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<p style="margin-top:0;margin-bottom:0">After an IRC discussion, I think Alexey and I are pretty much in agreement (on the general feasibility at least).</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0">I try to sketch the proposed idea again below, as the initial RFC was simply not descriptive enough.</p>
<p style="margin-top:0;margin-bottom:0">After that, I shortly summarize how I see these changes being developed and committed so that we</p>
<p style="margin-top:0;margin-bottom:0"> - never have any regressions,</p>
<p style="margin-top:0;margin-bottom:0"> - can make an educated decision before removing any existing code.</p>
<p></p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0">What we want to do:</p>
<p style="margin-top:0;margin-bottom:0">The intermediate goal is that the code generated by clang for the SPMD and non-SPMD (earlier denoted as "guarded") case</p>
<p style="margin-top:0;margin-bottom:0">is conceptually/structurally very similar. The current non-SPMD code is, however, a state machine generated into the user code
<br>
</p>
<p style="margin-top:0;margin-bottom:0">module. This state machine is very hard to analyze and optimize. If the code would look as the SPMD code but *behave the same</p>
<p style="margin-top:0;margin-bottom:0">way it does now*, we could "easily" switch from non-SPMD to SPMD version after a (late) analysis determined legality. To make</p>
<p style="margin-top:0;margin-bottom:0">the code look the same but behave differently, we propose to hide the semantic difference in the runtime library calls. That is,
<br>
</p>
<p style="margin-top:0;margin-bottom:0">the runtime calls emitted in the two modes are (slightly) different, or there is a flag which indicates the (initial) mode. If that mode</p>
is SPMD, the runtime behavior does not change compared to the way it is now. If that mode is non-SPMD, the runtime would
<p></p>
<p style="margin-top:0;margin-bottom:0">separate the master and worker threads, as we do it now in the user code module, and keep the workers in an internal state machine</p>
<p style="margin-top:0;margin-bottom:0">waiting for the master to provide them with work. Only the master would return from the runtime call and the mechanism</p>
<p style="margin-top:0;margin-bottom:0">to distribute work to the worker threads would (for now) stay the same.</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
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</p>
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<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0">Preliminary implementation (and integration) steps:<br>
</p>
<p style="margin-top:0;margin-bottom:0">1) Design and implement the necessary runtime extensions and determine feasibility.
<br>
</p>
<p style="margin-top:0;margin-bottom:0">2) Allow to Clang codegen to use the new runtime extensions if explicitly chosen by the user.</p>
<p style="margin-top:0;margin-bottom:0">2b) Performance comparison unoptimized new code path vs. original code path on test cases and real use cases.<br>
</p>
<p style="margin-top:0;margin-bottom:0">3) Implement the middle-end pass to analyze and optimize the code using the runtime extensions.</p>
<p style="margin-top:0;margin-bottom:0">3b) Performance comparison optimized new code path vs. original code path on real use cases.</p>
<p style="margin-top:0;margin-bottom:0">4) If no conceptual problem was found and 2b)/3b) determined that the new code path is superior, switch to the</p>
<p style="margin-top:0;margin-bottom:0"> new code path by default.</p>
<p style="margin-top:0;margin-bottom:0">5) If no regressions/complaints are reported after a grace period, remove the old code path from the clang front-end.</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<br>
<p></p>
<p style="margin-top:0;margin-bottom:0">Again, this is an early design RFC for which I welcome any feedback!<br>
</p>
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</p>
<p></p>
<p style="margin-top:0;margin-bottom:0">Thanks,</p>
<p style="margin-top:0;margin-bottom:0"> Johannes<br>
</p>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> Doerfert, Johannes Rudolf<br>
<b>Sent:</b> Tuesday, January 22, 2019 1:50:51 PM<br>
<b>To:</b> Alexey Bataev<br>
<b>Cc:</b> cfe-dev@lists.llvm.org; openmp-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [RFC] Late (OpenMP) GPU code "SPMD-zation"</font>
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<p style="margin-top:0; margin-bottom:0">What do you refer to with: "No, we don't". </p>
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</p>
<p style="margin-top:0; margin-bottom:0">Again, I do not propose to remove the SPMD "detection" in Clang. We will still identify SPMD mode based on the syntactic criteria we have now.</p>
<p style="margin-top:0; margin-bottom:0">The Clang analysis is also not affected. Thus, we will globalize/localize the same variables as we do now. I don't see why this should be any different.</p>
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<div id="x_divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Alexey Bataev via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Sent:</b> Tuesday, January 22, 2019 1:46:39 PM<br>
<b>To:</b> Doerfert, Johannes Rudolf<br>
<b>Cc:</b> llvm-dev; cfe-dev@lists.llvm.org; openmp-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [llvm-dev] [RFC] Late (OpenMP) GPU code "SPMD-zation"</font>
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<p>No, we don't. We need to perform the different kind of the analysis for SPMD mode constructs and Non-SPMD.</p>
<p>For SPMD mode we need to globalize only reduction/lastprivate variables. For Non-SPMD mode, we need to globalize all the private/local variables, that may escape their declaration context in the construct.<br>
</p>
<pre class="x_x_moz-signature" cols="72">-------------
Best regards,
Alexey Bataev</pre>
<div class="x_x_moz-cite-prefix">22.01.2019 14:29, Doerfert, Johannes Rudolf пишет:<br>
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<div id="x_x_divtagdefaultwrapper" dir="ltr" style="">We would still know that. We can do exactly the same reasoning as we do now.
<br>
<p style="margin-top:0; margin-bottom:0">I think the important question is, how different is the code generated for either mode and can we hide (most of) the differences in the runtime.</p>
<p style="margin-top:0; margin-bottom:0"><br>
</p>
<p style="margin-top:0; margin-bottom:0">If I understand you correctly, you say the data sharing code looks very different and the differences cannot be hidden, correct?</p>
<p style="margin-top:0; margin-bottom:0">It would be helpful for me to understand your point if you could give me a piece of OpenMP for which the data sharing in SPMD mode and "guarded"</p>
<p style="margin-top:0; margin-bottom:0">mode are as different as possible. I can compile it in both modes myself so high-level OpenMP is fine (I will disable SPMD mode manually in the source if necessary).</p>
<p style="margin-top:0; margin-bottom:0"><br>
</p>
<p style="margin-top:0; margin-bottom:0">Thanks,</p>
<p style="margin-top:0; margin-bottom:0"> Johannes</p>
<p style="margin-top:0; margin-bottom:0"><br>
</p>
<br>
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<div id="x_x_divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> llvm-dev
<a class="x_x_moz-txt-link-rfc2396E OWAAutoLink" href="mailto:llvm-dev-bounces@lists.llvm.org" id="LPlnk150696" previewremoved="true">
<llvm-dev-bounces@lists.llvm.org></a> on behalf of Alexey Bataev via llvm-dev <a class="x_x_moz-txt-link-rfc2396E OWAAutoLink" href="mailto:llvm-dev@lists.llvm.org" id="LPlnk314770" previewremoved="true">
<llvm-dev@lists.llvm.org></a><br>
<b>Sent:</b> Tuesday, January 22, 2019 13:10<br>
<b>To:</b> Doerfert, Johannes Rudolf<br>
<b>Cc:</b> Alexey Bataev; LLVM-Dev; Arpith Chacko Jacob; <a class="x_x_moz-txt-link-abbreviated OWAAutoLink" href="mailto:openmp-dev@lists.llvm.org" id="LPlnk325008" previewremoved="true">
openmp-dev@lists.llvm.org</a>; <a class="x_x_moz-txt-link-abbreviated OWAAutoLink" href="mailto:cfe-dev@lists.llvm.org" id="LPlnk183851" previewremoved="true">
cfe-dev@lists.llvm.org</a><br>
<b>Subject:</b> Re: [llvm-dev] [RFC] Late (OpenMP) GPU code "SPMD-zation"</font>
<div> </div>
</div>
<div>
<div>But we need to know the execution mode, SPMD or "guarded"<br>
<br>
</div>
<pre class="x_x_x_moz-signature" cols="72">-------------
Best regards,
Alexey Bataev</pre>
<div class="x_x_x_moz-cite-prefix">22.01.2019 13:54, Doerfert, Johannes Rudolf пишет:<br>
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We could still do that in clang, couldn't we?<br>
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<div dir="auto" style="direction:ltr; margin:0; padding:0; font-family:sans-serif; font-size:11pt; color:black">
Get <a href="https://aka.ms/ghei36" id="LPlnk791863" class="x_x_OWAAutoLink" previewremoved="true">
Outlook for Android</a></div>
<br>
</div>
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<div id="x_x_x_divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> Alexey Bataev
<a class="x_x_x_moz-txt-link-rfc2396E x_x_OWAAutoLink" href="mailto:a.bataev@outlook.com" id="LPlnk710514" previewremoved="true">
<a.bataev@outlook.com></a><br>
<b>Sent:</b> Tuesday, January 22, 2019 12:52:42 PM<br>
<b>To:</b> Doerfert, Johannes Rudolf; <a class="x_x_x_moz-txt-link-abbreviated x_x_OWAAutoLink" href="mailto:cfe-dev@lists.llvm.org" id="LPlnk92484" previewremoved="true">
cfe-dev@lists.llvm.org</a><br>
<b>Cc:</b> <a class="x_x_x_moz-txt-link-abbreviated x_x_OWAAutoLink" href="mailto:openmp-dev@lists.llvm.org" id="LPlnk406132" previewremoved="true">
openmp-dev@lists.llvm.org</a>; LLVM-Dev; Finkel, Hal J.; Alexey Bataev; Arpith Chacko Jacob<br>
<b>Subject:</b> Re: [RFC] Late (OpenMP) GPU code "SPMD-zation"</font>
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<div>
<p>The globalization for the local variables, for example. It must be implemented in the compiler to get the good performance, not in the runtime.</p>
<p><br>
</p>
<pre class="x_x_x_moz-signature" cols="72">-------------
Best regards,
Alexey Bataev</pre>
<div class="x_x_x_moz-cite-prefix">22.01.2019 13:43, Doerfert, Johannes Rudolf пишет:<br>
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Could you elaborate on what you refer to wrt data sharing. What do we currently do in the clang code generation that we could not effectively implement in the runtime, potentially with support of an llvm pass.<br>
<br>
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Thanks,<br>
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<div dir="auto" style="direction:ltr; margin:0; padding:0; font-family:sans-serif; font-size:11pt; color:black">
James<br>
<br>
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<div dir="auto" style="direction:ltr; margin:0; padding:0; font-family:sans-serif; font-size:11pt; color:black">
Get <a href="https://aka.ms/ghei36" id="LPlnk975037" class="x_x_OWAAutoLink" previewremoved="true">
Outlook for Android</a></div>
<br>
</div>
<hr tabindex="-1" style="display:inline-block; width:98%">
<div id="x_x_x_divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> Alexey Bataev
<a class="x_x_x_moz-txt-link-rfc2396E x_x_OWAAutoLink" href="mailto:a.bataev@outlook.com" id="LPlnk323824" previewremoved="true">
<a.bataev@outlook.com></a><br>
<b>Sent:</b> Tuesday, January 22, 2019 12:34:01 PM<br>
<b>To:</b> Doerfert, Johannes Rudolf; <a class="x_x_x_moz-txt-link-abbreviated x_x_OWAAutoLink" href="mailto:cfe-dev@lists.llvm.org" id="LPlnk882142" previewremoved="true">
cfe-dev@lists.llvm.org</a><br>
<b>Cc:</b> <a class="x_x_x_moz-txt-link-abbreviated x_x_OWAAutoLink" href="mailto:openmp-dev@lists.llvm.org" id="LPlnk56115" previewremoved="true">
openmp-dev@lists.llvm.org</a>; LLVM-Dev; Finkel, Hal J.; Alexey Bataev; Arpith Chacko Jacob<br>
<b>Subject:</b> Re: [RFC] Late (OpenMP) GPU code "SPMD-zation"</font>
<div> </div>
</div>
<div>
<p><br>
</p>
<pre class="x_x_x_moz-signature" cols="72">-------------
Best regards,
Alexey Bataev</pre>
<div class="x_x_x_moz-cite-prefix">22.01.2019 13:17, Doerfert, Johannes Rudolf пишет:<br>
</div>
<blockquote type="cite">
<pre class="x_x_x_moz-quote-pre">Where we are
------------
Currently, when we generate OpenMP target offloading code for GPUs, we
use sufficient syntactic criteria to decide between two execution modes:
1) SPMD -- All target threads (in an OpenMP team) run all the code.
2) "Guarded" -- The master thread (of an OpenMP team) runs the user
code. If an OpenMP distribute region is encountered, thus
if all threads (in the OpenMP team) are supposed to
execute the region, the master wakes up the idling
worker threads and points them to the correct piece of
code for distributed execution.
For a variety of reasons we (generally) prefer the first execution mode.
However, depending on the code, that might not be valid, or we might
just not know if it is in the Clang code generation phase.
The implementation of the "guarded" execution mode follows roughly the
state machine description in [1], though the implementation is different
(more general) nowadays.
What we want
------------
Increase the amount of code executed in SPMD mode and the use of
lightweight "guarding" schemes where appropriate.
How we get (could) there
------------------------
We propose the following two modifications in order:
1) Move the state machine logic into the OpenMP runtime library. That
means in SPMD mode all device threads will start the execution of
the user code, thus emerge from the runtime, while in guarded mode
only the master will escape the runtime and the other threads will
idle in their state machine code that is now just "hidden".
Why:
- The state machine code cannot be (reasonably) optimized anyway,
moving it into the library shouldn't hurt runtime but might even
improve compile time a little bit.
- The change should also simplify the Clang code generation as we
would generate structurally the same code for both execution modes
but only the runtime library calls, or their arguments, would
differ between them.
- The reason we should not "just start in SPMD mode" and "repair"
it later is simple, this way we always have semantically correct
and executable code.
- Finally, and most importantly, there is now only little
difference (see above) between the two modes in the code
generated by clang. If we later analyze the code trying to decide
if we can use SPMD mode instead of guarded mode the analysis and
transformation becomes much simpler.</pre>
</blockquote>
<p>The last item is wrong, unfortunately. A lot of things in the codegen depend on the execution mode, e.g. correct support of the data-sharing. Of course, we can try to generalize the codegen and rely completely on the runtime, but the performance is going
to be very poor.</p>
<p>We still need static analysis in the compiler. I agree, that it is better to move this analysis to the backend, at least after the inlining, but at the moment it is not possible. We need the support for the late outlining, which will allow to implement better
detection of the SPMD constructs + improve performance.<br>
</p>
<blockquote type="cite">
<pre class="x_x_x_moz-quote-pre"> 2) Implement a middle-end LLVM-IR pass that detects the guarded mode,
e.g., through the runtime library calls used, and that tries to
convert it into the SPMD mode potentially by introducing lightweight
guards in the process.
Why:
- After the inliner, and the canonicalizations, we have a clearer
picture of the code that is actually executed in the target
region and all the side effects it contains. Thus, we can make an
educated decision on the required amount of guards that prevent
unwanted side effects from happening after a move to SPMD mode.
- At this point we can more easily introduce different schemes to
avoid side effects by threads that were not supposed to run. We
can decide if a state machine is needed, conditionals should be
employed, masked instructions are appropriate, or "dummy" local
storage can be used to hide the side effect from the outside
world.
None of this was implemented yet but we plan to start in the immediate
future. Any comments, ideas, criticism is welcome!
Cheers,
Johannes
P.S. [2-4] Provide further information on implementation and features.
[1] <a class="x_x_x_moz-txt-link-freetext x_x_OWAAutoLink" href="https://ieeexplore.ieee.org/document/7069297" id="LPlnk545306" previewremoved="true">https://ieeexplore.ieee.org/document/7069297</a>
[2] <a class="x_x_x_moz-txt-link-freetext x_x_OWAAutoLink" href="https://dl.acm.org/citation.cfm?id=2833161" id="LPlnk848282" previewremoved="true">https://dl.acm.org/citation.cfm?id=2833161</a>
[3] <a class="x_x_x_moz-txt-link-freetext x_x_OWAAutoLink" href="https://dl.acm.org/citation.cfm?id=3018870" id="LPlnk111280" previewremoved="true">https://dl.acm.org/citation.cfm?id=3018870</a>
[4] <a class="x_x_x_moz-txt-link-freetext x_x_OWAAutoLink" href="https://dl.acm.org/citation.cfm?id=3148189" id="LPlnk967688" previewremoved="true">https://dl.acm.org/citation.cfm?id=3148189</a>
</pre>
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