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<p class="MsoNormal"><a name="_MailEndCompose"><span lang="EN-GB" style="font-size:10.0pt;font-family:"Verdana",sans-serif;color:#1F497D">Can you lift the level a little?<o:p></o:p></span></a></p>
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<p class="MsoNormal"><span lang="EN-GB" style="font-size:10.0pt;font-family:"Verdana",sans-serif;color:#1F497D">Why do you care whether an instruction is executed in a parallel region or not?<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB" style="font-size:10.0pt;font-family:"Verdana",sans-serif;color:#1F497D">What higher-level problem are you trying to solve?<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB" style="font-size:10.0pt;font-family:"Verdana",sans-serif;color:#1F497D">-- Jim<br>
<br>
James Cownie <james.h.cownie@intel.com><br>
SSG/DPD/TCAR (Technical Computing, Analyzers and Runtimes)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:10.0pt;font-family:"Verdana",sans-serif;color:#1F497D">Tel: +44 117 9071438</span><span lang="EN-GB" 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"> openmp-dev-bounces@cs.uiuc.edu [mailto:openmp-dev-bounces@cs.uiuc.edu]
<b>On Behalf Of </b>Simone Atzeni<br>
<b>Sent:</b> Wednesday, February 11, 2015 5:15 PM<br>
<b>To:</b> Ronan KERYELL<br>
<b>Cc:</b> openmp-dev@dcs-maillist2.engr.illinois.edu<br>
<b>Subject:</b> Re: [Openmp-dev] Knowing if an IR instruction will be executed by an OpenMP region<o:p></o:p></span></p>
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<p class="MsoNormal">Hi Ronan,<o:p></o:p></p>
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<p class="MsoNormal">thanks for you answer.<o:p></o:p></p>
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<p class="MsoNormal">I think I will try to implement your first suggestion, some inter procedural control flow analysis.<o:p></o:p></p>
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<p class="MsoNormal">I would like to do it statically and not involving any dynamic analysis.<o:p></o:p></p>
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<p class="MsoNormal">My basic idea is that given an instruction we can check if it belongs or not to an OpenMP microtask, if yes it will be executed<o:p></o:p></p>
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<p class="MsoNormal">in parallel, if not I recursively check if its parent function is called inside an OpenMP construct and so on.<o:p></o:p></p>
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<p class="MsoNormal">In order to do this I was taking a look to llvm::CallGraph (<a href="http://llvm.org/docs/doxygen/html/classllvm_1_1CallGraph.html#details">http://llvm.org/docs/doxygen/html/classllvm_1_1CallGraph.html#details</a>) <o:p></o:p></p>
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<p class="MsoNormal">do you think it could help?<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.<o:p></o:p></p>
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<p class="MsoNormal">Best,<o:p></o:p></p>
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<p class="MsoNormal">Simone<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Simone Atzeni<o:p></o:p></p>
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<p class="MsoNormal"><a href="mailto:simone.at@gmail.com">simone.at@gmail.com</a><o:p></o:p></p>
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<p class="MsoNormal">+1 (801) 696-8373<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 Feb 11, 2015, at 10:05, Ronan KERYELL <<a href="mailto:ronan-list@keryell.fr">ronan-list@keryell.fr</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Tue, 10 Feb 2015 11:19:39 +0000, "Cownie, James H" <<a href="mailto:james.h.cownie@intel.com">james.h.cownie@intel.com</a>> said:<o:p></o:p></p>
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<p class="MsoNormal">is there a way to understand if an LLVM IR instruction will be<br>
executed by an OpenMP constructs?<o:p></o:p></p>
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James> The simple answer is "No", you can't in general tell at<br>
James> compile time whether code will be executed in parallel. (It<br>
James> could be in a separate compilation unit, or even a dynamic<br>
James> library which is only dlopened at runtime).<br>
<br>
Yes, this is why the OpenMP API provides functions like<br>
int omp_in_parallel(void);<br>
to test it.<br>
<br>
James> Even without those practical examples, even knowing whether a<br>
James> given IR instruction will ever be executed is clearly the<br>
James> same as the halting problem...<br>
<br>
This is true but let's focus on the cases when we know it is not<br>
intractable... :-)<br>
<br>
You can apply some interprocedural control flow analysis in the compiler<br>
and in some cases be sure it is executed or not in parallel. Doing this<br>
in the LTO phase is a way to go.<br>
<br>
Another more pragmatic way is to rewrite some code bloc B by cloning it<br>
in the following way:<br>
<br>
if omp_in_parallel() {<br>
// We know that here B is executed in a parallel context<br>
B<br>
}<br>
else {<br>
// We know that here B is not executed in a parallel context<br>
B<br>
}<br>
and then go on with some optimizations on both paths.<br>
<br>
So this is a dynamic+static approach which may worth it if B is big enough.<br>
<br>
You may also use some partial evaluation late in the compilation phases<br>
to figure out that in some case omp_in_parallel() returns always true or<br>
false and then remove the dead code.<br>
-- <br>
Ronan KERYELL<br>
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