[SCEV] Extends the SCEVInitRewriter

[skatkov]

Summary

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.

[sanjoy]

lib/Analysis/ScalarEvolution.cpp

4097 ↗

(On Diff #135007)

I would factor this a bit differently --

• Don't pipe in IgnoreOtherLoops into the SCEVInitRewriter object. Instead rename Valid to SeenOtherLoops.
• Handle the IgnoreOtherLoops logic in SCEVInitRewriter:: rewrite itself as: return Rewriter.seenOtherLoops() && !IgnoreOtherLoops ? SE.getCouldNotCompute() : Result;.

[skatkov]

please take a look.

[sanjoy]

lgtm with the two other issues mentioned in D43499 addressed.

llvm/trunk/lib/Analysis/ScalarEvolution.cpp

while (!Worklist.empty()) {
     }
 
     PushDefUseChildren(I, Worklist);
   }
 }
 
 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
 /// condition of loop latch. If there is a match we assume a true value
 /// for the condition while building SCEV nodes.
 class SCEVBackedgeConditionFolder
     : public SCEVRewriteVisitor<SCEVBackedgeConditionFolder> {
 public: