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

Philip Reames via llvm-dev llvm-dev at lists.llvm.org
Fri Aug 2 13:14:01 PDT 2019

Thanks for all the hard work that went into getting it here.


Can you spell out what performance validation has been done? Informally, 
I know there's been quite a bit, but getting a summary in one place for 
later reference would be super helpful.


On 8/2/19 10:25 AM, 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?
> 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.
> Thanks,
> Leonard
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