[Attributor] Provide an edge-based interface in AAIsDead

This patch produces an edge-based interface in AAIsDead.
By this, we can query a set of basic blocks that are directly reachable from a given basic block.
This is specifically useful for implementation of AAReachability.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D85547

llvm/include/llvm/Transforms/IPO/Attributor.h

@@ -2646,6 +2646,12 @@ public:
     return F.hasPersonalityFn() && !canSimplifyInvokeNoUnwind(&F);
   }
 
+  /// Return if the edge from \p From BB to \p To BB is assumed dead.
+  /// This is specifically useful in AAReachability.
+  virtual bool isEdgeDead(const BasicBlock *From, const BasicBlock *To) const {
+    return false;
+  }
+
   /// See AbstractAttribute::getName()
   const std::string getName() const override { return "AAIsDead"; }
 

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

@@ -3093,6 +3093,10 @@ struct AAIsDeadFunction : public AAIsDead {
   /// See AbstractAttribute::updateImpl(...).
   ChangeStatus updateImpl(Attributor &A) override;
 
+  bool isEdgeDead(const BasicBlock *From, const BasicBlock *To) const override {
+    return !AssumedLiveEdges.count(std::make_pair(From, To));
+  }
+
   /// See AbstractAttribute::trackStatistics()
   void trackStatistics() const override {}
 
@@ -3170,6 +3174,9 @@ struct AAIsDeadFunction : public AAIsDead {
   /// Collection of instructions that are known to not transfer control.
   SmallSetVector<const Instruction *, 8> KnownDeadEnds;
 
+  /// Collection of all assumed live edges
+  DenseSet<std::pair<const BasicBlock *, const BasicBlock *>> AssumedLiveEdges;
+
   /// Collection of all assumed live BasicBlocks.
   DenseSet<const BasicBlock *> AssumedLiveBlocks;
 };
@@ -3340,6 +3347,9 @@ ChangeStatus AAIsDeadFunction::updateImpl(Attributor &A) {
                "Non-terminator expected to have a single successor!");
         Worklist.push_back(AliveSuccessor);
       } else {
+        // record the assumed live edge
+        AssumedLiveEdges.insert(
+            std::make_pair(I->getParent(), AliveSuccessor->getParent()));
         if (assumeLive(A, *AliveSuccessor->getParent()))
           Worklist.push_back(AliveSuccessor);
       }