zeek/src/script_opt/UseDefs.h

// See the file "COPYING" in the main distribution directory for copyright.

#pragma once

#include <unordered_map>
#include <unordered_set>
#include <vector>

#include "zeek/Expr.h"

namespace zeek::detail
	{

// UseDefs track which variables (identifiers) are used at or subsequent
// to a given (reduced) Statement.  They allow us to determine unproductive
// variable assignments (both to warn the user, and to prune temporaries),
// and also accesses to globals (so we know which ones need to be synchronized
// across function calls).

class UseDefSet;
using UDs = std::shared_ptr<UseDefSet>;

class UseDefSet
	{
public:
	UseDefSet() { }
	UseDefSet(const UDs& uds) { Replicate(uds); }

	void Replicate(const UDs& from) { use_defs = from->use_defs; }

	bool HasID(const ID* id) { return use_defs.find(id) != use_defs.end(); }

	void Add(const ID* id) { use_defs.insert(id); }
	void Remove(const ID* id) { use_defs.erase(id); }

	const std::unordered_set<const ID*>& IterateOver() const { return use_defs; }

	void Dump() const;
	void DumpNL() const
		{
		Dump();
		printf("\n");
		}

protected:
	std::unordered_set<const ID*> use_defs;
	};

class Reducer;

class UseDefs
	{
public:
	UseDefs(StmtPtr body, std::shared_ptr<Reducer> rc);

	// Does a full pass over the function body's AST.  We can wind
	// up doing this multiple times because when we use use-defs to
	// prune AST nodes, that in turn can change other use-defs.
	void Analyze();

	// True if we've computed use-defs for the given statement.
	bool HasUsage(const Stmt* s) const { return use_defs_map.find(s) != use_defs_map.end(); }
	bool HasUsage(const StmtPtr& s) const { return HasUsage(s.get()); }

	// Returns the use-defs for the given statement.
	UDs GetUsage(const Stmt* s) const { return FindUsage(s); }
	UDs GetUsage(const StmtPtr& s) const { return FindUsage(s.get()); }

	// Removes assignments corresponding to unused temporaries.
	// In the process, reports on locals that are assigned
	// but never used.  Returns the body, which may have been
	// changed if the original first statement has been pruned.
	StmtPtr RemoveUnused();

	void Dump();

private:
	// Makes one pass over the statements, removing assignments
	// corresponding to temporaries (because those can be propagted).
	// "iter" is the iteration count of how often we've done such passes,
	// with the first pass being numbered 1.
	//
	// Returns true if something was removed, false if not.
	bool RemoveUnused(int iter);

	// For a given identifier defined at a given statement, returns
	// whether it is unused.  If "report" is true, also reports
	// this fact.
	bool CheckIfUnused(const Stmt* s, const ID* id, bool report);

	// Propagates use-defs (backwards) across statement s,
	// given its successor's UDs.
	//
	// succ_stmt is the successor statement to this statement.
	// We only care about it for potential assignment statements,
	// (see the "successor" map below).
	//
	// second_pass is true when we revisit a set of statements
	// to propagate additional UDs generated by loop confluence.
	// If true, it prevents some redundant bookkeeping from occurring.
	UDs PropagateUDs(const StmtPtr& s, UDs succ_UDs, const StmtPtr& succ_stmt, bool second_pass)
		{
		return PropagateUDs(s.get(), std::move(succ_UDs), succ_stmt.get(), second_pass);
		}
	UDs PropagateUDs(const Stmt* s, UDs succ_UDs, const Stmt* succ_stmt, bool second_pass);

	UDs FindUsage(const Stmt* s) const;
	UDs FindSuccUsage(const Stmt* s) const;

	// Returns a new use-def corresponding to the variables
	// referenced in e.
	UDs ExprUDs(const Expr* e);

	// Helper method that adds in an expression's use-defs (if any)
	// to an existing set of UDs.
	void AddInExprUDs(UDs uds, const Expr* e);

	// Add an ID into an existing set of UDs.
	void AddID(UDs uds, const ID* id) const;

	// Returns a new use-def corresonding to the given one, but
	// with the definition of "id" removed.
	UDs RemoveID(const ID* id, const UDs& uds);

	// Similar, but updates the UDs in place.
	void RemoveUDFrom(UDs uds, const ID* id);

	// Adds in the additional UDs to the main UDs.  Always creates
	// a new use_def and updates main_UDs to point to it.
	void FoldInUDs(UDs& main_UDs, const UDs& u1, const UDs& u2 = nullptr);

	// Adds in the given UDs to those already associated with s.
	void UpdateUDs(const Stmt* s, const UDs& uds);

	// Returns a new use-def corresponding to the union of 2 or 3 UDs.
	UDs UD_Union(const UDs& u1, const UDs& u2, const UDs& u3 = nullptr) const;

	// Associate a (shallow) copy of the given UDs with the given
	// statement.
	UDs UseUDs(const Stmt* s, UDs uds);

	// Sets the given statement's UDs to a new UD set corresponding
	// to the union of the given UDs and those associated with the
	// given expression.
	UDs CreateExprUDs(const Stmt* s, const Expr* e, const UDs& uds);

	// The given statement takes ownership of the given UDs.
	UDs CreateUDs(const Stmt* s, UDs uds);

	// Maps each statement to its associated use-def identifiers
	// (which could be nil).
	std::unordered_map<const Stmt*, UDs> use_defs_map;

	// The following tracks statements whose use-defs are
	// currently copies of some other statement's use-defs.
	std::unordered_set<const Stmt*> UDs_are_copies;

	// Track the statements we've processed.  This lets us dump
	// things out in order, even though the main map is unordered.
	std::vector<const Stmt*> stmts;

	// For a given expression statement, maps it to its successor
	// (the statement that will execute after it).  We need this
	// because we track UDs present at the *beginning* of
	// a statement, not at its end; those at the end are
	// the same as those at the beginning of the successor.
	std::unordered_map<const Stmt*, const Stmt*> successor;

	// Loop bodies have two successors, and it's important to
	// track both because sometimes a relevant UD will be present
	// in only one or the other.
	std::unordered_map<const Stmt*, const Stmt*> successor2;

	StmtPtr body;
	std::shared_ptr<Reducer> rc;
	};

	} // zeek::detail