[MemAccessUtils] Visit access as unique storage uses.

Add a new member functino UniqueStorageUseVisitor::visitBeginAccess and
call it when GatherUniqueStorageUses runs.

Author: nate-chandler

include/swift/SIL/MemAccessUtils.h

@@ -1098,7 +1098,6 @@ class AccessPath {
   // ignores its subclass bits.
   AccessPath(AccessStorage storage, PathNode pathNode, int offset)
       : storage(storage), pathNode(pathNode), offset(offset) {
-    assert(storage.getKind() != AccessStorage::Nested);
     assert(pathNode.isValid() || !storage && "Access path requires a pathNode");
   }
 
@@ -1337,6 +1336,7 @@ namespace swift {
 /// to the storage within this function is derived from these roots.
 ///
 /// Gather the kinds of uses that are typically relevant to algorithms:
+/// - accesses    (specifically, begin_access insts)
 /// - loads       (including copies out of, not including inout args)
 /// - stores      (including copies into and inout args)
 /// - destroys    (of the entire aggregate)
@@ -1353,6 +1353,7 @@ struct UniqueStorageUseVisitor {
 
   virtual ~UniqueStorageUseVisitor() = default;
 
+  virtual bool visitBeginAccess(Operand *use) = 0;
   virtual bool visitLoad(Operand *use) = 0;
   virtual bool visitStore(Operand *use) = 0;
   virtual bool visitDestroy(Operand *use) = 0;

lib/SIL/Utils/MemAccessUtils.cpp

@@ -1941,7 +1941,8 @@ struct GatherUniqueStorageUses : public AccessUseVisitor {
 };
 
 bool UniqueStorageUseVisitor::findUses(UniqueStorageUseVisitor &visitor) {
-  assert(visitor.storage.isUniquelyIdentified());
+  assert(visitor.storage.isUniquelyIdentified() ||
+         visitor.storage.getKind() == AccessStorage::Kind::Nested);
 
   GatherUniqueStorageUses gather(visitor);
   return visitAccessStorageUses(gather, visitor.storage, visitor.function);
@@ -1972,6 +1973,9 @@ bool GatherUniqueStorageUses::visitUse(Operand *use, AccessUseType useTy) {
     }
   }
   switch (user->getKind()) {
+  case SILInstructionKind::BeginAccessInst:
+    return visitor.visitBeginAccess(use);
+
   case SILInstructionKind::DestroyAddrInst:
   case SILInstructionKind::DestroyValueInst:
     if (useTy == AccessUseType::Exact) {
@@ -1982,6 +1986,9 @@ bool GatherUniqueStorageUses::visitUse(Operand *use, AccessUseType useTy) {
   case SILInstructionKind::DebugValueInst:
     return visitor.visitDebugUse(use);
 
+  case SILInstructionKind::EndAccessInst:
+    return true;
+
   case SILInstructionKind::LoadInst:
   case SILInstructionKind::LoadWeakInst:
   case SILInstructionKind::LoadUnownedInst:

lib/SILOptimizer/Transforms/SSADestroyHoisting.cpp

@@ -142,6 +142,14 @@ struct KnownStorageUses : UniqueStorageUseVisitor {
     return true;
   }
 
+  bool visitBeginAccess(Operand *use) override {
+    auto *bai = cast<BeginAccessInst>(use->getUser());
+    for (auto *eai : bai->getEndAccesses()) {
+      storageUsers.insert(eai);
+    }
+    return true;
+  }
+
   bool visitLoad(Operand *use) override { return recordUser(use->getUser()); }
 
   bool visitStore(Operand *use) override { return recordUser(use->getUser()); }