LSR Fix: check SCEV expression safety before expansion.

All SCEV expressions used by LSR formulae must be safe to
expand. i.e. they may not contain UDiv unless we can prove nonzero
denominator.

Fixes PR11356: LSR hoists UDiv.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160205 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ScalarEvolutionExpander.cpp

@@ -1700,3 +1700,44 @@ unsigned SCEVExpander::replaceCongruentIVs(Loop *L, const DominatorTree *DT,
   }
   return NumElim;
 }
+
+namespace {
+// Search for a SCEV subexpression that is not safe to expand.  Any expression
+// that may expand to a !isSafeToSpeculativelyExecute value is unsafe, namely
+// UDiv expressions. We don't know if the UDiv is derived from an IR divide
+// instruction, but the important thing is that we prove the denominator is
+// nonzero before expansion.
+//
+// IVUsers already checks that IV-derived expressions are safe. So this check is
+// only needed when the expression includes some subexpression that is not IV
+// derived.
+//
+// Currently, we only allow division by a nonzero constant here. If this is
+// inadequate, we could easily allow division by SCEVUnknown by using
+// ValueTracking to check isKnownNonZero().
+struct SCEVFindUnsafe {
+  bool IsUnsafe;
+
+  SCEVFindUnsafe(): IsUnsafe(false) {}
+
+  bool follow(const SCEV *S) {
+    const SCEVUDivExpr *D = dyn_cast<SCEVUDivExpr>(S);
+    if (!D)
+      return true;
+    const SCEVConstant *SC = dyn_cast<SCEVConstant>(D->getRHS());
+    if (SC && !SC->getValue()->isZero())
+      return true;
+    IsUnsafe = true;
+    return false;
+  }
+  bool isDone() const { return IsUnsafe; }
+};
+}
+
+namespace llvm {
+bool isSafeToExpand(const SCEV *S) {
+  SCEVFindUnsafe Search;
+  visitAll(S, Search);
+  return !Search.IsUnsafe;
+}
+}