[PATCH] D106099: [DependenceAnalysis] Guard analysis using getPointerBase()

Thu Jul 15 15:28:49 PDT 2021

efriedma added inline comments.

================
Comment at: llvm/lib/Analysis/DependenceAnalysis.cpp:3556
   LLVM_DEBUG(dbgs() << "    DstSCEV = " << *DstSCEV << "\n");
+  if (SE->getPointerBase(SrcSCEV) != SE->getPointerBase(DstSCEV)) {
+    // If two pointers have different bases, trying to analyze indexes won't
----------------
efriedma wrote:
> bmahjour wrote:
> > We only get here if the two pointers `MustAlias`.  If my understanding of `MustAlias` is correct (that it means precise and complete overlapping of memory), and if the two references are in the same loop, then comparing indexes seems reasonable. In that case, the effect of the following analysis should be as if `src` and `dst` SCEVs have their base pointers replaced with the same value. Not sure what's the best way to deal with such cases. Does it make sense to introduce a mode in `getMinusSCEV` where different base pointers can be permitted? 
> `underlyingObjectsAlias(Src, Dst) == MustAlias` is basically a slightly fancier version `isIdentifiedObject(getUnderlyingObject(Src)) && isIdentifiedObject(getUnderlyingObject(Dst)) && getUnderlyingObject(Src) == getUnderlyingObject(Dst)`.  The issue here is that getPointerBase() is less powerful than getUnderlyingObject().
> 
> It is possible to cast the operands of getMinusSCEV to integers using getPtrToIntExpr(); in that case, you won't get a SCEVCouldNotCompute.  But then you end up with a SCEV containing something like `ptrtoint(SrcSCEVPointerBase) - ptrtoint(DstSCEVPointerBase)`, which is basically impossible to analyze.  So it seems simpler to just reject early.
Oh, also, there's been work to allow SCEV to look through LCSSA PHI nodes; that would allow DependenceAnalysis to analyze more cases. But that patch got reverted; not sure what the current status is.

Repository:
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  https://reviews.llvm.org/D106099/new/

https://reviews.llvm.org/D106099