[llvm] r225333 - [PM] Fix a pretty nasty bug where the new pass manager would invalidate

[Nathan Sidwell]

On 01/06/15 20:58, Chandler Carruth wrote:

> I think this is actually the issue that someone raised with me at the
> developer's meeting and in an email, but that we never really got to the
> bottom of. Having all the testing utilities made it much easier to dig
> down and uncover the core issue.

Yeah, that was me.

> The solution to this struck me as really strange at first, but the more
> I thought about it, the more natural it seemed. After a nice discussion
> with Duncan about it on IRC, it seemed even nicer. The idea is that
> invalidating an analysis *causes* it to be preserved! Preserving the
> lack of result is trivial. If it is recomputed, great. Until something
> *else* invalidates it again, we're good.

I think I understand the intent here, and it seems reasonable.

nathan