[PATCH] D101829: [SCEV] By more careful when traversing phis in isImpliedViaMerge.

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D101829#2740003 <https://reviews.llvm.org/D101829#2740003>, @reames wrote:

> In D101829#2739961 <https://reviews.llvm.org/D101829#2739961>, @fhahn wrote:
>
>> In D101829#2739843 <https://reviews.llvm.org/D101829#2739843>, @reames wrote:
>>
>>> 
>>
>> I might be glancing over something, but aren't we continuing the loop if `%call >s -1` and exit if `call <=s -1?
>
> No, you're entirely correct.  I'd swapped the branch order in my head when reading.
>
>> So in the iteration before we exit, `%call >s -1`, hence `%x >s -1` in the last iteration. In the last iteration, `%call <=s -1`, but `%x` still has the value from the previous iteration.
>
> There are two sub-cases here:
>
> 1. Exit on the first iteration - %x will be -1 (e.g. value on entry).
> 2. Exit on second or further iteration - %x will be >s -1.  (e.g. value on backedge, last iteration value of call)
>
> We can correctly conclude that x >=s -1, but not x >s -1.  Which I think was your original point right?  Sorry for the confusion.

Yes I think that matches my understanding of the issue. Plus we cannot conclude that `x >s -1` is always false, which happens for the test case IIUC.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D101829/new/

https://reviews.llvm.org/D101829