[PATCH] D23075: [IndVarSimplify] Extend trip count instead of truncating IV in LFTR, when original IV does not overflow

[Ehsan Amiri via llvm-commits]

amehsan added inline comments.

================
Comment at: lib/Transforms/Scalar/IndVarSimplify.cpp:2023
@@ +2022,3 @@
+        }
+        if (Iter->second->HasNUW && !Iter->second->IsSigned) {
+          extended = true;
----------------
sanjoy wrote:
> I think there is a way to make this stateless (though I don't know if SCEV is smart enough to make that work viably for all the cases you care about): you were going to emit a backedge condition of `trunc(IV) == ExitCnt`.  (Please double check this logic) since `sext` and `zext` are one-to-one, this is equivalent to both `zext(trunc(IV)) == zext(ExitCnt)` and `sext(trunc(IV)) == sext(ExitCnt)`.
> 
> `zext(trunc(IV)) == zext(ExitCnt)` is profitable if `zext(trunc(IV))` == `IV` (in which case the RHS is `zext(ExitCnt)`) and `sext(trunc(IV)) == sext(ExitCnt)` is profitable if `sext(trunc(IV))` == `IV` (in which case the RHS is `sext(ExitCnt)`).  You can check these equivalences (i.e. `exit(trunc(T))` == `T`) using SCEV.
> 
> I think this will be a better solution overall, if it works. :)
I agree this is better. Will try it to see if it works.


https://reviews.llvm.org/D23075