[llvm-dev] [RFC] Expanding the scope of ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER

[Arthur Eubanks via llvm-dev]

On Fri, Dec 4, 2020 at 3:28 PM Philip Reames <listmail at philipreames.com>
wrote:

> You are proposing to move code for the new pass manager into conditional
> compilation.  I am strongly opposed.
>
> As for the overall status of the NPM, I find the continued delay in
> switching to be extremely problematic for the health of project long term.
> I understand the "X doesn't work yet" problem, but a) X is fairly small,
> and b) the folks involved in maintaining X need to pay the cost of
> supporting the old pass manager.  I do want to be careful and state
> explicitly that I'm expressing opinion here, not making an actual
> proposal.  I may get around to the later eventually, but this is not it.
>
> (two minor response inline)
>
> Philip
> On 12/4/20 3:19 PM, Arthur Eubanks wrote:
>
> Implementing this proposal does not in any way stop the flip of the
> flag, they are completely unrelated. This increases the scope of the new
> pass manager since much of lld's use of LTO is currently unconditionally
> using the legacy PM and flipping the flag wouldn't change that.
>
> "the default" for me only means opt and clang.  It doesn't mean llc, or
> any other tool which happens to use the old pm.  If we need clang to select
> the old pass manager at the command line when invoking LTO, that doesn't
> really bug me.
>
This essentially proposes changing "the default" to include more. The
functionality was already there, just nobody has been using it. There are
no lld/LTO tests that newly fail when turning it on for the various lld LTO
uses, so I don't see the issue with this. It doesn't make it harder to flip
the flag.

I've been spending the last half year trying to get check-llvm green when
opt is using the new pass manager by default. We're actually very close,
but not quite there. This <https://bugs.llvm.org/show_bug.cgi?id=46649> is
the overarching "flip the NPM flag" bug, and this
<https://bugs.llvm.org/show_bug.cgi?id=46651> is the bug for getting
check-llvm to work with opt using the NPM. If you'd like to help with some
of the last few remaining issues that'd be awesome :). Once that's done,
and some small remaining blockers like
https://bugs.llvm.org/show_bug.cgi?id=46858 have been investigated, I'd
like to flip the flag. I'm still spending all of my time trying to get it
flipped.
(some more remaining issues off the top of my head: coroutines don't work
<https://bugs.llvm.org/show_bug.cgi?id=48190> due to the NPM CGSCC infra
not properly supporting outlining, LSR doesn't preserve LCSSA in some edge
cases, the AMDGPU stuff below)

>
> There are some things that the new pass manager doesn't currently support.
> For example, all of AMDGPU would be broken with the switch to the new pass
> manager since currently AMDGPU's passes aren't injected into the pipeline.
> I'm working on the (few) remaining issues and do plan to flip the switch
> soon.
>
> I find this hard to believe.  Are you possibly talking about llc/codegen?
> If so, that's well out of scope for what I'm talking about.  If not, can
> you point to a bug so I can see an example?
>
https://bugs.llvm.org/show_bug.cgi?id=47244
Backends can inject passes into the LLVM IR pipeline, for example to lower
custom intrinsics. I was just about to send out a separate request to the
AMDGPU community to port their stuff to the NPM, there'll be more details
there.
You can see this by setting the -enable-new-pm flag in opt to true by
default, then running check-llvm to see some AMDGPU tests fail because of
this missing functionality.
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