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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. <br>
</p>
<p>Philip<br>
</p>
<div class="moz-cite-prefix">On 7/22/20 2:39 PM, Arthur Eubanks via
llvm-dev wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAPW48sopEcvadGK36NxP=gQiY8z40bcLs_ScRFOSJMZxXw-oYw@mail.gmail.com">
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<div>Hi all,</div>
<div><br>
</div>
<div>I wanted to give a quick update on the status of NPM for
the IR optimization pipeline and ask some questions.</div>
<div><br>
</div>
<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>
<div><br>
</div>
<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"
moz-do-not-send="true">https://bugs.llvm.org/show_bug.cgi?id=46651</a>
(some have been fixed since that was posted).</div>
<div><br>
</div>
<div>This has led to real bugs in NPM being discovered and fixed
(e.g. some optnone issues).</div>
<div><br>
</div>
<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>
<div><br>
</div>
<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>
<div><br>
</div>
<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>
<div><br>
</div>
<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>
<div><br>
</div>
<div>Thoughts?</div>
<div><br>
</div>
This is split off from <a
href="http://lists.llvm.org/pipermail/llvm-dev/2020-July/143395.html"
target="_blank" moz-do-not-send="true">http://lists.llvm.org/pipermail/llvm-dev/2020-July/143395.html</a>.
<div><br>
</div>
<div><br>
</div>
</div>
<br>
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