[llvm-dev] [RFC] Changing the default pass manager for the optimization pipeline

[Arthur Eubanks via llvm-dev]

There are a couple of failures that I hadn't noticed showing up in the
presubmit, as well as some internally reported performance regressions due
to NPM-related changes, so this will likely get pushed back.

On Wed, Jan 27, 2021 at 9:03 AM Philip Reames <listmail at philipreames.com>
wrote:

> +1 to the strategy and timeline.  This has been a long time in the works
> and I'm thrilled to see us approaching this major milestone.
>
> minor comment inline below
>
> Philip
> On 1/26/21 9:17 AM, Arthur Eubanks via llvm-dev wrote:
>
> Hi all,
>
> We've been fixing the various remaining issues in order to turn on the new
> pass manager for the optimization pipeline, and it's about time to turn it
> on. (Thanks to everyone who has helped with testing and fixing the new pass
> manager!)
>
> https://reviews.llvm.org/D95380 is the change that would happen, which
> sets the CMake flag -DENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=ON by default.
> This affects anything that uses the LLVM_ENABLE_NEW_PASS_MANAGER macro,
> which includes opt's handling of the `opt -instcombine` syntax, clang, and
> ThinLTO in lld drivers. This does not affect the backend target-specific
> codegen pipeline since that's mostly not been ported to use the new PM
> infrastructure yet.
>
> Here <https://bugs.llvm.org/show_bug.cgi?id=46649> is the umbrella bug
> for turning on the new PM with blockers. The main one is loop unswitching
> on divergent loop conditions is unsafe
> <https://bugs.llvm.org/show_bug.cgi?id=48819>, which is being looked
> into. There's also the LLVM C API and bugpoint that still use the legacy
> PM, which can be ported later on and only block the removal of the legacy
> PM. The C API can be worked through (we may need to introduce replacements
> to the legacy pass manager APIs), but bugpoint will be tricky since it has
> so many legacy PM-specific hacks and we may need to trim it down if we want
> it to work with the new PM. Anyway, I don't think any of the remaining
> blockers are large enough to block the switch (but comments welcome).
>
> I see no problem with having these two remain on the legacy pass manager
> for the moment.  I do think we should expose a new C API for the NewPM and
> not try to shove the new one into the same API as the old one, but that's a
> weakly held opinion and easily discussed later.
>
>
> I'd like to turn on the new PM by default soonish, after the 12.x branch.
> Perhaps roughly a week from now barring any major newly discovered
> regressions?
>
> As for potential issues only uncovered after the switch, if there is a
> large issue I will roll it back, but for smaller issues I'd rather ask
> users to pin to the legacy PM while we fix the issues, either via the CMake
> flag -DENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=OFF, or the corresponding
> compiler flags, like -flegacy-pass-manager for clang.
>
> Any concerns/comments?
>
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