[PATCH] [LAA] Try to prove non-wrapping of pointers if SCEV cannot

[Sanjoy Das]

================
Comment at: lib/Analysis/LoopAccessAnalysis.cpp:540
@@ +539,3 @@
+  // using a non-wrapping operation.
+  if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(NonConstIndex))
+    if ((OBO->hasNoUnsignedWrap() || OBO->hasNoSignedWrap()) &&
----------------
anemet wrote:
> sanjoy wrote:
> > What if `OBO` is `nuw` (and not `nsw`) and `OpAR` is `nsw` (and not `nuw`)?  Or vice-versa?
> Yeah, this certainly required more thinking on my behalf, thanks for pressing it.
> 
> So to document why this was wrong, let's take this counterexample:
> 
> Consider the AddRec {0,+,100} <nuw> in i8.  The first three iterations of that yields: 0, 100, 200.
> 
> Putting this through a *signed* add of 3, the input is now interpreted as signed: 0, 100, -56.
> 
> No signed overflow on the result (3, 103, -53), yet the result is wrapped. 
> 
> Similarly, we can't take <nuw>-only for index.  Index is interpreted as signed.  With the above (i8) {0,+,100} example we'd get a wrapping range even though it may be inbounds for the array. 
> 
> Let me know if you or others disagree or have further comments.  Otherwise I'll update the patch accordingly.
I think you're right w.r.t. the (no-)wrapping logic.  I have not worked on LAA so I cannot comment on how `isNoWrapAddRec` is used.

http://reviews.llvm.org/D10472

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