zeek/src/script_opt/Reduce.h

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

#pragma once

#include "zeek/Expr.h"
#include "zeek/Scope.h"
#include "zeek/Stmt.h"
#include "zeek/Traverse.h"

namespace zeek::detail
	{

class Expr;
class TempVar;

class Reducer
	{
public:
	Reducer() { }

	StmtPtr Reduce(StmtPtr s);

	void SetReadyToOptimize() { opt_ready = true; }

	void SetCurrStmt(const Stmt* stmt) { curr_stmt = stmt; }

	ExprPtr GenTemporaryExpr(const TypePtr& t, ExprPtr rhs);

	NameExprPtr UpdateName(NameExprPtr n);
	bool NameIsReduced(const NameExpr* n) const;

	void UpdateIDs(IDPList* ids);
	bool IDsAreReduced(const IDPList* ids) const;

	void UpdateIDs(std::vector<IDPtr>& ids);
	bool IDsAreReduced(const std::vector<IDPtr>& ids) const;

	IDPtr UpdateID(IDPtr id);
	bool ID_IsReduced(const IDPtr& id) const { return ID_IsReduced(id.get()); }
	bool ID_IsReduced(const ID* id) const;

	// This is called *prior* to pushing a new inline block, in
	// order to generate the equivalent of function parameters.
	NameExprPtr GenInlineBlockName(const IDPtr& id);

	int NumNewLocals() const { return new_locals.size(); }

	// Returns the name of a temporary for holding the return
	// value of the block, or nil if the type indicates there's
	// o return value.
	NameExprPtr PushInlineBlock(TypePtr type);
	void PopInlineBlock();

	// Whether it's okay to split a statement into two copies for if-else
	// expansion.  We only allow this to a particular depth because
	// beyond that a function body can get too large to analyze.
	bool BifurcationOkay() const { return bifurcation_level <= 12; }
	int BifurcationLevel() const { return bifurcation_level; }

	void PushBifurcation() { ++bifurcation_level; }
	void PopBifurcation() { --bifurcation_level; }

	int NumTemps() const { return temps.size(); }

	// True if this name already reflects the replacement.
	bool IsNewLocal(const NameExpr* n) const { return IsNewLocal(n->Id()); }
	bool IsNewLocal(const ID* id) const;

	bool IsTemporary(const ID* id) const { return FindTemporary(id) != nullptr; }

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

	// True if the Reducer is being used in the context of a second
	// pass over for AST optimization.
	bool Optimizing() const { return ! IsPruning() && opt_ready; }

	// A predicate that indicates whether a given reduction pass
	// is being made to prune unused statements.
	bool IsPruning() const { return omitted_stmts.size() > 0; }

	// A predicate that returns true if the given statement should
	// be removed due to AST optimization.
	bool ShouldOmitStmt(const Stmt* s) const
		{
		return omitted_stmts.find(s) != omitted_stmts.end();
		}

	// Provides a replacement for the given statement due to
	// AST optimization, or nil if there's no replacement.
	StmtPtr ReplacementStmt(const StmtPtr& s) const
		{
		auto repl = replaced_stmts.find(s.get());
		if ( repl == replaced_stmts.end() )
			return nullptr;
		else
			return repl->second;
		}

	// Tells the reducer to prune the given statement during the
	// next reduction pass.
	void AddStmtToOmit(const Stmt* s) { omitted_stmts.insert(s); }

	// Tells the reducer to replace the given statement during the
	// next reduction pass.
	void AddStmtToReplace(const Stmt* s_old, StmtPtr s_new)
		{
		replaced_stmts[s_old] = std::move(s_new);
		}

	// Tells the reducer that it can reclaim the storage associated
	// with the omitted statements.
	void ResetAlteredStmts()
		{
		omitted_stmts.clear();
		replaced_stmts.clear();
		}

	// Given the LHS and RHS of an assignment, returns true
	// if the RHS is a common subexpression (meaning that the
	// current assignment statement should be deleted).  In
	// that case, has the side effect of associating an alias
	// for the LHS with the temporary variable that holds the
	// equivalent RHS.
	//
	// Assumes reduction (including alias propagation) has
	// already been applied.
	bool IsCSE(const AssignExpr* a, const NameExpr* lhs, const Expr* rhs);

	// Returns a constant representing folding of the given expression
	// (which must have constant operands).
	ConstExprPtr Fold(ExprPtr e);

	// Notes that the given expression has been folded to the
	// given constant.
	void FoldedTo(ExprPtr orig, ConstExprPtr c);

	// Given an lhs=rhs statement followed by succ_stmt, returns
	// a (new) merge of the two if they're of the form tmp=rhs, var=tmp;
	// otherwise, nil.
	StmtPtr MergeStmts(const NameExpr* lhs, ExprPtr rhs, const StmtPtr& succ_stmt);

	// Update expressions with optimized versions.  They are distinct
	// because the first two (meant for calls in a Stmt reduction
	// context) will also Reduce the expression, whereas the last
	// one (meant for calls in an Expr context) does not, to avoid
	// circularity.
	ExprPtr OptExpr(Expr* e);
	ExprPtr OptExpr(const ExprPtr& e) { return OptExpr(e.get()); }

	// This one for expressions appearing in an Expr context.
	ExprPtr UpdateExpr(ExprPtr e);

protected:
	// True if two Val's refer to the same underlying value.  We gauge
	// this conservatively (i.e., for complicated values we just return
	// false, even if with a lot of work we could establish that they
	// are in fact equivalent.)
	bool SameVal(const Val* v1, const Val* v2) const;

	// Track that the variable "var" will be a replacement for
	// the "orig" expression.  Returns the replacement expression
	// (which is is just a NameExpr referring to "var").
	ExprPtr NewVarUsage(IDPtr var, const Expr* orig);

	void BindExprToCurrStmt(const ExprPtr& e);
	void BindStmtToCurrStmt(const StmtPtr& s);

	// Returns true if op1 and op2 represent the same operand, given
	// the reaching definitions available at their usages (e1 and e2).
	bool SameOp(const Expr* op1, const Expr* op2);
	bool SameOp(const ExprPtr& op1, const ExprPtr& op2) { return SameOp(op1.get(), op2.get()); }

	// True if e1 and e2 reflect identical expressions in the context
	// of using a value computed for one of them in lieu of computing
	// the other.  (Thus, for example, two record construction expressions
	// are never equivalent even if they both specify exactly the same
	// record elements, because each invocation of the expression produces
	// a distinct value.)
	bool SameExpr(const Expr* e1, const Expr* e2);

	// Finds a temporary, if any, whose RHS matches the given "rhs", using
	// the reaching defs associated with the assignment "a".  The context
	// is that "rhs" is currently being assigned to temporary "lhs_tmp"
	// (nil if the assignment isn't to a temporary), and we're wondering
	// whether we can skip that assignment because we already have the
	// exact same value available in a previously assigned temporary.
	IDPtr FindExprTmp(const Expr* rhs, const Expr* a,
	                  const std::shared_ptr<const TempVar>& lhs_tmp);

	// Tests whether an expression computed at e1 (and assigned to "id")
	// remains valid for substitution at e2.
	bool ExprValid(const ID* id, const Expr* e1, const Expr* e2) const;

	// Inspects the given expression for identifiers, adding any
	// observed to the given vector.  Assumes reduced form, so only
	// NameExpr's and ListExpr's are of interest - does not traverse
	// into compound expressions.
	void CheckIDs(const Expr* e, std::vector<const ID*>& ids) const;

	IDPtr GenTemporary(const TypePtr& t, ExprPtr rhs);
	std::shared_ptr<TempVar> FindTemporary(const ID* id) const;

	// Retrieve the identifier corresponding to the new local for
	// the given expression.  Creates the local if necessary.
	IDPtr FindNewLocal(const IDPtr& id);
	IDPtr FindNewLocal(const NameExprPtr& n) { return FindNewLocal(n->IdPtr()); }

	// Generate a new local to use in lieu of the original (seen
	// in an inlined block).  The difference is that the new
	// version has a distinct name and has a correct frame offset
	// for the current function.
	IDPtr GenLocal(const IDPtr& orig);

	// This is the heart of constant propagation.  Given an identifier,
	// if its value is constant at the given location then returns
	// the corresponding ConstExpr with the value.
	const ConstExpr* CheckForConst(const IDPtr& id, int stmt_num) const;

	// Tracks the temporary variables created during the reduction/
	// optimization process.
	std::vector<std::shared_ptr<TempVar>> temps;

	// Temps for which we've processed their associated expression
	// (and they didn't wind up being aliases).
	std::vector<std::shared_ptr<const TempVar>> expr_temps;

	// Lets us go from an identifier to an associated temporary
	// variable, if it corresponds to one.
	std::unordered_map<const ID*, std::shared_ptr<TempVar>> ids_to_temps;

	// Local variables created during reduction/optimization.
	IDSet new_locals;

	// Mapping of original identifiers to new locals.  Used to
	// rename local variables when inlining.
	std::unordered_map<const ID*, IDPtr> orig_to_new_locals;

	// Tracks expressions we've folded, so that we can recognize them
	// for constant propagation.
	std::unordered_map<const Expr*, ConstExprPtr> constant_exprs;

	// Holds onto those expressions so they don't become invalid
	// due to memory management.
	std::vector<ExprPtr> folded_exprs;

	// Which statements to elide from the AST (because optimization
	// has determined they're no longer needed).
	std::unordered_set<const Stmt*> omitted_stmts;

	// Maps statements to replacements constructed during optimization.
	std::unordered_map<const Stmt*, StmtPtr> replaced_stmts;

	// Tracks whether we're inside an inline block, and if so then
	// how deeply.
	int inline_block_level = 0;

	// Tracks how deeply we are in "bifurcation", i.e., duplicating
	// code for if-else cascades.  We need to cap this at a certain
	// depth or else we can get functions whose size blows up
	// exponentially.
	int bifurcation_level = 0;

	// Tracks which (non-temporary) variables had constant
	// values used for constant propagation.
	IDSet constant_vars;

	// Statement at which the current reduction started.
	StmtPtr reduction_root = nullptr;

	// Statement we're currently working on.
	const Stmt* curr_stmt = nullptr;

	bool opt_ready = false;
	};

// Helper class that walks an AST to determine whether it's safe
// to substitute a common subexpression (which at this point is
// an assignment to a variable) created using the assignment
// expression at position "start_e", at the location specified by
// the expression at position "end_e".
//
// See Reducer::ExprValid for a discussion of what's required
// for safety.

class CSE_ValidityChecker : public TraversalCallback
	{
public:
	CSE_ValidityChecker(const std::vector<const ID*>& ids, const Expr* start_e, const Expr* end_e);

	TraversalCode PreStmt(const Stmt*) override;
	TraversalCode PostStmt(const Stmt*) override;
	TraversalCode PreExpr(const Expr*) override;

	// Returns the ultimate verdict re safety.
	bool IsValid() const
		{
		if ( ! is_valid )
			return false;

		if ( ! have_end_e )
			reporter->InternalError("CSE_ValidityChecker: saw start but not end");
		return true;
		}

protected:
	// Returns true if an assignment involving the given identifier on
	// the LHS is in conflict with the given list of identifiers.
	bool CheckID(const std::vector<const ID*>& ids, const ID* id, bool ignore_orig) const;

	// Returns true if the assignment given by 'e' modifies an aggregate
	// with the same type as that of one of the identifiers.
	bool CheckAggrMod(const std::vector<const ID*>& ids, const Expr* e) const;

	// The list of identifiers for which an assignment to one of them
	// renders the CSE unsafe.
	const std::vector<const ID*>& ids;

	// Where in the AST to start our analysis.  This is the initial
	// assignment expression.
	const Expr* start_e;

	// Where in the AST to end our analysis.
	const Expr* end_e;

	// If what we're analyzing is a record element, then its offset.
	// -1 if not.
	int field;

	// The type of that record element, if any.
	TypePtr field_type;

	// The verdict so far.
	bool is_valid = true;

	// Whether we've encountered the start/end expression in
	// the AST traversal.
	bool have_start_e = false;
	bool have_end_e = false;

	// Whether analyzed expressions occur in the context of
	// a statement that modifies an aggregate ("add" or "delete").
	bool in_aggr_mod_stmt = false;
	};

// Used for debugging, to communicate which expression wasn't
// reduced when we expected them all to be.
extern const Expr* non_reduced_perp;
extern bool checking_reduction;

// Used to report a non-reduced expression.
extern bool NonReduced(const Expr* perp);

	} // zeek::detail