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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi Chandler<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="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The transformation I added is not “completely target-specific”. I don’t see why other targets will not benefit from it especially if cost models
are reasonable.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> The transformation only kicks in at the last pass before CodeGen. So IR is the same as if you do it the first thing in CodeGen.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">3.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> There is an agreement that it will be moved into CodeGen or library (plug-in?) when inventories of similar transformations build up.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">4.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">If you strongly object, I can add an addition pass of SimplifyCFG as the first thing in the CodeGen. But this will really bump up our compilation
time since we are using on-line compilation model and due to the lack of function call supports in GPU, every function call including library functions are inlined.<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">-Mei<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 style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Chandler Carruth [mailto:chandlerc@google.com]
<br>
<b>Sent:</b> Friday, July 26, 2013 9:38 PM<br>
<b>To:</b> Evan Cheng; Andy Trick<br>
<b>Cc:</b> Ye, Mei; llvm-commits@cs.uiuc.edu<br>
<b>Subject:</b> Re: [patch] simplifyCFG for review<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">Hey Evan, Andy, and others.<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 Mon, Jul 15, 2013 at 2:10 PM, Evan Cheng <<a href="mailto:evan.cheng@apple.com" target="_blank">evan.cheng@apple.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">I am ok with adding this transformation into simplifycfg <span class="il">
and</span> have it being controlled by a target hook. I think it's a reasonable compromise for now. In the future, once we start to accumulate more GPU specific transformations, I'd prefer they get factored out into a separate passes (or even libraries).<o:p></o:p></p>
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So, I'm really surprised to see this get committed without a discussion on llvmdev about the fundamental issues presented by this *radical* design shift in simplifycfg.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">A while back, I added a target hook to simplifycfg to do a target-specific transform (I called it TTI and a cost model, but it's the same thing). You, Jakob, Andy, and a bunch of other people objected. You presented compelling arguments,
and we went the other way. In fact, you convinced me that keeping the same canonical CFG structure in the IR-level passes was really important for canonicalization and other optimization passes. I now share what I thought were your objections to such IR-level
CFG transforms.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">And I'm now confused as this patch adds a significantly more invasive target hook to do a completely target-specific transform to the CFG. This patch completely changes the idea of canonical CFG structures at the IR level for one target
compared to the other target. I don't really know how to even work on simplify-cfg any more because I don't know what the interactions here. This really needs a design discussion not tucked away in a 40-email long patch review thread. Could one of you or Mei
start that? In particular, I would like a really compelling reason why this isn't done in a target specific lowering pass, ideally not at the IR level at all.<o:p></o:p></p>
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