[LLVMdev] Heads up: Pass Manager changes will be starting shortly

reed kotler rkotler at mips.com
Sat Sep 14 16:15:39 PDT 2013

On 09/14/2013 04:05 PM, Chandler Carruth wrote:
> On Sat, Sep 14, 2013 at 3:57 PM, Reed Kotler <rkotler at mips.com 
> <mailto:rkotler at mips.com>> wrote:
>     Hi Chandler,
>     What changes are you planning to make?
>     Are you going to submit a proposal and RFC?
> There have been several RFCs...
> What's your specific question?

Which RFCs cover this?

Will the new system allow for passes to be inserted dynamically after 

I have some delicate things that I did to make this ability to switch 
between mips 32 and mips16 ( they are different instruction sets) that 
relies on the pass manager working the way it does now. I'm not sure 
what you are planning to do and how it might affect what I did. It was 
very tricky to get that to work and a lot of things depend on it working 

I don't think that we need to see how this will be implemented but I 
think that it would be helpful to list what you are trying to accomplish 
at a high level and what new features will be there and how the behavior 
will change from now.

> I'm planning to essentially try to build a new pass management layer 
> that doesn't suffer from the problems the existing one does. I could 
> describe every interface I plan in prose and email it out, but I'm not 
> sure that's really the most effective way to do it. My plan has been 
> to work on implementing the new system in-tree, and let folks code 
> review it and pick it apart as usual. When folks are happy with it, 
> and it works, then we can look at switching over.
>     Reed
>     On 09/14/2013 03:46 PM, Chandler Carruth wrote:
>         I just wanted to give everyone a head up. I'm starting work on
>         the pass
>         manager based on numerous list discussions and some IRC
>         discussions.
>         The first steps will be marking the existing code as "legacy" and
>         clearing a path to build new facilities here. From there I'll
>         start
>         building the new facilities without enabling them.
>         Some explicit legacy support goals:
>         1) I'm going to build a bridge so that an existing Pass can be
>         inserted
>         into the new pass manager with some adaptors and everything
>         will just
>         work. This may require touching the code that sets up the pass
>         manager,
>         but not the code that defines a pass. This will work even once
>         the new
>         pass manager bits are enabled if at all possible.
>         2) If you have code that includes the current PassManager headers,
>         nothing should break right away, but when the new manager
>         infrastructure
>         is enabled, I'll likely remove the old PassManager headers,
>         breaking
>         this code. My goal is to only break code that directly
>         interacts with
>         the management layer.
>         3) I'm going to play namespace games so that we don't end up with
>         PassManagerV2 and other silly names. The legacy headers will
>         paper over
>         this to keep legacy code compiling without change.
>         -Chandler
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