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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"> In my mind at least, “whole program” means no dynamic libraries, so the only<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">external functions are simple runtime support, do you have a suggested term for that ?<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>
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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> Tuesday, July 12, 2016 12:31 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">On Jul 12, 2016, at 12:20 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 am looking for an understanding of 1) IPRA in general, 2) IPRA in LLVM.</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">Whether I want to use LTO or not is a separate issue.</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">1) I currently believe it is a true statement that:</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 all external functions are known to not call back into the “whole-program”</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"> Being compiled, then IPRA is free to do anything at all to the functions being</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"> Compiled, not limited to only “upgrades” calling convention changes, but</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"> Also allowing “downgrades” calling convention changes as well.</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">Do you think my current belief #1 is correct ?</span><o:p></o:p></p>
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<p class="MsoNormal">Yes, with some extra assumptions (you don’t use dlsym for instance, and you won’t link to another file with a global initializer that can call any of these).<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 expressed this earlier (which include the other issues I mentioned just before) as “we can turn the linkage of every function into local” (or private, or static, whatever denomination you prefer).<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">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"><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">“whole-program” here is a misnomer since there are external functions, but I don’t</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">Have a better term for this.</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">I believe you can talk about “main module”, i.e. the module defines the entry point for the program.<o:p></o:p></p>
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<p class="MsoNormal">Note LLVM can’t make assumption about the lack of dlsym() or global initializer in other module for example, so the linkage type of functions is what tells us about the possibility to call back or not.<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:"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">“upgrades” means some scratch regs are converted to save</span><o:p></o:p></p>
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<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">(the callee either doesn’t touch them at all, or does do save/restore)</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">“downgrades” means some save regs are converted to scratch</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 callee no longer does save/restore to some registers, and does clobber them)</span><o:p></o:p></p>
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