[LLVMdev] BasicAA unable to analyze recursive PHI nodes

[Daniel Berlin]

> Not a huge change here although the numbers tend to be more on the side
> of a slowdown. This is interesting - you'd think that better alias
> analysis shouldn't have that effect. Any idea what could be causing
> this?


LLVM's alias analysis is good but not amazing.
If you improve it significantly (as you appear to have :P), you give
the compiler more freedom to do things.
What it chooses to do with that freedom  may not always be good :)

Basically, with any significant alias analysis change comes a lot of
tuning of heuristics/etc, or looking through regressions, to determine
what's going on now.  *Usually* it's "something used to not be able to
do a ton of <whatever>, and now it can.  The cost model for <whatever>
needs to be worked on so it doesn't go too crazy".