[llvm-dev] New pass manager for optimization pipeline status and questions

[Arthur Eubanks via llvm-dev]

Hi all,

I wanted to give a quick update on the status of NPM for the IR
optimization pipeline and ask some questions.

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'`.

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 https://bugs.llvm.org/show_bug.cgi?id=46651 (some have been fixed since
that was posted).

This has led to real bugs in NPM being discovered and fixed (e.g. some
optnone issues).

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).

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.

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-").

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?

Thoughts?

This is split off from
http://lists.llvm.org/pipermail/llvm-dev/2020-July/143395.html.
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