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

[Arthur Eubanks via llvm-dev]

This has been submitted as https://reviews.llvm.org/D95380. Please file
bugs for any regressions.

On Tue, Feb 2, 2021 at 12:25 PM Arthur Eubanks <aeubanks at google.com> wrote:

> 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?
>>
>> _______________________________________________
>> LLVM Developers mailing listllvm-dev at lists.llvm.orghttps://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20210203/85498a39/attachment-0001.html>