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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">At this point, the intrinsic has already been selected, as it was done in its grandparent function(visitCall). This function is only generating the SDNodes that corresponds
to the intrinsic and the question is should the call be used to determine whether the chain should be generated, or should the intrinsic be used instead? Since Tablegen uses the intrinsic to do so, I believe normalizing the behavior here is the correct approahc.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Micah<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Evan Cheng [mailto:evan.cheng@apple.com]
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<b>Sent:</b> Tuesday, November 06, 2012 3:42 PM<br>
<b>To:</b> Villmow, Micah<br>
<b>Cc:</b> llvmdev@cs.uiuc.edu List<br>
<b>Subject:</b> Re: [LLVMdev] Bug in SelectionDAG visitTargetIntrinsic<o:p></o:p></span></p>
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<p class="MsoNormal"> void SelectionDAGBuilder::visitTargetIntrinsic(const CallInst &I,<o:p></o:p></p>
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<p class="MsoNormal">- unsigned Intrinsic) {<o:p></o:p></p>
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<p class="MsoNormal">- bool HasChain = !I.doesNotAccessMemory();<o:p></o:p></p>
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<p class="MsoNormal">- bool OnlyLoad = HasChain && I.onlyReadsMemory();<o:p></o:p></p>
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<p class="MsoNormal">+ unsigned Intrinsic) {<o:p></o:p></p>
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<p class="MsoNormal">+ // Info is set by getTgtMemInstrinsic<o:p></o:p></p>
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<p class="MsoNormal">+ TargetLowering::IntrinsicInfo Info;<o:p></o:p></p>
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<p class="MsoNormal">+ bool IsTgtIntrinsic = TLI.getTgtMemIntrinsic(Info, I, Intrinsic);<o:p></o:p></p>
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<p class="MsoNormal">+ bool HasChain = Info.readMem || Info.writeMem;<o:p></o:p></p>
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<p class="MsoNormal">+ bool OnlyLoad = HasChain && Info.readMem;<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">This doesn't seem right. If a call is marked ReadNone, it doesn't seem legal to select it to an intrinsic that read / write memory. By definition a "ReadNone" function cannot touch memory, no? <o:p></o:p></p>
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<p class="MsoNormal">Evan<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 Nov 6, 2012, at 1:45 PM, "Villmow, Micah" <<a href="mailto:Micah.Villmow@amd.com">Micah.Villmow@amd.com</a>> wrote:<o:p></o:p></p>
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<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span class="apple-converted-space"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> </span></span><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">Villmow,
Micah<span class="apple-converted-space"> </span><br>
<b>Sent:</b><span class="apple-converted-space"> </span>Tuesday, November 06, 2012 1:37 PM<br>
<b>To:</b><span class="apple-converted-space"> </span>'<a href="mailto:llvm-dev@cs.uiuc.edu"><span style="color:purple">llvm-dev@cs.uiuc.edu</span></a>'<br>
<b>Cc:</b><span class="apple-converted-space"> </span>Guo, Xiaoyi<br>
<b>Subject:</b><span class="apple-converted-space"> </span>Bug in SelectionDAG visitTargetIntrinsic</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">We ran into a problem where specifying IntrNoMem was causing our instruction selection to fail with target specific intrinsics. After looking into the code and ISel debug
it looks like tablegen and SelectionDAG are using different criteria to generate code for intrinsic_w_chain vs intrinsic_wo_chain.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">In CodeGenDAGPatterns.cpp, tablegen decides based on whether IntrNoMem is set or not. However with SelectionDAG, whether to use a chain or not is determined by the call site
attributes and not by the intrinsic.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">So, we can get the situation where the call site has a different attribute than the intrinsic, and this causes selection dag to fail.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">I believe that this is wrong and that whether a chain should be generated or not should come from only the intrinsic and not the call site. Since the mapping of call -> intrinsic
is by function name only, it should not matter if the readnone attribute is set or not as that is irrelevant to the code generator. Only what is set in the tablegen definition should be determine how the intrinsic is generated.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">So, I'm proposing the following patch. What this patch does is instead of relying on the call site to determine if a chain is required, use instead the read/write attributes
of the intrinsic from the backend instead. There is not much documentation on target intrinsics and no other backend uses them in this manner.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">This patch sound good?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Thanks,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Micah<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:13.5pt;font-family:"Helvetica","sans-serif""><target_intrinsic_incorrect_chain.txt>_______________________________________________<br>
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<a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev"><span style="color:purple">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</span></a><o:p></o:p></span></p>
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