diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index d7fe09c61a..3b0cd3acfd 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -323,6 +323,7 @@ set(MAIN_SRCS
     plugin/Plugin.cc
 
     script_opt/DefItem.cc
+    script_opt/DefSetsMgr.cc
     script_opt/Expr.cc
     script_opt/Inline.cc
     script_opt/ProfileFunc.cc
diff --git a/src/script_opt/DefSetsMgr.cc b/src/script_opt/DefSetsMgr.cc
new file mode 100644
index 0000000000..27229f9cb6
--- /dev/null
+++ b/src/script_opt/DefSetsMgr.cc
@@ -0,0 +1,63 @@
+// See the file "COPYING" in the main distribution directory for copyright.
+
+#include "zeek/script_opt/DefSetsMgr.h"
+
+
+namespace zeek::detail {
+
+
+DefSetsMgr::DefSetsMgr()
+	{
+	pre_min_defs = make_intrusive<ReachingDefSet>(item_map);
+	post_min_defs = make_intrusive<ReachingDefSet>(item_map);
+
+	pre_max_defs = make_intrusive<ReachingDefSet>(item_map);
+	post_max_defs = make_intrusive<ReachingDefSet>(item_map);
+	}
+
+
+void DefSetsMgr::CreatePostDef(const ID* id, DefinitionPoint dp, bool min_only)
+	{
+	auto di = item_map.GetID_DI(id);
+	CreatePostDef(di, dp, min_only);
+	}
+
+void DefSetsMgr::CreatePostDef(std::shared_ptr<DefinitionItem> di,
+				DefinitionPoint dp, bool min_only)
+	{
+	auto where = dp.OpaqueVal();
+
+	if ( ! post_min_defs->HasRDs(where) )
+		{
+		// We haven't yet started creating post RDs for this
+		// statement/expression, so create them.
+		auto pre = GetPreMinRDs(where);
+		SetPostFromPre(where);
+		}
+
+	if ( ! min_only && ! post_max_defs->HasRDs(where) )
+		{
+		auto pre = GetPreMaxRDs(where);
+		SetPostFromPre(where);
+		}
+
+	CreateDef(di, dp, false, min_only);
+	}
+
+void DefSetsMgr::CreateDef(std::shared_ptr<DefinitionItem> di,
+				DefinitionPoint dp, bool is_pre, bool min_only)
+	{
+	auto where = dp.OpaqueVal();
+	RDSetPtr min_defs = is_pre ? pre_min_defs : post_min_defs;
+
+	min_defs->AddOrReplace(where, di.get(), dp);
+
+	if ( min_only )
+		return;
+
+	RDSetPtr& max_defs = is_pre ? pre_max_defs : post_max_defs;
+	max_defs->AddOrReplace(where, di.get(), dp);
+	}
+
+
+} // zeek::detail
diff --git a/src/script_opt/DefSetsMgr.h b/src/script_opt/DefSetsMgr.h
new file mode 100644
index 0000000000..435f6f2b39
--- /dev/null
+++ b/src/script_opt/DefSetsMgr.h
@@ -0,0 +1,213 @@
+// See the file "COPYING" in the main distribution directory for copyright.
+
+#pragma once
+
+#include "zeek/script_opt/DefItem.h"
+#include "zeek/script_opt/DefPoint.h"
+#include "zeek/script_opt/ReachingDefs.h"
+
+
+namespace zeek::detail {
+
+
+// Class for managing collections of reaching definitions associated
+// with AST nodes.
+//
+// Each node has "pre" RDs reflecting the reaching definitions active
+// before the node executes, and "post" RDs reflecting the state after
+// executing.
+//
+// In addition, we track both *minimal* RDs (those guaranteed to exist)
+// and *maximal* RDs (those that _could_ exist).
+//
+// To illustrate both of these notions with an example, consider this
+// scripting code:
+//
+//	local x = 5;
+//	if ( predicate() )
+//		x = 9;
+//	foobar();
+//
+// The "x = 5" node has empty pre-RDs, and minimal and maximal post-RDs
+// of {x = 5 <node-at-line-1>}.  The "if" node and its interior call
+// to predicate() both inherit those post-RDs as their pre-RDs, and
+// these are also the post-RDs for predicate().
+//
+// The assignment "x = 9" inherits those post-RDs as its pre-RDs.
+// When it executes, has leaves both minimal and maximal post-RDs
+// of { x = 9 <node-at-line-3> }.
+//
+// The post-RDs for the "if" statement (and thus the pre-RDs for the foobar()
+// call) have a minimal set of { x = SOMETHING }, and a maximal set of
+// { x = 5 <node-at-line-1>, x = 9 <node-at-line-3> }.  The minimal set
+// here captures the notion that "x is definitely assigned to a value
+// at this point, but it's uncertain just what that value is".
+
+class DefSetsMgr {
+public:
+	DefSetsMgr();
+
+	// Returns the minimal or maximal pre-RDs associated with a given node.
+	RDPtr GetPreMinRDs(const Obj* o) const
+		{ return GetRDs(pre_min_defs, o); }
+	RDPtr GetPreMaxRDs(const Obj* o) const
+		{ return GetRDs(pre_max_defs, o); }
+
+	// Same, but for post-RDs.
+	RDPtr GetPostMinRDs(const Obj* o) const
+		{
+		if ( HasPostMinRDs(o) )
+			return GetRDs(post_min_defs, o);
+		else
+			return GetPreMinRDs(o);
+		}
+	RDPtr GetPostMaxRDs(const Obj* o) const
+		{
+		if ( HasPostMaxRDs(o) )
+			return GetRDs(post_max_defs, o);
+		else
+			return GetPreMaxRDs(o);
+		}
+
+	// Initialize a node's pre-RDs to be empty.
+	void SetEmptyPre(const Obj* o)
+		{
+		auto empty_rds = make_intrusive<ReachingDefs>();
+		SetPreMinRDs(o, empty_rds);
+		SetPreMaxRDs(o, empty_rds);
+		empty_rds.release();
+		}
+
+	// Inherit a node's pre-RDs from those of another node.
+	void SetPreFromPre(const Obj* target, const Obj* source)
+		{
+		SetPreMinRDs(target, GetPreMinRDs(source));
+		SetPreMaxRDs(target, GetPreMaxRDs(source));
+		}
+
+	// Inherit a node's pre-RDs from the post-RDs of another node.
+	void SetPreFromPost(const Obj* target, const Obj* source)
+		{
+		SetPreMinRDs(target, GetPostMinRDs(source));
+		SetPreMaxRDs(target, GetPostMaxRDs(source));
+		}
+
+	// Set the post-RDs for a given node to the given min/max values.
+	void SetPostRDs(const Obj* o, RDPtr min_rd, RDPtr max_rd)
+		{
+		SetPostMinRDs(o, min_rd);
+		SetPostMaxRDs(o, max_rd);
+		}
+
+	// Propagate the node's pre-RDs to also be its post-RDs.
+	void SetPostFromPre(const Obj* o)
+		{
+		SetPostMinRDs(o, GetPreMinRDs(o));
+		SetPostMaxRDs(o, GetPreMaxRDs(o));
+		}
+
+	// Inherit a node's post-RDs from another node's pre-RDs.
+	void SetPostFromPre(const Obj* target, const Obj* source)
+		{
+		SetPostMinRDs(target, GetPreMinRDs(source));
+		SetPostMaxRDs(target, GetPreMaxRDs(source));
+		}
+
+	// Inherit a node's post-RDs from another node's post-RDs.
+	void SetPostFromPost(const Obj* target, const Obj* source)
+		{
+		SetPostMinRDs(target, GetPostMinRDs(source));
+		SetPostMaxRDs(target, GetPostMaxRDs(source));
+		}
+
+	// Fine-grained control for setting RDs.
+	void SetPreMinRDs(const Obj* o, RDPtr rd)
+		{ pre_min_defs->SetRDs(o, rd); }
+	void SetPreMaxRDs(const Obj* o, RDPtr rd)
+		{ pre_max_defs->SetRDs(o, rd); }
+
+	void SetPostMinRDs(const Obj* o, RDPtr rd)
+		{ post_min_defs->SetRDs(o, rd); }
+	void SetPostMaxRDs(const Obj* o, RDPtr rd)
+		{ post_max_defs->SetRDs(o, rd); }
+
+	// Used for confluence: add a set of RDs into those already
+	// associated with a node's pre-RDs / post-RDs.  Only applies
+	// to maximal RDs.
+	void MergeIntoPre(const Obj* o, const RDPtr& rds)
+		{ pre_max_defs->AddRDs(o, rds); }
+	void MergeIntoPost(const Obj* o, const RDPtr& rds)
+		{ post_max_defs->AddRDs(o, rds); }
+
+	// The same, but merging a node's own maximal post-RDs into
+	// its maximal pre-RDs.
+	void MergePostIntoPre(const Obj* o)
+		{ MergeIntoPre(o, GetPostMaxRDs(o)); }
+
+
+	// The following predicates look up whether a given node exists
+	// in the given pre/post minimal/maximal RDs.
+	bool HasPreMinRDs(const Obj* o) const
+		{ return pre_min_defs && pre_min_defs->HasRDs(o); }
+	bool HasPreMaxRDs(const Obj* o) const
+		{ return pre_max_defs && pre_max_defs->HasRDs(o); }
+
+	bool HasPostMinRDs(const Obj* o) const
+		{ return post_min_defs && post_min_defs->HasRDs(o); }
+	bool HasPostMaxRDs(const Obj* o) const
+		{ return post_max_defs && post_max_defs->HasRDs(o); }
+
+	// True if the given node has a minimal pre-RD associated
+	// with the given identifier.
+	bool HasPreMinRD(const Obj* o, const ID* id) const
+		{ return pre_min_defs && pre_min_defs->HasRD(o, id); }
+
+	// True if at the given node, there's a single *unambiguous*
+	// pre RD for the given identifier.
+	bool HasSinglePreMinRD(const Obj* o, const ID* id) const
+		{ return pre_min_defs && pre_min_defs->HasSingleRD(o, id); }
+
+
+	// Methods for creating new pre/post RDs.  If min_only is true,
+	// then only done for minimal RDs.
+	void CreatePreDef(std::shared_ptr<DefinitionItem> di,
+				DefinitionPoint dp, bool min_only)
+		{ CreateDef(di, dp, true, min_only); }
+	void CreatePostDef(const ID* id, DefinitionPoint dp, bool min_only);
+	void CreatePostDef(std::shared_ptr<DefinitionItem> di,
+				DefinitionPoint dp, bool min_only);
+
+	std::shared_ptr<DefinitionItem> GetExprDI(const Expr* e)
+		{ return item_map.GetExprDI(e); }
+	std::shared_ptr<DefinitionItem> GetID_DI(const ID* id)
+		{ return item_map.GetID_DI(id); }
+	const DefinitionItem* GetConstID_DI(const ID* id) const
+		{ return item_map.GetConstID_DI(id); }
+        const DefinitionItem* GetConstID_DI(const DefinitionItem* di,
+						const char* field_name) const
+		{ return item_map.GetConstID_DI(di, field_name); }
+
+private:
+	void CreateDef(std::shared_ptr<DefinitionItem> di, DefinitionPoint dp,
+			bool is_pre, bool min_only);
+
+	RDPtr GetRDs(const RDSetPtr& defs, const Obj* o) const
+		{
+		return defs->FindRDs(o);
+		}
+
+	// Mappings of minimal reaching defs pre- and post- execution
+	// of the given node.
+	RDSetPtr pre_min_defs;
+	RDSetPtr post_min_defs;
+
+	// Mappings of maximal reaching defs pre- and post- execution
+	// of the given node.
+	RDSetPtr pre_max_defs;
+	RDSetPtr post_max_defs;
+
+	DefItemMap item_map;
+};
+
+
+} // zeek::detail