[llvm] [IVDescriptors] Implement MonotonicDescriptor (PR #214490)

[Benjamin Maxwell via llvm-commits]

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@@ -482,6 +483,44 @@ class InductionDescriptor {
   SmallVector<const SCEVPredicate *, 2> NoWrapPredicates;
 };
 
+/// A struct for saving information about monotonic variables.
+/// Monotonic variable can be considered as a "conditional" induction variable:
+/// its update happens only on loop iterations for which a certain predicate is
+/// satisfied. In this implementation the predicate is represented as an edge in
+/// loop CFG: variable is updated if this edge is executed on current loop
+/// iteration.
+class MonotonicDescriptor {
+public:
+  using Edge = std::pair<BasicBlock *, BasicBlock *>;
+
+  MonotonicDescriptor() = default;
+
+  const SmallPtrSetImpl<PHINode *> &getChain() const { return Chain; }
+  Instruction *getStepInst() const { return StepInst; }
+  Edge getPredicateEdge() const { return PredEdge; }
+  const SCEVAddRecExpr *getExpr() const { return Expr; }
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MacDue wrote:

I've added some extra docs for the fields/methods of the descriptor :+1: 

https://github.com/llvm/llvm-project/pull/214490