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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Hi,<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 have updated the review here:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><a href="https://reviews.llvm.org/D21723">https://reviews.llvm.org/D21723</a><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">As Sameer pointed out, the motivation is:<o:p></o:p></span></p>
<p class="MsoNormal">In OpenCL 2.x, two atomic operations on the same atomic object need to have the same scope to prevent a data race. This derives from the definition of "inclusive scope" in OpenCL 2.x. Encoding OpenCL 2.x scope as metadata in LLVM IR would
be a problem because there cannot be a "safe default value" to be used when the metadata is dropped. If the "largest" scope is used as the default, then the optimizer must guarantee that the metadata is dropped from every atomic operation in the whole program,
or not dropped at all.<o:p></o:p></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">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Konstantin<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"><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"> Sameer Sahasrabuddhe [mailto:sameer@sbuddhe.net]
<br>
<b>Sent:</b> Sunday, July 10, 2016 4:06 AM<br>
<b>To:</b> Philip Reames <listmail@philipreames.com><br>
<b>Cc:</b> Mehdi Amini <mehdi.amini@apple.com>; Liu, Yaxun (Sam) <Yaxun.Liu@amd.com>; Ke Bai <kebai613@gmail.com>; Mekhanoshin, Stanislav <Stanislav.Mekhanoshin@amd.com>; Sumner, Brian <Brian.Sumner@amd.com>; llvm-dev@lists.llvm.org; Zhuravlyov, Konstantin
<Konstantin.Zhuravlyov@amd.com>; Tye, Tony <Tony.Tye@amd.com><br>
<b>Subject:</b> Re: [llvm-dev] Memory scope proposal<o:p></o:p></span></p>
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<p class="MsoNormal">On Mon, Jul 4, 2016 at 5:09 AM, Philip Reames via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">I will comment - as one of the few people actually working on llvm's atomic implementation with any regularity - that I am opposed to extending the instructions without a strong motivating case. I don't care anywhere near as much about
metadata based schemes, but extending the instruction semantics imposes a much larger burden on the rest of the community. That burden has to be well justified and supported. <o:p></o:p></p>
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In OpenCL 2.x, two atomic operations on the same atomic object need to have the same scope to prevent a data race. This derives from the definition of "inclusive scope" in OpenCL 2.x. Encoding OpenCL 2.x scope as metadata in LLVM IR would be a problem because
there cannot be a "safe default value" to be used when the metadata is dropped. If the "largest" scope is used as the default, then the optimizer must guarantee that the metadata is dropped from every atomic operation in the whole program, or not dropped at
all.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Hence the original attempt to extend LLVM atomic instructions with a broader scope field.
<o:p></o:p></p>
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<p class="MsoNormal">Sameer.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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