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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"> I’m compiling embedded applications which are small enough to do<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">whole-program-compilation. There’s no advantage in breaking them up into<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">separate compilation pieces and linking them, even though in source form<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">they are composed of a couple of separate source files.<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">So for me the compilation unit is always the entire program (and includes main())<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Except for some hand-coded-assembly-language support functions that are “external”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">to the compilation unit and in my case never call back into the compilation unit,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">IE they are always “leaf” functions from the point of view of the compilation unit’s call-graph.<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">Hence I would like a clang function attribute that says this function is “leaf”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">So that IPRA can know that none of the functions it is compiling is ever called<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">From outside this compilation unit.<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">And I apologize to everyone for confusingly using the term “compilation unit”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">When I meant “whole program”.<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"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Yes I am aware of the fact that if you change a function’s calling convention<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">By converting some scratch regs into save regs (for example because they aren’t even touched)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Then you are safe to call it from either the default calling convention or the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Optimized calling convention. This is the safe thing to do, and is why I will<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Only use “preserves_most” and “preserves_all” optimized calling conventions,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">As those will have been implemented by a back-end writer who is aware of<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">All these compilations (as opposed to the “registermask=” calling convention<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Which is much less safe)<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">I do however feel that IPRA in the whole-program case should not be restricted to<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Only scratch-becoming-save changes, I don’t have any data to support the notion,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">But it begs to be investigated, unless someone can somehow prove that it can’t help<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Performance.<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"><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>
<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"><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> Monday, July 11, 2016 7:06 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"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Jul 11, 2016, at 6:45 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"><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">Vivek,</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"> Here’s the way I see it, let me know if you agree or disagree,</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">You cannot optimize a function’s calling convention (register-usage) unless</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">You can see and change every caller,</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">That’s true only if you want to “downgrade” the guarantees, i.e. if you want to reduce the callee-saved registers.<o:p></o:p></p>
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<p class="MsoNormal">You can freely provide more information to limit the amount of caller-saved registers to a partial list of call-sites, which is in practice changing the “local" calling convention while keeping it compatible with the public one.<o:p></o:p></p>
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<p class="MsoNormal"><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;color:#1F497D">and you only know this for non-static functions</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 you know that all calls to external functions cannot call back into the current</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">compilation unit.</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’m not sure why you consider calls to external functions and call back? If you don’t see main() (the common case) you don’t need a call to an external function to have a possible call to an externally visible function in the current module.<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;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 gives you the info necessary to change the call-site to the external 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"> </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">So you don’t need #2 to do RA around the call-site to the external function, instead</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">You need #2 before you can change any non-static function’s 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">within the current compilation unit, assuming you have this information for all</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">external functions.</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">If I understand the case you have in mind, it is only when you see the main() function in the current module and you’re trying to prove that an externally visible function could not be called from outside the module basically?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">It seems to me that this is a bit orthogonal to IPRA: multiple optimizations (IPRA included) work best when functions are deduced local, non-recursive, are not tail called (for IPRA in particular), and don’t have their address taken. <o:p></o:p></p>
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<p class="MsoNormal">The “infer-func-attr” and “globalopt” passes try to do their best to make this happen, especially during LTO.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The attribute case that Vivek is adding seems more murky though.<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"><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;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">To be more concrete, let foo() be a non-static function in the current compilation</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">Unit, any calls to foo() from external functions will have to use the “default”</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">Calling convention, so foo’s calling convention cannot be changed. We have to</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">Know that none of the external functions can call-back to the compilation unit</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">(they are “leaf” functions relative to the compilation unit) before we can change</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">Foo()’s 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"> </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">Also, the issue of escaping-pointer-to-function is made clear by the example</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">Of the atexit() and exit() library functions, IE even static functions can end up</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 called by external functions. So exit() can never be declared “leaf”, and</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">To get the benefit of IPRA it needs to be within the compilation unit, either</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">By whole-program compilation or by LTO, if it is used.</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"> </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"><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"> </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"> </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"> </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">vivek
pandya [<a href="mailto:vivekvpandya@gmail.com">mailto:vivekvpandya@gmail.com</a>]<span class="apple-converted-space"> </span><br>
<b>Sent:</b><span class="apple-converted-space"> </span>Friday, July 08, 2016 9:26 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>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>>; Tim Amini Golling <<a href="mailto:mehdi.amini@apple.com">mehdi.amini@apple.com</a>><br>
<b>Subject:</b><span class="apple-converted-space"> </span>Re: Re:[llvm-dev] IPRA, interprocedural register allocation, question</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">On Sat, Jul 9, 2016 at 8:15 AM, Lawrence, Peter <<a href="mailto:c_plawre@qca.qualcomm.com" target="_blank"><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">Vivek,</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"> IIUC it seems that we need two pieces of information to do IPRA,</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. what registers the callee clobbers</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. what the callee does to the call-graph</span><o:p></o:p></p>
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<p class="MsoNormal">Yes I think this is enough, but in your case we don't require #2 <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">And it is #2 that we are missing when we define an external 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">Even when we declare it with a preserves or a regmask attribute,</span><o:p></o:p></p>
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<p class="MsoNormal">Because I think once we have effect of attribute at IR/MI level then we can just parse it and populate register usage information vector for declared function and then we can propagate reg mask on each call site encountered.<o:p></o:p></p>
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<p class="MsoNormal">But I am not user will it be easy to get new attribute working or we may need to hack clang for that too.<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 would also like to have thoughts from my mentors (Mehdi Amini and Hal Finkel) about this.<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">So what I / we need is another attribute that says this is a leaf 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">At least in my case all I’m really concerned with are leaf functions</span><o:p></o:p></p>
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<p class="MsoNormal">I am stating with a simple function declaration which have a custom attribute.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">-Vivek<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">Thoughts ?</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"> </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"><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">vivek
pandya [mailto:</span><a href="mailto:vivekvpandya@gmail.com" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:purple">vivekvpandya@gmail.com</span></a><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">]<span class="apple-converted-space"> </span><br>
<b>Sent:</b><span class="apple-converted-space"> </span>Friday, July 08, 2016 10:24 AM<br>
<b>To:</b><span class="apple-converted-space"> </span>Lawrence, Peter <</span><a href="mailto:c_plawre@qca.qualcomm.com" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:purple">c_plawre@qca.qualcomm.com</span></a><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">><br>
<b>Cc:</b><span class="apple-converted-space"> </span>llvm-dev <</span><a href="mailto:llvm-dev@lists.llvm.org" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:purple">llvm-dev@lists.llvm.org</span></a><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">>;<span class="apple-converted-space"> </span></span><a href="mailto:llvm-dev-request@lists.llvm.org" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:purple">llvm-dev-request@lists.llvm.org</span></a><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><br>
<b>Subject:</b><span class="apple-converted-space"> </span>Re: Re:[llvm-dev] IPRA, interprocedural register allocation, question</span><o:p></o:p></p>
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<p class="MsoNormal">On Fri, Jul 8, 2016 at 1:42 PM, vivek pandya <<a href="mailto:vivekvpandya@gmail.com" target="_blank"><span style="color:purple">vivekvpandya@gmail.com</span></a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Fri, Jul 8, 2016 at 9:47 AM, Lawrence, Peter <<a href="mailto:c_plawre@qca.qualcomm.com" target="_blank"><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">Vivek,</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 into these function attributes in the clang docs</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"> Preserve_most</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"> Preserve_all</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">They are not available in the 3.6.2 that I am currently using, but I hope they exist in 3.8</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">These should provide enough info to solve my problem,</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">at the MC level calls to functions with these attributes</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">with be code-gen’ed through different “calling conventions”,</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">and CALL instructions to them should have different register USE and DEF info,</span><o:p></o:p></p>
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<p class="MsoNormal">Yes I believe that preserve_most or preserve_all should help you even with out IPRA. But just to note IPRA can even help further for example on X86 preserve_most cc will not preserve R11 (this can be verified from X86CallingConv.td and
X86RegisterInfo.cpp) how ever IPAR calculates regmask based on the actual register usage and if procedure with preserve_most cc does not use R11 and none callsite inside of function body then IPRA will mark R11 as preserved. Also IPRA produces RegMask which
is super set of RegMask due to calling convention.<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 that __attribute__ ((registermask = ....)) can provide more flexibility compare to preserve_all or preserve_most CC in some case. So believe that we should try it out.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">-Vivek<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">This CALL instruction register USE and DEF info should already be useful</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">to the intra-procedural register allocator (allowing values live across these</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">calls to be in what are otherwise caller-save registers),</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">at least that’s how I read the MC dumps, every call instruction seems to have</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">every caller-save register flagged as “imp-def”, IE implicitly-defined by the instruction,</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">and hopefully what is considered a caller-save register at a call-site is defined by the callee.</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">And this should be the information that IPRA takes advantage of in its bottom-up analysis.</span><o:p></o:p></p>
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<p class="MsoNormal">Yes that is expected help from IPRA. <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">Which leads me to this question, when compiling an entire whole program at one time,</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">so there is no linking and no LTO, will there ever be IPRA that works within LLC for this scenario,</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">and is this an objective of your project, or are you focusing only on LTO ?</span><o:p></o:p></p>
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<p class="MsoNormal">The current IPRA infrastructure works at compile time so it's scope of optimization is restricted to a compilation unit. So IPRA can only construct correct register usage information if the procedure's code is generated by same compiler
instance that means we can't optimize library calls or procedure defined in other module. This is because we can't keep register usage information data across two different compiler instance. <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 if we consider LTO, it eliminates above limitation by making a large IR module from smaller modules before generating code and thus we can have register usage information (at lest) for procedure which was previously defined in other
module, because now with LTO every thing is in one module. So that also clarifies that IPRA does not do anything at link time.<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 coming to LLC, it can use IPRA and optimize for functions defined in current module. So yes while compiling whole program ( a single huge .bc file) IPRA can be used with LLC. Also just note that if a software is written in separate
files per module (which is very common) and still you want to maximize benefits of IPRA, then we can use llvm-link tool to combine several .bc files to produce a huge .bc file and use that with LLC to get maximum benefits. <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 know this is not the typical “linux” scenario (dynamic linking of not only standard libraries,</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 also sometimes even application libraries, and lots of static linking because of 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">size), but it is a typical “embedded” scenario, which is where I am currently.</span><o:p></o:p></p>
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<p class="MsoNormal">I don't understand this use case but we can have further improvement in IPRA for example if you have several libraries which has already compiled and codegen, but you are able to provide information of register usage for the functions of
that libraries than we can think about an approach were we can store register usage information into a file (which will obviously increase compile time) and use that information across different compiler instances so that we can provide register usage information
with out having actual code while compiling.<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">Other thoughts or comments ?</span><o:p></o:p></p>
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<p class="MsoNormal">I am looking for some ideas that can improve current IPRA. So if you feel anything relevant please let me know we can discuss and implement feasible ideas.<o:p></o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Vivek <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"><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">vivek
pandya [mailto:</span><a href="mailto:vivekvpandya@gmail.com" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:purple">vivekvpandya@gmail.com</span></a><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">]<span class="apple-converted-space"> </span><br>
<b>Sent:</b><span class="apple-converted-space"> </span>Wednesday, July 06, 2016 2:09 PM<br>
<b>To:</b><span class="apple-converted-space"> </span>llvm-dev <</span><a href="mailto:llvm-dev@lists.llvm.org" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:purple">llvm-dev@lists.llvm.org</span></a><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">>;<span class="apple-converted-space"> </span></span><a href="mailto:llvm-dev-request@lists.llvm.org" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:purple">llvm-dev-request@lists.llvm.org</span></a><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">;
Lawrence, Peter <</span><a href="mailto:c_plawre@qca.qualcomm.com" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:purple">c_plawre@qca.qualcomm.com</span></a><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">><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"> <o:p></o:p></p>
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<p class="MsoNormal">Hello Peter,<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">Thanks to pointing out this interesting case. <o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Vivek,<br>
I have an application where many of the leaf functions are<br>
Hand-coded assembly language, because they use special IO instructions<br>
That only the assembler knows about. These functions typically don't<br>
Use any registers besides the incoming argument registers, IE they don't<br>
Need to use any additional callee-save nor caller-save registers.<o:p></o:p></p>
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<p class="MsoNormal">If inline asm template has specified clobbered list properly than IPRA is able to use that information and it propagates correct register mask (and that also means that skipping clobbers list while IPRA enabled may broke executable)<o:p></o:p></p>
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<p class="MsoNormal">For example in following code:<o:p></o:p></p>
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<p class="MsoNormal">int gcd( int a, int b ) {<o:p></o:p></p>
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<p class="MsoNormal"> int result ;<o:p></o:p></p>
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<p class="MsoNormal"> /* Compute Greatest Common Divisor using Euclid's Algorithm */<o:p></o:p></p>
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<p class="MsoNormal"> __asm__ __volatile__ ( "movl %1, %%r15d;"<o:p></o:p></p>
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<p class="MsoNormal"> "movl %2, %%ecx;"<o:p></o:p></p>
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<p class="MsoNormal"> "CONTD: cmpl $0, %%ecx;"<o:p></o:p></p>
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<p class="MsoNormal"> "je DONE;"<o:p></o:p></p>
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<p class="MsoNormal"> "xorl %%r13d, %%r13d;"<o:p></o:p></p>
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<p class="MsoNormal"> "idivl %%ecx;"<o:p></o:p></p>
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<p class="MsoNormal"> "movl %%ecx, %%r15d;"<o:p></o:p></p>
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<p class="MsoNormal"> "movl %%r13d, %%ecx;"<o:p></o:p></p>
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<p class="MsoNormal"> "jmp CONTD;"<o:p></o:p></p>
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<p class="MsoNormal"> "DONE: movl %%r15d, %0;" : "=g" (result) : "g" (a), "g" (b) : "ecx" ,"r13", "r15"<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"> return result ;<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 calculates and propagates correct regmask in which it marks CH, CL, ECX .. clobbered and R13, R15 is not marked clobbered as it is callee saved and LLVM code generators also insert spill/restores code for them.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">Is there any way in your IPRA interprocedural register allocation project that<br>
The user can supply this information for external functions ?<o:p></o:p></p>
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<p class="MsoNormal">By external word do you here mean function defined in other module than being used? In that case as IPRA can operate on only one module at time register usage propagation is not possible. But there is a work around for this problem. You
can use IPRA with link time optimization enabled because the way LLVM LTO works it creates a big IR modules out of source files and them optimize and codegen it so in that case IPRA can have actual register usage info (if function will be compiled in current
module). <o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">In case you want to experiment with IPRA please apply <a href="http://reviews.llvm.org/D21395" target="_blank"><span style="color:purple">http://reviews.llvm.org/D21395</span></a><span class="apple-converted-space"> </span>this patch before
you begin.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">-Vivek<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">Perhaps using some form of __attribute__ ?<br>
Maybe __attribute__ ((registermask = ....)) ?<br>
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--Peter Lawrence.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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