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

[Ryotaro Kasuga via llvm-commits]

================
@@ -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;
+  const Value *Ptr = nullptr;
+};
+
+using MinMaxType = std::pair<const SCEV *, const SCEV *>;
+const MinMaxType Bottom = {nullptr, nullptr};
+
+/// A visitor that calculates the possible minimum and maximum values of SCEVs.
+/// This class assumes that the given SCEV is constant or monotonic.
+struct SCEVMinMaxCalculator
+    : public SCEVVisitor<SCEVMinMaxCalculator, MinMaxType> {
+  SCEVMinMaxCalculator(ScalarEvolution *SE, const Loop *OutermostLoop)
+      : SE(SE), OutermostLoop(OutermostLoop) {}
+
+  MinMaxType visitAddExpr(const SCEVAddExpr *Expr);
+  MinMaxType visitAddRecExpr(const SCEVAddRecExpr *Expr);
+  MinMaxType visitMulExpr(const SCEVMulExpr *Expr);
+  MinMaxType visitSignExtendExpr(const SCEVSignExtendExpr *Expr);
+
+  MinMaxType visitUnknown(const SCEVUnknown *Expr) {
+    return constantHelper(Expr);
+  }
+  MinMaxType visitConstant(const SCEVConstant *Expr) {
+    return constantHelper(Expr);
+  }
+  MinMaxType visitVScale(const SCEVVScale *Expr) {
+    return constantHelper(Expr);
+  }
+
+  MinMaxType visitZeroExtendExpr(const SCEVZeroExtendExpr *) { return Bottom; }
+  MinMaxType visitPtrToIntExpr(const SCEVPtrToIntExpr *) { return Bottom; }
+  MinMaxType visitTruncateExpr(const SCEVTruncateExpr *) { return Bottom; }
+  MinMaxType visitUDivExpr(const SCEVUDivExpr *) { return Bottom; }
+  MinMaxType visitSMaxExpr(const SCEVSMaxExpr *) { return Bottom; }
+  MinMaxType visitUMaxExpr(const SCEVUMaxExpr *) { return Bottom; }
+  MinMaxType visitSMinExpr(const SCEVSMinExpr *) { return Bottom; }
+  MinMaxType visitUMinExpr(const SCEVUMinExpr *) { return Bottom; }
+  MinMaxType visitSequentialUMinExpr(const SCEVSequentialUMinExpr *) {
+    return Bottom;
+  }
+  MinMaxType visitCouldNotCompute(const SCEVCouldNotCompute *) {
+    return Bottom;
+  }
+
+  MinMaxType constantHelper(const SCEV *C) { return MinMaxType(C, C); }
+
+private:
+  ScalarEvolution *SE;
+  const Loop *OutermostLoop;
+};
+
+} // anonymous namespace
+
+MonotonicityType
+SCEVSignedMonotonicityChecker::visitAddExpr(const SCEVAddExpr *Expr) {
+  if (!Expr->hasNoSignedWrap())
+    return MonotonicityType::MaySignedWrap;
+
+  MonotonicityType Result = MonotonicityType::Constant;
+  for (const SCEV *Op : Expr->operands()) {
+    switch (visit(Op)) {
+    case MonotonicityType::MaySignedWrap:
+      return MonotonicityType::MaySignedWrap;
+    case MonotonicityType::Constant:
+      break;
+    case MonotonicityType::Monotonic:
+      // Monotonic + Monotonic might be constant, so at the moment return
+      // MaySignedWrap.
+      // TODO: Should we separate Monotonically increasing and decreasing? Or
+      // SCEV is always simplified enough so that we don't have to consider such
+      // cases?
+      if (Result == MonotonicityType::Monotonic)
+        return MonotonicityType::MaySignedWrap;
+      Result = MonotonicityType::Constant;
+      break;
+    }
+  }
+  return Result;
+}
+
+MonotonicityType
+SCEVSignedMonotonicityChecker::visitMulExpr(const SCEVMulExpr *Expr) {
+  if (!Expr->hasNoSignedWrap())
+    return MonotonicityType::MaySignedWrap;
+
+  // Same as visitAddExpr.
+  MonotonicityType Result = MonotonicityType::Constant;
+  for (const SCEV *Op : Expr->operands()) {
+    switch (visit(Op)) {
+    case MonotonicityType::MaySignedWrap:
+      return MonotonicityType::MaySignedWrap;
+    case MonotonicityType::Constant:
+      break;
+    case MonotonicityType::Monotonic:
+      if (Result == MonotonicityType::Monotonic)
+        return MonotonicityType::MaySignedWrap;
+      Result = MonotonicityType::Constant;
+      break;
+    }
+  }
+  return Result;
+}
+
+MonotonicityType
+SCEVSignedMonotonicityChecker::visitAddRecExpr(const SCEVAddRecExpr *Expr) {
+  if (!Expr->isAffine())
+    return MonotonicityType::MaySignedWrap;
+
+  const SCEV *Start = Expr->getStart();
+  const SCEV *Step = Expr->getStepRecurrence(*SE);
+
+  MonotonicityType StartRes = visit(Start);
+  if (StartRes == MonotonicityType::MaySignedWrap)
+    return MonotonicityType::MaySignedWrap;
+
+  MonotonicityType StepRes = visit(Step);
+  if (StepRes != MonotonicityType::Constant || !SE->isKnownNonZero(Step))
+    return MonotonicityType::MaySignedWrap;
+
+  bool IsNSW = [&] {
+    if (Expr->hasNoSignedWrap())
+      return true;
+
+    if (Ptr) {
+      // TODO: This seems incorrect. Maybe we should check the reachability from
+      // the GEP to the target instruction. E.g., in the following case, maybe
+      // no-wrap is not guaranteed:
+      //
+      //  entry:
+      //    ...
+      //    %gep = getelementptr inbounds i32, ptr %ptr, i32 %addrec
+      //    br i1 %cond, label %store, label %sink
+      //
+      //   store:
+      //     store i32 42, ptr %ptr
+      //     br label %sink
+      //
+      //   sink:
+      //     ...
+      //
+      auto *GEP = dyn_cast<GetElementPtrInst>(Ptr);
+      if (GEP && GEP->hasNoUnsignedSignedWrap())
+        return true;
+    }
+
+    return false;
+  }();
+
+  if (!IsNSW) {
+    if (!SE->isKnownNegative(Step))
+      // If the coefficient can be positive value, ensure that the AddRec is
+      // monotonically increasing.
+      if (!SE->isKnownOnEveryIteration(ICmpInst::ICMP_SGE, Expr, Start))
+        return MonotonicityType::MaySignedWrap;
+
+    if (!SE->isKnownPositive(Step))
+      // If the coefficient can be positive value, ensure that the AddRec is
+      // monotonically decreasing.
+      if (!SE->isKnownOnEveryIteration(ICmpInst::ICMP_SLE, Expr, Start))
+        return MonotonicityType::MaySignedWrap;
+  }
+
+  return MonotonicityType::Monotonic;
+}
+
+MonotonicityType SCEVSignedMonotonicityChecker::visitZeroExtendExpr(
+    const SCEVZeroExtendExpr *Expr) {
+  return visit(Expr->getOperand());
+}
+
+MonotonicityType SCEVSignedMonotonicityChecker::visitSignExtendExpr(
+    const SCEVSignExtendExpr *Expr) {
+  return visit(Expr->getOperand());
+}
+
+MonotonicityType
+SCEVSignedMonotonicityChecker::visitUnknown(const SCEVUnknown *Expr) {
+  return SE->isLoopInvariant(Expr, OutermostLoop)
+             ? MonotonicityType::Constant
+             : MonotonicityType::MaySignedWrap;
----------------
kasuga-fj wrote:

Just to align with the current behavior.

https://github.com/llvm/llvm-project/blob/db024764c18413a3b2d3cebe2bb06a09eeef507d/llvm/lib/Analysis/DependenceAnalysis.cpp#L854-L867

But I'm not sure whether checking invariance under the current loop is sufficient or not.

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