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I am not in favour of just flipping the switch and then deal with all the fall-out, because <span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">we see major regressions that would be unacceptable for our users.
Thus, not only would this be very disruptive, also our releases are based on a certain trunk versions, so we would need to revert back to the legacy PM downstream and thus diverge from upstream which wouldn't be ideal for us. About the regressions, s</span><span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">ee
the message/thread that I kicked off earlier (</span><a href="http://lists.llvm.org/pipermail/llvm-dev/2020-July/143646.html" style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">http://lists.llvm.org/pipermail/llvm-dev/2020-July/143646.html</a><span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">)
which was quickly followed up by this thread. </span></div>
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I would like to see here if we are interesting in defining a few criteria that must be met before we switch:</div>
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<li>Correctness, which obviously always must come first: looks like this is covered by bots that are running with the NPM, and by downstream users. From the latest messages, I am getting we are there, or nearly there.</li><li>Performance (i.e. optimising for speed), </li><li>Code-size.</li></ol>
<div>With 1) correctness box covered and ticked, is now the time to look at codegen quality: 2) performance and 3) code-size? Would it be reasonable that we create a plan or timeline to address this, and thus allow time that these issues can be addressed?</div>
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<div>We are now ready to start tuning the NPM for code-size. Perhaps we are late to the NPM party (but that was a priority and bandwidth issue), but perhaps with correctness fixed this is actually the right time. I only ran numbers for code-size, and haven't
even looked at performance numbers yet, which we would also need to do and takes time.</div>
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<div>Cheers,</div>
<div>Sjoerd.</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Eric Christopher via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Sent:</b> 23 July 2020 02:05<br>
<b>To:</b> Philip Reames <listmail@philipreames.com>; Alina Sbirlea <asbirlea@google.com>; Chandler Carruth <chandlerc@gmail.com><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] New pass manager for optimization pipeline status and questions</font>
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<div dir="ltr">FWIW I'm in favor of this direction while making sure that we keep focus on removing the vestiges of the old pass manager for the code health impact to the project.
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<div>-eric</div>
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<div dir="ltr" class="x_gmail_attr">On Wed, Jul 22, 2020 at 3:15 PM Philip Reames via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br>
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<p>(I'm probably going to derail your thread, sorry about that.)</p>
<p>I think at this point, we should just bite the bullet and make the switch to NPM by default for Clang's optimization pipeline. Today.</p>
<p>Why? Because many of our downstream consumers have already switched. Google has. We (Azul) have. I think I've heard the same for a couple other major contributors. Why does this matter? Testing. At the current moment, the vast majority of testing
the project gets is exercising NPM, not LPM.</p>
<p>NPM is functionally complete for Clang optimization. There might be a few missing cases around the sanitizers, but last I heard those were on the edge of being fixed.</p>
<p>I think we should make the switch, and deal with any fall out as regressions. If we made the change immediately after a release branch, we'd have several months to address any major issues before the next release.
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<p>Philip<br>
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<div>On 7/22/20 2:39 PM, Arthur Eubanks via llvm-dev wrote:<br>
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<div>Hi all,</div>
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<div>I wanted to give a quick update on the status of NPM for the IR optimization pipeline and ask some questions.</div>
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<div>In the past I believe there were thoughts that NPM was basically ready because all of check-llvm and check-clang passed when -DENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=ON was specified. But that CMake flag did not apply to opt and any tests running something
like `opt -foo-pass -bar-pass` (which is the vast majority of check-llvm tests) were still using the legacy PM. The intended way to use NPM was to use the -passes flag, e.g. `opt -passes='foo,bar'`.</div>
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<div>I've added a -enable-new-pm flag to opt to force running NPM passes even when `opt -foo-pass` is used. This is because I didn't want to go through every single test and figure out which ones should be using both -foo-pass and -passes=foo. Switching on
-enable-new-pm currently leads to ~1800 check-llvm failures. I've documented the failed tests count per directory in <a href="https://bugs.llvm.org/show_bug.cgi?id=46651" target="_blank">https://bugs.llvm.org/show_bug.cgi?id=46651</a> (some have been fixed
since that was posted).</div>
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<div>This has led to real bugs in NPM being discovered and fixed (e.g. some optnone issues).</div>
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<div>But a large portion of the remaining failures are because codegen-only passes haven't been ported to NPM yet. That's fine for the optimization pipeline NPM transition since it doesn't affect the optimization pipeline, but it does present an issue with
the approach of the -enable-new-pm flag (which would by default become true alongside the NPM transition). Lots of tests are testing codegen-specific passes via opt (e.g. `opt -amdgpu-lower-intrinsics`) and they can't use NPM (yet).</div>
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<div>I was thinking either we have a way of identifying codegen-only passes and revert back to the legacy PM in opt whenever we see one, or we go back to considering the originally intended approach of adding an equivalent `-passes=` RUN to all tests that should
be also running under NPM.</div>
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<div>I'm not sure of a nice and clean solution to identify codegen-only passes. We could go and update every instance of INITIALIZE_PASS to take another parameter indicating if it's codegen-only. Or we could just have a central list somewhere where we check
if the pass is in some hardcoded list or has some prefix (e.g. "x86-").</div>
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<div>The approach of adding equivalent `-passes=` RUN lines to all relevant tests seems daunting, but not exactly sure how daunting. Maybe it's possible to script something and see what fails? We'd still need some way to identify codegen-only passes to make
sure we don't miss anything, and we'd need to distinguish between analyses and normal passes. Also, it would slow down test execution since we'd run a lot more tests twice, but maybe that's not such a big deal? Maybe it's good to have most tests running against
the legacy PM even when NPM is on by default?</div>
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<div>Thoughts?</div>
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This is split off from <a href="http://lists.llvm.org/pipermail/llvm-dev/2020-July/143395.html" target="_blank">http://lists.llvm.org/pipermail/llvm-dev/2020-July/143395.html</a>.
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