[SCEV] Change the SCEV Predicates interfaces for conversion to AddRecExpr to return SCEVAddRecExpr* instead of SCEV*

[sbaranga]

Summary

This changes the conversion functions from SCEV * to SCEVAddRecExpr from
ScalarEvolution and PredicatedScalarEvolution to return a SCEVAddRecExpr*
instead of a SCEV* (which removes the need of most clients to do a
dyn_cast right after calling these functions).

We also don't add new predicates if the transformation was not successful.

This is not entirely a NFC (as it can theoretically remove some predicates
from LAA when we have an unknown dependece), but I couldn't find an obvious
regression test for it.

[sanjoy]

lgtm with minor nits

lib/Analysis/ScalarEvolution.cpp

9916

Use auto * here, to make it obvious that it is a pointer.

10141

Use auto * here (to make it obvious that this is a pointer).

[sbaranga]

Replace auto with auto *.

[sbaranga]

Thanks! r264161

include/llvm/Analysis/ScalarEvolution.h

public:
     getWrapPredicate(const SCEVAddRecExpr *AR,
                      SCEVWrapPredicate::IncrementWrapFlags AddedFlags);
 
     /// Re-writes the SCEV according to the Predicates in \p A.
     const SCEV *rewriteUsingPredicate(const SCEV *S, const Loop *L,
                                       SCEVUnionPredicate &A);
     /// Tries to convert the \p S expression to an AddRec expression,
     /// adding additional predicates to \p Preds as required.
-    const SCEV *convertSCEVToAddRecWithPredicates(const SCEV *S, const Loop *L,
-                                                  SCEVUnionPredicate &Preds);
+    const SCEVAddRecExpr *
+    convertSCEVToAddRecWithPredicates(const SCEV *S, const Loop *L,
 
   private:
     /// Compute the backedge taken count knowing the interval difference, the
     /// stride and presence of the equality in the comparison.
     const SCEV *computeBECount(const SCEV *Delta, const SCEV *Stride,
                                bool Equality);
 
     /// Verify if an linear IV with positive stride can overflow when in a
public:
     /// SCEV predicate.
     /// The order of transformations applied on the expression of V returned
     /// by ScalarEvolution is guaranteed to be preserved, even when adding new
     /// predicates.
     const SCEV *getSCEV(Value *V);
     /// \brief Adds a new predicate.
     void addPredicate(const SCEVPredicate &Pred);
     /// \brief Attempts to produce an AddRecExpr for V by adding additional
-    /// SCEV predicates.
-    const SCEV *getAsAddRec(Value *V);
+    /// SCEV predicates. If we can't transform the expression into an
+    /// AddRecExpr we return nullptr and not add additional SCEV predicates
+    /// to the current context.
+    const SCEVAddRecExpr *getAsAddRec(Value *V);
     /// \brief Proves that V doesn't overflow by adding SCEV predicate.
     void setNoOverflow(Value *V, SCEVWrapPredicate::IncrementWrapFlags Flags);
     /// \brief Returns true if we've proved that V doesn't wrap by means of a
     /// SCEV predicate.
     bool hasNoOverflow(Value *V, SCEVWrapPredicate::IncrementWrapFlags Flags);
     /// \brief Returns the ScalarEvolution analysis used.
     ScalarEvolution *getSE() const { return &SE; }
     /// We need to explicitly define the copy constructor because of FlagsMap.

lib/Analysis/LoopAccessAnalysis.cpp

DEBUG(dbgs() << "LAA: Bad stride - Not a pointer to a scalar type" << *Ptr
                  << "\n");
     return 0;
   }
 
   const SCEV *PtrScev = replaceSymbolicStrideSCEV(PSE, StridesMap, Ptr);
 
   const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrScev);
   if (Assume && !AR)
-    AR = dyn_cast<SCEVAddRecExpr>(PSE.getAsAddRec(Ptr));
+    AR = PSE.getAsAddRec(Ptr);
 
   if (!AR) {
     DEBUG(dbgs() << "LAA: Bad stride - Not an AddRecExpr pointer " << *Ptr
                  << " SCEV: " << *PtrScev << "\n");
     return 0;
   }
 
   // The accesss function must stride over the innermost loop.

lib/Analysis/ScalarEvolution.cpp

 };
 } // end anonymous namespace
 
 const SCEV *ScalarEvolution::rewriteUsingPredicate(const SCEV *S, const Loop *L,
                                                    SCEVUnionPredicate &Preds) {
   return SCEVPredicateRewriter::rewrite(S, L, *this, Preds, false);
 }
 
-const SCEV *
+const SCEVAddRecExpr *
 ScalarEvolution::convertSCEVToAddRecWithPredicates(const SCEV *S, const Loop *L,
                                                    SCEVUnionPredicate &Preds) {
-  return SCEVPredicateRewriter::rewrite(S, L, *this, Preds, true);
+  SCEVUnionPredicate TransformPreds;
+  S = SCEVPredicateRewriter::rewrite(S, L, *this, TransformPreds, true);
+  auto AddRec = dyn_cast<SCEVAddRecExpr>(S);
+
+  if (!AddRec)
+    return nullptr;
+
+  // Since the transformation was successful, we can now transfer the SCEV
+  // predicates.
+  Preds.add(&TransformPreds);
+  return AddRec;
 }
 
 /// SCEV predicates
 SCEVPredicate::SCEVPredicate(const FoldingSetNodeIDRef ID,
                              SCEVPredicateKind Kind)
     : FastID(ID), Kind(Kind) {}
 
 SCEVEqualPredicate::SCEVEqualPredicate(const FoldingSetNodeIDRef ID,
bool PredicatedScalarEvolution::hasNoOverflow(
   auto II = FlagsMap.find(V);
 
   if (II != FlagsMap.end())
     Flags = SCEVWrapPredicate::clearFlags(Flags, II->second);
 
   return Flags == SCEVWrapPredicate::IncrementAnyWrap;
 }
 
-const SCEV *PredicatedScalarEvolution::getAsAddRec(Value *V) {
+const SCEVAddRecExpr *PredicatedScalarEvolution::getAsAddRec(Value *V) {
   const SCEV *Expr = this->getSCEV(V);
-  const SCEV *New = SE.convertSCEVToAddRecWithPredicates(Expr, &L, Preds);
+  auto New = SE.convertSCEVToAddRecWithPredicates(Expr, &L, Preds);
+
+  if (!New)
+    return nullptr;
+
   updateGeneration();
   RewriteMap[SE.getSCEV(V)] = {Generation, New};
   return New;
 }
 
 PredicatedScalarEvolution::
 PredicatedScalarEvolution(const PredicatedScalarEvolution &Init) :
   RewriteMap(Init.RewriteMap), SE(Init.SE), L(Init.L), Preds(Init.Preds),
   Generation(Init.Generation) {
   for (auto I = Init.FlagsMap.begin(), E = Init.FlagsMap.end(); I != E; ++I)
     FlagsMap.insert(*I);
 }