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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Mehdi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> My bad, I said “3.8 trunk” when I should have said “trunk”<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">--Peter.<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:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> mehdi.amini@apple.com [mailto:mehdi.amini@apple.com]
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<b>Sent:</b> Wednesday, July 13, 2016 4:48 PM<br>
<b>To:</b> Lawrence, Peter <c_plawre@qca.qualcomm.com><br>
<b>Cc:</b> vivek pandya <vivekvpandya@gmail.com>; llvm-dev <llvm-dev@lists.llvm.org>; llvm-dev-request@lists.llvm.org; Hal Finkel <hfinkel@anl.gov><br>
<b>Subject:</b> Re: [llvm-dev] IPRA, interprocedural register allocation, question<o:p></o:p></span></p>
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<p class="MsoNormal">You mentioned 3.8 earlier, and I doubt the code you are referring to here is in there, it is probably in trunk.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Now, anyway the patch wasn’t actually committed yet, it is as of r275347.<o:p></o:p></p>
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<p class="MsoNormal">— <o:p></o:p></p>
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<p class="MsoNormal">Mehdi<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 Jul 13, 2016, at 4:28 PM, Lawrence, Peter <<a href="mailto:c_plawre@qca.qualcomm.com">c_plawre@qca.qualcomm.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Mehdi,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> I’m seeing lots of “upgrading” logic,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> If (UseIPRA)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> createPass(new DummyCGSCCPass);</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (UseIPRA)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> if (UseIPRA)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">???</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">--Peter.</span><o:p></o:p></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span class="apple-converted-space"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span></span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><a href="mailto:mehdi.amini@apple.com">mehdi.amini@apple.com</a>
[<a href="mailto:mehdi.amini@apple.com">mailto:mehdi.amini@apple.com</a>]<span class="apple-converted-space"> </span><br>
<b>Sent:</b><span class="apple-converted-space"> </span>Wednesday, July 13, 2016 4:26 PM<br>
<b>To:</b><span class="apple-converted-space"> </span>Lawrence, Peter <<a href="mailto:c_plawre@qca.qualcomm.com">c_plawre@qca.qualcomm.com</a>><br>
<b>Cc:</b><span class="apple-converted-space"> </span>vivek pandya <<a href="mailto:vivekvpandya@gmail.com">vivekvpandya@gmail.com</a>>; llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>>;
<a href="mailto:llvm-dev-request@lists.llvm.org">llvm-dev-request@lists.llvm.org</a>; Hal Finkel <<a href="mailto:hfinkel@anl.gov">hfinkel@anl.gov</a>><br>
<b>Subject:</b><span class="apple-converted-space"> </span>Re: [llvm-dev] IPRA, interprocedural register allocation, question</span><o:p></o:p></p>
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<p class="MsoNormal">On Jul 13, 2016, at 4:24 PM, Lawrence, Peter <<a href="mailto:c_plawre@qca.qualcomm.com"><span style="color:purple">c_plawre@qca.qualcomm.com</span></a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Mehdi,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> I am perusing the 3.8 trunk sources, and don’t find evidence where I</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">would expect it for LLVM “downgrading” a function’s calling convention.</span><o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">IPRA is a project started ~ 2 months ago, there is nothing like that in 3.8 (neither downgrading, nor upgrading).<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">— <o:p></o:p></p>
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<p class="MsoNormal">Mehdi<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">PrologEpilogEmitter() { “CodeGen/”</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> TFI->determineCalleeSaves() { “Target/XYZ/”</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> TargetFrameLowering::determineCalleeSaves() { “CodeGen/”</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> Return <<< some object derived from “*CallingConv.td” >>>; “build/lib/Target/XYX/”</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> ...</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> SavedRegs.set(Reg); // to “add” a reg, EG for ‘hasFP’, ETC</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">That comes typically from “*CallingConv.td” hence is not function-specific.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">The only time SavedRegs.reset() is ever called (which is rarely to begin with)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">are for target-specific, calling-conventions-specific reasons, never function-specific.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Perhaps I’m looking in the wrong place ?</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">But I think while we both agree that in principle LLVM could “downgrade” a function,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Given that it can provably see every call-site to it, it does not seem like this is actually</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Happening, unless I’m missing something ???</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">(even if true I’m not claiming we’re missing an important case, I don’t have any</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Logical arguments either way and don’t have any evidence either way. I’m just</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Trying to understand what LLVM actually does or does not do).</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">--Peter Lawrence.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">2) it seems that LLVM currently limits itself to “upgrades” calling convention changes,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">The reason being so that not all call sites are required to be changed,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">therefore calls through function pointers can use the default calling convention</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">If for example there is insufficient analysis to know for sure what functions can be</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">called from that site.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Is my understanding #2 of IPRA in LLVM correct ?</span><o:p></o:p></p>
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<p class="MsoNormal"> <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 don’t believe this is correct, currently IPRA will limit itself to this for function that can be called from another module.<o:p></o:p></p>
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<p class="MsoNormal">I will freely change the calling convention, including downgrades, when it knows that it can see all call sites (+ extra conditions, like no recursion being involved I think).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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