >= y (e.g. ARM). Does the ISD::SUBCARRY picks one interpretation (and uses it for combiners on top of this node) or leaves the interpretation
to the target.<br>
<br>
Maybe my whole question does not make sense and even if only one interpretation is chosen this does not impact the target?<br>
<br>
Thank you very much,<br>
Roger<br>
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