[PATCH] D32663: [SCEV] createAddRecFromPHI: Optimize for the most common case.

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy added a comment.

In https://reviews.llvm.org/D32663#745401, @mzolotukhin wrote:

> Thanks for taking a look, I've updated the patch.
>
> The change in `2014-08-29-CompactUnwind.ll` is caused by the fact that SCEV started to produce a simpler expression for `%iv.i`, which allowed LSR to optimize the test harder.


Hm, I suspect this is because `BEValue` would be a `SCEVUnknown` in that case (== the symbolic phi) in the slow path since `%iv.i+0` would be simplified to `%iv.i`.

So that means this change isn't quite NFC.  Can you please add a dedicated test case checking the difference in behavior above ^?

> The test seems to exercise instructions positions, so I tried to keep them untouched with this change. It's still a question why the original SCEV algorithm failed to construct the simpler expression for this case, I'll take a look at it later if I find a time.
> 
> Michael




https://reviews.llvm.org/D32663