[SCEV] Extends the SCEVInitRewriter

The patch introduces an additional parameter IgnoreOtherLoops to SCEVInitRewriter.
if it is equal to true then rewriter will not invalidate result in case
SCEV depends on other loops then specified during creation.

The patch does not change the default behavior.
This is a preparation for re-writing isKnownPredicate utility as
described in https://reviews.llvm.org/D42417.

Reviewers: sanjoy, mkazantsev, reames
Reviewed By: sanjoy
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D43498

llvm-svn: 326067

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -4085,37 +4085,49 @@ void ScalarEvolution::forgetSymbolicName(Instruction *PN, const SCEV *SymName) {
 
 namespace {
 
+/// Takes SCEV S and Loop L. For each AddRec sub-expression, use its start
+/// expression in case its Loop is L. If it is not L then
+/// if IgnoreOtherLoops is true then use AddRec itself
+/// otherwise rewrite cannot be done.
+/// If SCEV contains non-invariant unknown SCEV rewrite cannot be done.
 class SCEVInitRewriter : public SCEVRewriteVisitor<SCEVInitRewriter> {
 public:
-  static const SCEV *rewrite(const SCEV *S, const Loop *L,
-                             ScalarEvolution &SE) {
+  static const SCEV *rewrite(const SCEV *S, const Loop *L, ScalarEvolution &SE,
+                             bool IgnoreOtherLoops = false) {
     SCEVInitRewriter Rewriter(L, SE);
     const SCEV *Result = Rewriter.visit(S);
-    return Rewriter.isValid() ? Result : SE.getCouldNotCompute();
+    if (Rewriter.hasSeenLoopVariantSCEVUnknown())
+      return SE.getCouldNotCompute();
+    return Rewriter.hasSeenOtherLoops() && !IgnoreOtherLoops
+               ? SE.getCouldNotCompute()
+               : Result;
   }
 
   const SCEV *visitUnknown(const SCEVUnknown *Expr) {
     if (!SE.isLoopInvariant(Expr, L))
-      Valid = false;
+      SeenLoopVariantSCEVUnknown = true;
     return Expr;
   }
 
   const SCEV *visitAddRecExpr(const SCEVAddRecExpr *Expr) {
-    // Only allow AddRecExprs for this loop.
+    // Only re-write AddRecExprs for this loop.
     if (Expr->getLoop() == L)
       return Expr->getStart();
-    Valid = false;
+    SeenOtherLoops = true;
     return Expr;
   }
 
-  bool isValid() { return Valid; }
+  bool hasSeenLoopVariantSCEVUnknown() { return SeenLoopVariantSCEVUnknown; }
+
+  bool hasSeenOtherLoops() { return SeenOtherLoops; }
 
 private:
   explicit SCEVInitRewriter(const Loop *L, ScalarEvolution &SE)
       : SCEVRewriteVisitor(SE), L(L) {}
 
   const Loop *L;
-  bool Valid = true;
+  bool SeenLoopVariantSCEVUnknown = false;
+  bool SeenOtherLoops = false;
 };
 
 /// This class evaluates the compare condition by matching it against the