[llvm-dev] Switching to the New Pass Manager by Default

Finkel, Hal J. via llvm-dev llvm-dev at lists.llvm.org
Fri Aug 2 10:45:40 PDT 2019

On 8/2/19 12:25 PM, Leonard Chan via llvm-dev wrote:
Hello all,

As of now, all LLVM and Clang tests have been updated/addressed to run under the new/experimental pass manager (at least the ones that failed when using the new PM).

For those who aren't aware of what the new pass manager (PM) is, the tl;dr is that this will serve as a replacement for the legacy PM, and promises faster build times by restructuring how passes are run over IR units. For some time, when someone wants to create a new pass, they may need to implement it for both pass managers since the legacy one is what is enabled by default. Passes that were initially made under the new PM have also slowly been ported over time.

Now that LLVM 9.0.0 has branched, we have about 6 months before the next release. We think it would be a good idea to take this opportunity and make the new PM the default one which gives us enough time to work out any kinks that might come out of this switch before LLVM 10.0.0 branch point. We suspect that there may be other LLVM projects that will be affected by this, probably from unported passes.

Does anyone have any opinions on this?

A huge thank you to everyone who has worked on this! This is great news. You're exactly right: we should change the default early in the release cycle. +1 to doing this.


Off the top of my head, the next immediate work would be to update the docs with instructions on how to write or port a new PM pass, and address any breakages for other LLVM projects.


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Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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