[LLVMdev] PassManager Woes

[dag at cray.com]

"Daniel Stewart" <stewartd at codeaurora.org> writes:

> You haven't mentioned if your passes are implementers of an interface
> (like AliasAnalysis).

No, they're just regular FunctionPasses.

> What seems to matter as far as having a pass free'd from memory are
> the LastUsers. The LastUsers are set by required passes. So if a pass
> says it requires another pass, the required pass sticks around. The
> last user is the last pass that required some other particular pass.
> It appears in your case that F requires A and C. The PassManager will
> indicate that F is the last user of A. The fact that C doesn't
> preserve A is seemingly meaningless.  Therefore A will be run,
> followed by C, followed by F.

That's just broken.  But I had come to a similar conclusion over the
weekend.  The obvious question is, if I remove the dependency from F how
do I guarantee the information is available when it runs?

> This brings up a question I had about the meaning of "preserve". Some
> places I read (here on the mailing list + the docs) seem to indicate
> that preserving is about keeping something around because you may want
> to use it again. While other places seem to indicate that it is about
> whether a pass is invalidated. These are very two different things,
> and, as far as I can tell, the code seems to implement neither of
> these ideas.

Spot on.

> To solve your issue, can you use a barrier pass? This might force the
> passes to re-run when you need them.

What's a barrier pass?

                              -David