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<p class="MsoNormal">I’m not sure what the ulp is for the full fdiv expansions, so I’m not sure<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-Matt<o:p></o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">"Fang, Changpeng" <Changpeng.Fang@amd.com><br>
<b>Date: </b>Thursday, January 30, 2020 at 18:22<br>
<b>To: </b>Changpeng Fang via Phabricator <reviews@reviews.llvm.org>, "Arsenault, Matthew" <Matthew.Arsenault@amd.com>, Brian Sumner <Brian.Sumner@amd.com><br>
<b>Cc: </b>Konstantin Zhuravlyov <Konstantin.Zhuravlyov@amd.com>, "jv356@scarletmail.rutgers.edu" <jv356@scarletmail.rutgers.edu>, Wei Ding <wei.ding2@amd.com>, "nhaehnle@gmail.com" <nhaehnle@gmail.com>, "Liu, Yaxun (Sam)" <Yaxun.Liu@amd.com>, "Stuttard, David"
<David.Stuttard@amd.com>, "tpr.ll@botech.co.uk" <tpr.ll@botech.co.uk>, Tony Tye <Tony.Tye@amd.com>, "hiraditya@msn.com" <hiraditya@msn.com>, "Kerbow, Austin" <Austin.Kerbow@amd.com>, "llvm-commits@lists.llvm.org" <llvm-commits@lists.llvm.org>, "jun.l@samsung.com"
<jun.l@samsung.com><br>
<b>Subject: </b>Re: [PATCH] D73588: AMDGPU: Enhancement on FDIV lowering in AMDGPUCodeGenPrepare<o:p></o:p></span></p>
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<p style="margin:15.0pt"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#0078D7">[AMD Official Use Only - Internal Distribution Only]<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">>// Faster 2.5 ULP division that does not support denormals.<br>
>SDValue SITargetLowering::lowerFDIV_FAST(SDValue Op, SelectionDAG &DAG) const {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Also, it is the case that fdiv.fast generates "Faster 2.5 ULP division that does not support denormals"?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">So we should still use 2.5ULP for fdiv.fast, and 1.0ULP for rcp?
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<p class="MsoNormal"><b><span style="color:black">From:</span></b><span style="color:black"> Changpeng Fang via Phabricator <reviews@reviews.llvm.org><br>
<b>Sent:</b> Thursday, January 30, 2020 3:16 PM<br>
<b>To:</b> Fang, Changpeng <Changpeng.Fang@amd.com>; Arsenault, Matthew <Matthew.Arsenault@amd.com>; Sumner, Brian <Brian.Sumner@amd.com><br>
<b>Cc:</b> Zhuravlyov, Konstantin <Konstantin.Zhuravlyov@amd.com>; jv356@scarletmail.rutgers.edu <jv356@scarletmail.rutgers.edu>; wei.ding2@amd.com <wei.ding2@amd.com>; nhaehnle@gmail.com <nhaehnle@gmail.com>; Liu, Yaxun (Sam) <Yaxun.Liu@amd.com>; Stuttard,
David <David.Stuttard@amd.com>; tpr.ll@botech.co.uk <tpr.ll@botech.co.uk>; Tye, Tony <Tony.Tye@amd.com>; hiraditya@msn.com <hiraditya@msn.com>; Kerbow, Austin <Austin.Kerbow@amd.com>; llvm-commits@lists.llvm.org <llvm-commits@lists.llvm.org>; jun.l@samsung.com
<jun.l@samsung.com><br>
<b>Subject:</b> [PATCH] D73588: AMDGPU: Enhancement on FDIV lowering in AMDGPUCodeGenPrepare</span>
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<p class="MsoNormal" style="margin-bottom:12.0pt">[CAUTION: External Email]<br>
<br>
cfang marked 2 inline comments as done.<br>
cfang added inline comments.<br>
<br>
<br>
================<br>
Comment at: llvm/lib/Target/AMDGPU/AMDGPUCodeGenPrepare.cpp:714<br>
const bool UseFDivFast = Ty->isFloatTy() && !NeedHighAccuracy &&<br>
- !FastUnsafeRcpLegal;<br>
+ !CanReassociateFDiv;<br>
<br>
----------------<br>
arsenm wrote:<br>
> cfang wrote:<br>
> > arsenm wrote:<br>
> > > fdiv.fast doesn't' care about the reassociation<br>
> > You are right. This is just the optimization priority issue.<br>
> ><br>
> > If we can reassociate fdiv, x/y -> x * rcp(y) is faster than fdiv.fast so we don't do fdiv.fast.<br>
> The comment and variable name are misleading, as no reassociate is going on here. This needs an explanation here<br>
I am going to write an explanation here.<br>
But I am confused about fdiv.fast intrinsic:<br>
1.0/x -> fdiv.fast (1.0, x) when denormals are supported. Because I think does not support fdiv.fast.<br>
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