[llvm] [DA] Check monotonicity for subscripts (PR #154527)

[Ryotaro Kasuga via llvm-commits]

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@@ -3308,6 +3308,322 @@ void DependenceInfo::updateDirection(Dependence::DVEntry &Level,
     llvm_unreachable("constraint has unexpected kind");
 }
 
+namespace {
+
+enum class MonotonicityType {
+  MaySignedWrap, ///< The expression contains arithmetic operations that may
+                 ///< cause signed wrap.
+  Constant,      ///< The expression is constant. If a SCEV is classified as
+                 ///< Constant, it also implies that it doesn't contain any
+                 ///< arithmetic operations that may cause signed wrap.
+  Monotonic, ///< The expression is monotonically increasing or decreasing. This
+             ///< is exclusive of Constant. That is, we say an SCEV is Monotonic
+             ///< iff it contains at least one AddRec where its step reccurence
+             ///< value is non-zero.
+};
+
+/// A visitor that checks the signed monotonicity of SCEVs.
+struct SCEVSignedMonotonicityChecker
+    : public SCEVVisitor<SCEVSignedMonotonicityChecker, MonotonicityType> {
+  /// \p Ptr is the pointer that the SCEV is associated with, if any. It may be
+  /// used for the inferrence.
+  SCEVSignedMonotonicityChecker(ScalarEvolution *SE, const Loop *OutermostLoop,
+                                const Value *Ptr = nullptr)
+      : SE(SE), OutermostLoop(OutermostLoop), Ptr(Ptr) {}
+
+  MonotonicityType visitAddExpr(const SCEVAddExpr *Expr);
+  MonotonicityType visitAddRecExpr(const SCEVAddRecExpr *Expr);
+  MonotonicityType visitMulExpr(const SCEVMulExpr *Expr);
+  MonotonicityType visitUnknown(const SCEVUnknown *Expr);
+  MonotonicityType visitZeroExtendExpr(const SCEVZeroExtendExpr *Expr);
+  MonotonicityType visitSignExtendExpr(const SCEVSignExtendExpr *Expr);
+
+  MonotonicityType visitConstant(const SCEVConstant *) {
+    return MonotonicityType::Constant;
+  }
+  MonotonicityType visitVScale(const SCEVVScale *) {
+    return MonotonicityType::Constant;
+  }
+
+  // TODO: Handle more cases.
+  MonotonicityType visitPtrToIntExpr(const SCEVPtrToIntExpr *) {
+    return MonotonicityType::MaySignedWrap;
+  }
+  MonotonicityType visitTruncateExpr(const SCEVTruncateExpr *) {
+    return MonotonicityType::MaySignedWrap;
+  }
+  MonotonicityType visitUDivExpr(const SCEVUDivExpr *) {
+    return MonotonicityType::MaySignedWrap;
+  }
+  MonotonicityType visitSMaxExpr(const SCEVSMaxExpr *) {
+    return MonotonicityType::MaySignedWrap;
+  }
+  MonotonicityType visitUMaxExpr(const SCEVUMaxExpr *) {
+    return MonotonicityType::MaySignedWrap;
+  }
+  MonotonicityType visitSMinExpr(const SCEVSMinExpr *) {
+    return MonotonicityType::MaySignedWrap;
+  }
+  MonotonicityType visitUMinExpr(const SCEVUMinExpr *) {
+    return MonotonicityType::MaySignedWrap;
+  }
+  MonotonicityType visitSequentialUMinExpr(const SCEVSequentialUMinExpr *) {
+    return MonotonicityType::MaySignedWrap;
+  }
+  MonotonicityType visitCouldNotCompute(const SCEVCouldNotCompute *) {
+    return MonotonicityType::MaySignedWrap;
+  }
+
+private:
+  ScalarEvolution *SE;
+  const Loop *OutermostLoop;
----------------
kasuga-fj wrote:

I feel it's safer to make this outermost. Otherwise, unexpected behavior might occur in cases like below

https://github.com/llvm/llvm-project/blob/b5ca793172e604255cb4d57ab13e232cc712228a/llvm/test/Analysis/DependenceAnalysis/monotonic.ll#L336-L379

https://github.com/llvm/llvm-project/pull/154527