[WIP][Attributor] AAReachability Attribute

Summary: Working towards Johannes's suggestion for fixme, in Attributor's Noalias attribute deduction.
(ii) Check whether the value is captured in the scope using AANoCapture.
FIXME: This is conservative though, it is better to look at CFG and
// check only uses possibly executed before this call site.

A Reachability abstract attribute answers the question "does execution at point A potentially reach point B". If this question is answered with false for all other uses of the value that might be captured, we know it is not *yet* captured and can continue with the noalias deduction. Currently, information AAReachability provides is completely pessimistic.

    Reviewers: jdoerfert

    Reviewed By: jdoerfert

    Subscribers: uenoku, sstefan1, hiraditya, llvm-commits

    Differential Revision: https://reviews.llvm.org/D70233

include/llvm/Transforms/IPO/Attributor.h

@@ -1682,6 +1682,37 @@ struct AAWillReturn
   static const char ID;
 };
 
+/// An abstract interface to determine reachability of point A to B.
+struct AAReachability : public StateWrapper<BooleanState, AbstractAttribute>,
+                        public IRPosition {
+  AAReachability(const IRPosition &IRP) : IRPosition(IRP) {}
+
+  /// Returns true if 'From' instruction is assumed to reach, 'To' instruction.
+  /// Users should provide two positions they are interested in, and the class
+  /// determines (and caches) reachability.
+  bool isAssumedReachable(const Instruction *From,
+                          const Instruction *To) const {
+    return true;
+  }
+
+  /// Returns true if 'From' instruction is known to reach, 'To' instruction.
+  /// Users should provide two positions they are interested in, and the class
+  /// determines (and caches) reachability.
+  bool isKnownReachable(const Instruction *From, const Instruction *To) const {
+    return true;
+  }
+
+  /// Return an IR position, see struct IRPosition.
+  const IRPosition &getIRPosition() const override { return *this; }
+
+  /// Create an abstract attribute view for the position \p IRP.
+  static AAReachability &createForPosition(const IRPosition &IRP,
+                                           Attributor &A);
+
+  /// Unique ID (due to the unique address)
+  static const char ID;
+};
+
 /// An abstract interface for all noalias attributes.
 struct AANoAlias
     : public IRAttribute<Attribute::NoAlias,

lib/Transforms/IPO/Attributor.cpp

@@ -2051,6 +2051,34 @@ struct AAWillReturnCallSite final : AAWillReturnImpl {
   void trackStatistics() const override { STATS_DECLTRACK_CS_ATTR(willreturn); }
 };
 
+/// -------------------AAReachability Attribute--------------------------
+
+struct AAReachabilityImpl : AAReachability {
+  AAReachabilityImpl(const IRPosition &IRP) : AAReachability(IRP) {}
+
+  const std::string getAsStr() const override {
+    // TODO: Return the number of reachable queries.
+    return "reachable";
+  }
+
+  /// See AbstractAttribute::initialize(...).
+  void initialize(Attributor &A) override {
+    indicatePessimisticFixpoint();
+  }
+
+  /// See AbstractAttribute::updateImpl(...).
+  ChangeStatus updateImpl(Attributor &A) override {
+    return indicatePessimisticFixpoint();
+  }
+};
+
+struct AAReachabilityFunction final : public AAReachabilityImpl {
+  AAReachabilityFunction(const IRPosition &IRP) : AAReachabilityImpl(IRP) {}
+
+  /// See AbstractAttribute::trackStatistics()
+  void trackStatistics() const override { STATS_DECLTRACK_FN_ATTR(reachable); }
+};
+
 /// ------------------------ NoAlias Argument Attribute ------------------------
 
 struct AANoAliasImpl : AANoAlias {
@@ -5680,6 +5708,7 @@ const char AANonNull::ID = 0;
 const char AANoRecurse::ID = 0;
 const char AAWillReturn::ID = 0;
 const char AANoAlias::ID = 0;
+const char AAReachability::ID = 0;
 const char AANoReturn::ID = 0;
 const char AAIsDead::ID = 0;
 const char AADereferenceable::ID = 0;
@@ -5799,6 +5828,7 @@ CREATE_ALL_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAIsDead)
 CREATE_ALL_ABSTRACT_ATTRIBUTE_FOR_POSITION(AANoFree)
 
 CREATE_FUNCTION_ONLY_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAHeapToStack)
+CREATE_FUNCTION_ONLY_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAReachability)
 
 CREATE_NON_RET_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAMemoryBehavior)