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<p class="MsoNormal">Thanks for the feedbacks and questions, Wei. See my <span style="color:#4472C4">
replies inline</span>.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">Wei Mi <wmi@google.com><br>
<b>Date: </b>Friday, August 7, 2020 at 5:32 PM<br>
<b>To: </b>Wenlei He <wenlei@fb.com><br>
<b>Cc: </b>"llvm-dev@lists.llvm.org" <llvm-dev@lists.llvm.org>, Xinliang David Li <davidxl@google.com>, Hongtao Yu <hoy@fb.com><br>
<b>Subject: </b>Re: [RFC] Context-sensitive Sample PGO with Pseudo-Instrumentation<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Times New Roman",serif">Thanks for the proposal and the performance improvement over existing AutoFDO is impressive. <o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Fri, Aug 7, 2020 at 11:28 AM Wenlei He <<a href="mailto:wenlei@fb.com" target="_blank">wenlei@fb.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span style="color:black">Hi All,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">Our team at Facebook is building a new context-sensitive Sample PGO as an alternative to the existing AutoFDO. We’d like
to share our motivation, propose a new design, and reveal preliminary results on benchmarks. We will refer to the proposed design as CSSPGO in this RFC.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">The new CSSPGO leverages simultaneous LBR and stack sampling to construct a full context-sensitive profile. It doesn’t rely
on previous inlining like today’s AutoFDO to get context-sensitive profile, and it also doesn’t need a separate post-inline context-sensitive profile like CSPGO. In addition, we introduced pseudo-instrumentation for more accurate mapping from binary samples
back to IR, similar to instrumentation PGO, but without any measure-able runtime overhead that is usually associated with instrumentation.</span> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">We have a functioning implementation for the new CSSPGO now. Initial results on SPEC2006 shows ~2% geomean performance win
on top of AutoFDO (with MonoLTO and NewPM) and ~4% .text size reduction at the same time.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:4.0pt"><b><span style="font-size:17.0pt;font-family:"Arial",sans-serif;color:black">Motivation</span></b><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#1C1E21">AutoFDO is a big success as it lowers the entry barrier for PGO significantly while still delivering substantial performance
boost. However, there’s still a gap between AutoFDO and its instrumentation counterpart. From several failed internal attempts to improve AutoFDO, we realized that the bottleneck of AutoFDO lies in its profile quality. With the current level of profile quality,
it’s difficult to reap more performance win because good heuristics are often limited by inferior profile. That prompted a systemic effort to investigate and improve AutoFDO framework. Specifically, we’re trying to handle the two biggest sources of profile
quality issues:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<ol style="margin-top:0in" start="1" type="1">
<li class="MsoNormal" style="color:black;margin-top:12.0pt;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1;vertical-align:baseline">
<span style="font-family:"Arial",sans-serif">AutoFDO relies on a limited context-sensitive profile collected based on previous inlining. Therefore it can only replay or prune the previous inlining. With the main CGSCC inliner, post-inline counts are not accurate
due to scaling of context-less profile, which affects the effectiveness of later passes such as profile-guided code layout.</span><o:p></o:p></li><li class="MsoNormal" style="color:black;mso-margin-top-alt:auto;margin-bottom:12.0pt;mso-list:l0 level1 lfo1;vertical-align:baseline">
<span style="font-family:"Arial",sans-serif">Dwarf line and discriminator info aren’t always well-maintained throughout the compilation, thus using them as anchors to map binary samples back to the IR can sometimes be inaccurate, which leads to inferior profile
quality and limits PGO performance.</span><o:p></o:p></li></ol>
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<p class="MsoNormal"><span style="font-family:"Times New Roman",serif">Acknowledge to issues. We also found an issue that current AFDO profile doesn't keep edge information and that leads to nonoptimal profile in some cases. Since profile format is needed to
be redesigned for component 1, I am thinking whether it is possible to extend the profile format in a way so it can incorporate edge information as well.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Times New Roman",serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="color:#4472C4">[wenlei] Yes, we have implemented an “add-on” that could encode edges in addition to probes/blocks in .pseudo_probe section, and we also have a way to represent edges in new profile. But that’s not critical for
the framework and initial evaluation, which is why it’s not mentioned in this RFC. We did that mostly for enabling offline count inference algorithm experiments. We will share more details on that later. Curious what is the issue you saw due to lack of edge
info?<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Times New Roman",serif">About pseudo probe, seemly you doesn't mention in this proposal but does it still provides the ability to solve the source drift issue you mentioned before? If it does, how it is achieved?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="color:#4472C4">[wenlei] Pseudo-probe handles source drift reasonably well, and has good resilience against source changes. It can tolerate any source changes that doesn’t alter CFG, so the issues we ran into with line-based
approach where deleting a comment lead to big regression isn’t going to happen with pseudo-probe. For changes that does alter CFG, we could also employ fuzzy CFG matching in future. Bottom line is using probe and CFG as profile carrier inherently has richer
info, so it’s easier for PGO to see through the source changes and can still make sense of a stale profile. (We didn’t expand on the source drift issue in initial RFC, but I just mentioned that part in my reply to David, as secondary motivation for pseudo-probe.)<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
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<o:p> </o:p></li></ol>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">To lift the above limitations, we’d like to propose an alternative design that consists of two components: 1) Context-sensitive
sample PGO, 2) Sample to IR mapping using pseudo probes. The goal is to further improve sample PGO performance while maintaining usability and keeping training runtime overhead at zero. In addition, we hope the CSSPGO framework can also open up opportunities
for new optimizations with more stringent requirements on profile quality. </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"><span style="font-family:"Times New Roman",serif">I like both ideas, and those two components can be orthogonal? For the first component, I hope the existing debug information based AutoFDO can be benefited from it as well, with some extension
to the current profile format. <o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="color:#4472C4">[wenlei] Thanks. Yes, they’re orthogonal. But we need both for peak performance, and we want to focus tuning effort on the combination. Also see my reply to David’s questions.<o:p></o:p></span></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:4.0pt"><b><span style="font-size:17.0pt;font-family:"Arial",sans-serif;color:black">Context-sensitive Sample PGO</span></b><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">The effectiveness of BOLT, Propeller and CSPGO all demonstrated the importance of context-sensitive profile for PGO. However
there are two limitations with the existing approaches.</span><o:p></o:p></p>
<ol style="margin-top:0in" start="1" type="1">
<li class="MsoNormal" style="color:black;margin-top:12.0pt;mso-margin-bottom-alt:auto;mso-list:l2 level1 lfo3;vertical-align:baseline">
<span style="font-family:"Arial",sans-serif">The current solutions focus on leveraging a context-sensitive profile to attain an accurate post-inline profile that helps achieve a better code layout, but do not use the context-sensitive profile to drive better
inlining.</span><o:p></o:p></li><li class="MsoNormal" style="color:black;mso-margin-top-alt:auto;margin-bottom:12.0pt;mso-list:l2 level1 lfo3;vertical-align:baseline">
<span style="font-family:"Arial",sans-serif">The current solutions involve multiple training processes and profiles (e.g. a post-inline profile for CSPGO, or a post-link profile for BOLT and Propeller), which incurs higher operational cost and complicates the
build and release workflow.</span><o:p></o:p></li></ol>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">We propose a full context-sensitive sample profiling infrastructure that utilizes both LBR and call stack samples at the
same time to synthesize a profile with a full context sensitivity. The key advantage is that rather than relying on previous inlining or a separate profile, the profile collected with the new approach will have full calling contexts recovered from both inlined
and not inlined call sites. To achieve an accurate post-inline profile, a separate profile is no longer needed. Instead, the post-inline profile can be directly derived from adjusting the input profile based on all inline decisions. The richer context-sensitive
profile also enables better inline decisions. The infrastructure has two key components listed below.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:4.0pt"><b><span style="font-size:13.0pt;font-family:"Arial",sans-serif;color:black">Synthesizing context-sensitive LBR with a virtual unwinder</span></b><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">To make sample PGO’s input profile context aware, we need to know the call stack of each LBR fall through path. That is
done by sampling LBR and call stack simultaneously. With that, each sample will contain a call stack in addition to LBR entries. We use level 2 PEBS to control sampling skid so that the leaf frame from stack sample aligns with leaf frame from LBR. The raw
call stack sample describes the calling context for the leaf LBR entry. In addition, by unwinding “call” and “return” (including implicit ones from inlinee) from LBR entries backwards on top of raw stack samples, we can recover the calling context for each
of the LBR entries from the sample, thus synthesizing context-sensitive LBR profile.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Times New Roman",serif">What if the stack unwinding is not intact? For example, tail call optimization may cause unwinding issue currently in perf. framepointer or call frame information may not be properly maintained. <o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="color:#4472C4">[wenlei] That’s a good question. Currently, we have frame pointer optimization (FPO) and tail call optimization disabled for experiments. FPO is disabled for our production builds as well, so it’s not a problem
for us. For tail call, we’ll need to evaluate the cost-benefit and see what we can do. We know there’s heuristic to recover single missing frame due to tail call, which we haven’t implemented yet; beyond that, perhaps we can revisit leveraging dwarf unwinding,
or live with either imperfect profile or tail call disabled. We also implemented special case for sample that lands in prolog and epilog where frame chain isn’t ready. However, even with both FPO and tail call disabled, we still see truncated stack samples,
which we’re investigating. But the perf results are with profiles containing truncated/imperfect stack samples, so it looks like a small portion of imperfect profile doesn’t impact the effectiveness of CSSPGO too much.
<o:p></o:p></span></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">We can then generate context-sensitive sample PGO profile using the context-sensitive LBR profile. In the new profile, a
function’s profile becomes a collection of profiles, each representing a profile for a given calling context.</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"><span style="font-family:"Times New Roman",serif">Will the profile size be significantly larger?<o:p></o:p></span></p>
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<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="color:#4472C4">[wenlei] Currently text CS profile is 1-10x larger. But there’re ways to bring them it and we’re working on it: 1) trim cold context, 2) leverage compression from extended binary (should be effective for context
strings that has duplicated long C++ mangle names), 3) consider fixed-length integer context presentation, e.g. rolling hash. Also see my replies to David’s question on this.<o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:4.0pt"><b><span style="font-size:13.0pt;font-family:"Arial",sans-serif;color:black">Context-sensitive FDO/PGO framework in LLVM</span></b><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">In order to leverage context-sensitive profile for inlining, and to maintain accurate post-inline counts, we introduced </span><span style="font-family:"Courier New";color:black">SampleContextTracker</span><span style="font-family:"Arial",sans-serif;color:black"> which
is a layer sitting in between input profile and the profile used to annotate CFG for optimizations. We also introduced the notion of base profile which </span><span style="font-family:"Arial",sans-serif;color:#1C1E21">is the merged profile for function’s profiles
from any outstanding (not inlined) context, </span><span style="font-family:"Arial",sans-serif;color:black">and context profile which is a function's profile for a given calling context. The framework includes four simple APIs for updating and query profiles:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">Query API:</span><o:p></o:p></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoNormal" style="color:black;margin-top:12.0pt;mso-margin-bottom-alt:auto;mso-list:l3 level1 lfo4;vertical-align:baseline">
<span style="font-family:"Courier New"">getBaseSamplesFor</span><span style="font-family:"Arial",sans-serif">: Query base profile by function name.</span><o:p></o:p></li><li class="MsoNormal" style="color:black;mso-margin-top-alt:auto;margin-bottom:12.0pt;mso-list:l3 level1 lfo4;vertical-align:baseline">
<span style="font-family:"Courier New"">getContextSamplesFor</span><span style="font-family:"Arial",sans-serif">: Query context profile by calling context and function name.</span><o:p></o:p></li></ul>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">Update API:</span><o:p></o:p></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoNormal" style="color:black;margin-top:12.0pt;mso-margin-bottom-alt:auto;mso-list:l4 level1 lfo5;vertical-align:baseline">
<span style="font-family:"Courier New"">MarkContextSamplesInlined</span><span style="font-family:"Arial",sans-serif">: When a function is inlined for a given calling context, we need to mark the context profile for that context as inlined. This is to make sure
we don't include inlined context profile when synthesizing the base profile.</span><o:p></o:p></li><li class="MsoNormal" style="color:black;mso-margin-top-alt:auto;margin-bottom:12.0pt;mso-list:l4 level1 lfo5;vertical-align:baseline">
<span style="font-family:"Courier New"">PromoteMergeContextSamplesTree</span><span style="font-family:"Arial",sans-serif">: When a function is not inlined for a given calling context, we need to promote the context profile tree to be top-level context. This
preserves the child context under that function so later inline decisions for calls originating from that not inlined function will still be driven by an accurate context profile.</span><o:p></o:p></li></ul>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">These APIs are used by </span><span style="font-family:"Courier New";color:black">SampleProfileLoader</span><span style="font-family:"Arial",sans-serif;color:black">’s
inlining, for better inline decisions and better post-inline counts. For optimal results, the new infrastructure needs to work with a top-down FDO inliner. We added top-down FDO inlining under a switch, and the switch is turned on by default in upstream recently.
There’re a few other improvements for the FDO inliner that we plan to upstream soon.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:4.0pt"><b><span style="font-size:17.0pt;font-family:"Arial",sans-serif;color:black">Pseudo-instrumentation for sample to IR mapping</span></b><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">Being able to profile production binaries is a key advantage of AutoFDO over Instrumentation PGO, but it also comes with
a big challenge. While using line number and discriminator as anchor for profile mapping incurs zero run time overhead for AutoFDO, it’s not as accurate as instrumented probes. This is because the instrumented probes are part of the IR, rather than metadata
attached to the IR like </span><span style="font-family:"Courier New";color:black">!dbg</span><span style="font-family:"Arial",sans-serif;color:black">. That has two implications: 1) it’s easier to maintain IR than metadata for optimization passes; 2) probe
blocks some CFG transformations that can mess up profile correlation.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">With the proposed pseudo instrumentation, we can achieve most of the benefit of instrumentation PGO in little runtime overhead.
We instrument each basic block with a pseudo probe associated with the block Id. Unlike in PGO instrumentation where a counter is implemented as a persisting operation such as atomic read/write or runtime helper call, a pseudo probe is implemented as a dedicated
intrinsic call with </span><span style="font-family:"Courier New";color:black">IntrInaccessibleMemOnly</span><span style="font-family:"Arial",sans-serif;color:black"> flag. The intrinsic comes with </span><span style="font-family:"Arial",sans-serif;color:#1C1E21">most
of the semantics of a PGO </span><span style="font-family:"Arial",sans-serif;color:black">counter</span><span style="font-family:"Arial",sans-serif;color:#1C1E21"> but is much less optimization-</span><span style="font-family:"Arial",sans-serif;color:black">intrusive. </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">The pseudo probe intrinsic calls are on the IR throughout the optimization and code generation pipeline and are materialized
as a piece of binary data stored in a separate </span><span style="font-family:"Courier New";color:black">.pseudo_probe</span><span style="font-family:"Arial",sans-serif;color:black"> data section. The section is then used to map binary samples back to blocks
of CFG during profile generation. There are also no real machine instructions generated for a pseudo probe and the</span><span style="font-family:"Courier New";color:black">.pseudo_probe</span><span style="font-family:"Arial",sans-serif;color:black"> section
won’t be loaded into memory at runtime, therefore they should incur very little runtime overhead. As a fact, we see no measure-able performance impact from pseudo-instrumentation itself on SPEC2006 or big internal workload.</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:4.0pt"><b><span style="font-size:13.0pt;font-family:"Arial",sans-serif;color:black">Pseudo-instrumentation integration and Pass Ordering</span></b><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">One implication from pseudo-probe instrumentation is that the profile is now sensitive to CFG changes. We now defect stale
profiles for sample PGO via CFG checksum, instead of just using it. However, the potential downside is that CFG may change between different versions of the compiler even if the source code is unchanged. To solve that problem, we perform the pseudo instrumentation
very early in the pre-LTO pipeline, before any CFG transformation. This ensures that the CFG instrumented and annotated is stable. We added </span><span style="font-family:"Courier New";color:black">SampleProfileProber</span><span style="font-family:"Arial",sans-serif;color:black"> that
performs the pseudo instrumentation and runs independent of profile annotation.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">A new switch </span><span style="font-family:"Courier New";color:black">-fpseudo-probe-for-profiling</span><span style="font-family:"Arial",sans-serif;color:black"> is
added to enable sample PGO with pseudo instrumentation, similar to </span><span style="font-family:"Courier New";color:black">-fdebug-info-for-profiling</span><span style="font-family:"Arial",sans-serif;color:black"> for AutoFDO. Input profile is still provided
through the same switch used by today’s AutoFDO, namely </span><span style="font-family:"Courier New";color:black">-fprofile-sample-use</span><span style="font-family:"Arial",sans-serif;color:black">, and the profile loader will automatically decide how to
load and annotate profile depending on whether input profile is dwarf-based or pseudo-probe based.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:4.0pt"><b><span style="font-size:17.0pt;font-family:"Arial",sans-serif;color:black">New profile format and profile generation</span></b><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">We extend current profile format in order to be able to represent a full context-sensitive profile and also encode pseudo-probe
info. This is done without drastically diverging from today’s AutoFDO profile format so that existing tools and libraries can be reused with minor changes (e.g. </span><span style="font-family:"Courier New";color:black">llvm-profdata</span><span style="font-family:"Arial",sans-serif;color:black">,
profiler reader and writer).</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">For a context-sensitive profile, we extend the profile format by changing the function profile header line to include calling
context in addition to function name. With today’s AutoFDO, we have a single profile header for each function to represent its accumulative profile. E.g. This is the profile header for </span><span style="font-family:"Courier New";color:black">foo</span><span style="font-family:"Arial",sans-serif;color:black">,
with 1290 total samples, and 74 header samples.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Courier New";color:black">foo:1290:74</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">For CSSPGO, we will have multiple profile headers for a single function’s profile, each representing profile for a specific
calling context as shown below. CSSPGO profile header is bracketed to differentiate from today’s AutoFDO.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Courier New";color:black">[main:12 @ bar:3 @ foo]:279:54</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Courier New";color:black">[main:19 @ zoo:7 @ foo]:1011:20</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">With calling context encoded in the function header, we no longer need a nested function profile for inlinees. Instead,
a context profile will be represented uniformly using context strings in the function profile header, regardless of whether the calls in the context are inlined or not. The flat structure makes sure that context profile is easily indexable. The change is mostly
transparent to tools like </span><span style="font-family:"Courier New";color:black">llvm-profdata</span><span style="font-family:"Arial",sans-serif;color:black">. Support for binary profile format has not been added yet, but should be easy to do.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">For pseudo-probe, we repurposed the line to count map of AutoFDO profile to be block Id to count map. This only changes
the interpretation of profile content rather than the representation, hence all reader/writer helpers can be reused.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">There's a new profile generation tool, </span><span style="font-family:"Courier New";color:black">llvm-profgen</span><span style="font-family:"Arial",sans-serif;color:black">,
with the virtual winder implemented for context-sensitive profiling, and uses the </span><span style="font-family:"Courier New";color:black">.pseudo_probe</span><span style="font-family:"Arial",sans-serif;color:black"> section to map binary profile to pre-opt
CFG profile. Since profile generation is a critical piece of the workflow, we’d like to propose to include the tool as part of LLVM, alongside with </span><span style="font-family:"Courier New";color:black">llvm-profdata</span><span style="font-family:"Arial",sans-serif;color:black">.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:4.0pt"><b><span style="font-size:17.0pt;font-family:"Arial",sans-serif;color:black">Preliminary Results</span></b><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">To quantitatively assess profile quality improvement brought by pseudo-instrumentation, we introduce a profile quality metric.
We measure the metric by first annotating an optimized binary with the MIR block execution counts derived from a profile. The binary is then sampled and the counts are compared against the annotation. The weighted relative delta is used as an indicator for
profile quality (lower is better). </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">Table below shows the profile quality metric for SPEC2006. We can see from the numbers that the profile quality of pseudo-instrumentation
sample PGO is much better than AutoFDO and close to instrumentation PGO.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span style="color:black"> </span><o:p></o:p></p>
<table class="MsoNormalTable" border="0" cellspacing="0" cellpadding="0" width="624" style="width:6.5in;border-collapse:collapse">
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<td valign="top" style="border:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-family:"Arial",sans-serif;color:black">Profile quality metric</span><o:p></o:p></p>
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<td valign="top" style="border:solid black 1.0pt;border-left:none;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-family:"Arial",sans-serif;color:black">Baseline AutoFDO</span><o:p></o:p></p>
</td>
<td valign="top" style="border:solid black 1.0pt;border-left:none;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-family:"Arial",sans-serif;color:black">Instrumentation PGO</span><o:p></o:p></p>
</td>
<td valign="top" style="border:solid black 1.0pt;border-left:none;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:black">Sample PGO w/ Pseudo Instrumentation</span><o:p></o:p></p>
</td>
</tr>
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<td valign="top" style="border:solid black 1.0pt;border-top:none;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">SPEC2006</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="right" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:right">
<span style="font-family:"Arial",sans-serif;color:black">24.58%</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="right" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:right">
<span style="font-family:"Arial",sans-serif;color:black">15.70%</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="right" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:right">
<span style="font-family:"Arial",sans-serif;color:black">16.21%</span><o:p></o:p></p>
</td>
</tr>
</tbody>
</table>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">We also measured performance and code size on SPEC2006 with CSSPGO. The measurement was done with MonoLTO and new pass manager,
with tuning for FDO inliner to accommodate context-sensitive profile, and used training dataset for both pass1 and pass2. The result shows ~2% performance win on top of today’s AutoFDO, with ~4% .text reduction, see table below. </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<table class="MsoNormalTable" border="0" cellspacing="0" cellpadding="0" width="624" style="width:6.5in;border-collapse:collapse">
<tbody>
<tr style="height:21.0pt">
<td rowspan="2" valign="top" style="border:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt;height:21.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-family:"Arial",sans-serif;color:black">SPEC2006</span><o:p></o:p></p>
</td>
<td colspan="3" valign="top" style="border:solid black 1.0pt;border-left:none;padding:5.0pt 5.0pt 5.0pt 5.0pt;height:21.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-family:"Arial",sans-serif;color:black">Performance</span><o:p></o:p></p>
</td>
<td colspan="3" valign="top" style="border:solid black 1.0pt;border-left:none;padding:5.0pt 5.0pt 5.0pt 5.0pt;height:21.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-family:"Arial",sans-serif;color:black">Code Size</span><o:p></o:p></p>
</td>
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<tr style="height:21.0pt">
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt;height:21.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">AutoFDO over LTO</span><o:p></o:p></p>
</td>
<td width="79" valign="top" style="width:59.0pt;border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt;height:21.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">CSSPGO</span><o:p></o:p></p>
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">Over LTO</span><o:p></o:p></p>
</td>
<td width="89" valign="top" style="width:66.55pt;border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt;height:21.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:black">CSSPGO over AutoFDO</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt;height:21.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">AutoFDO over LTO</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt;height:21.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">CSSPGO</span><o:p></o:p></p>
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">Over LTO</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt;height:21.0pt">
<p class="MsoNormal" align="center" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:center">
<span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:black">CSSPGO over AutoFDO</span><o:p></o:p></p>
</td>
</tr>
<tr>
<td valign="top" style="border:solid black 1.0pt;border-top:none;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">Geomean Delta %</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="right" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:right">
<span style="font-family:"Arial",sans-serif;color:#6AA84F">6.80%</span><o:p></o:p></p>
</td>
<td width="79" valign="top" style="width:59.0pt;border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="right" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:right">
<span style="font-family:"Arial",sans-serif;color:#6AA84F">8.70%</span><o:p></o:p></p>
</td>
<td width="89" valign="top" style="width:66.55pt;border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="right" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:right">
<span style="font-family:"Arial",sans-serif;color:#6AA84F">2.04%</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="right" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:right">
<span style="font-family:"Arial",sans-serif;color:#CC0000">11.17%</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="right" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:right">
<span style="font-family:"Arial",sans-serif;color:#CC0000">6.66%</span><o:p></o:p></p>
</td>
<td valign="top" style="border-top:none;border-left:none;border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:5.0pt 5.0pt 5.0pt 5.0pt">
<p class="MsoNormal" align="right" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:right">
<span style="font-family:"Arial",sans-serif;color:#CC0000">4.06%</span><o:p></o:p></p>
</td>
</tr>
</tbody>
</table>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">While the SPEC2006 benchmark suite is different from large workloads, we think the results demonstrated the potential of
CSSPGO and served its purpose for proof of concept. We plan to continue tuning and start to evaluate larger internal workloads soon, and we’d like to upstream our work. Feedbacks are welcomed!</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">Thanks,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:black">Wenlei & Hongtao</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:black"> </span><o:p></o:p></p>
</div>
</div>
</blockquote>
</div>
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