X-Git-Url: http://git.tdb.fi/?p=libs%2Fgl.git;a=blobdiff_plain;f=source%2Fglsl%2Foptimize.cpp;h=9ab8ef70ef94dfc235e08ecee479792f9ae9cf2b;hp=c1431f2b6e9d9e33ed522cd853bcd326bbb3414a;hb=7335009e18ecbf53ad9f59d64eed2ed5abbe7b8b;hpb=041ba4b1acd55337239c5ce24cc310118c621206

diff --git a/source/glsl/optimize.cpp b/source/glsl/optimize.cpp
index c1431f2b..9ab8ef70 100644
--- a/source/glsl/optimize.cpp
+++ b/source/glsl/optimize.cpp
@@ -193,7 +193,7 @@ bool FunctionInliner::apply(Stage &s)
 	return r_any_inlined;
 }
 
-void FunctionInliner::visit_and_inline(RefPtr<Expression> &ptr)
+void FunctionInliner::visit(RefPtr<Expression> &ptr)
 {
 	r_inline_result = 0;
 	ptr->visit(*this);
@@ -216,26 +216,10 @@ void FunctionInliner::visit(Block &block)
 	}
 }
 
-void FunctionInliner::visit(UnaryExpression &unary)
-{
-	visit_and_inline(unary.expression);
-}
-
-void FunctionInliner::visit(BinaryExpression &binary)
-{
-	visit_and_inline(binary.left);
-	visit_and_inline(binary.right);
-}
-
-void FunctionInliner::visit(MemberAccess &memacc)
-{
-	visit_and_inline(memacc.left);
-}
-
 void FunctionInliner::visit(FunctionCall &call)
 {
 	for(NodeArray<Expression>::iterator i=call.arguments.begin(); i!=call.arguments.end(); ++i)
-		visit_and_inline(*i);
+		visit(*i);
 
 	FunctionDeclaration *def = call.declaration;
 	if(def)
@@ -259,29 +243,12 @@ void FunctionInliner::visit(FunctionCall &call)
 	}
 }
 
-void FunctionInliner::visit(ExpressionStatement &expr)
-{
-	visit_and_inline(expr.expression);
-}
-
-void FunctionInliner::visit(VariableDeclaration &var)
-{
-	if(var.init_expression)
-		visit_and_inline(var.init_expression);
-}
-
 void FunctionInliner::visit(FunctionDeclaration &func)
 {
 	SetForScope<FunctionDeclaration *> set_func(current_function, &func);
 	TraversingVisitor::visit(func);
 }
 
-void FunctionInliner::visit(Conditional &cond)
-{
-	visit_and_inline(cond.condition);
-	cond.body.visit(*this);
-}
-
 void FunctionInliner::visit(Iteration &iter)
 {
 	/* Visit the initialization statement before entering the loop body so the
@@ -296,12 +263,6 @@ void FunctionInliner::visit(Iteration &iter)
 	iter.body.visit(*this);
 }
 
-void FunctionInliner::visit(Return &ret)
-{
-	if(ret.expression)
-		visit_and_inline(ret.expression);
-}
-
 
 ExpressionInliner::ExpressionInfo::ExpressionInfo():
 	expression(0),
@@ -388,35 +349,36 @@ void ExpressionInliner::visit(Block &block)
 {
 	TraversingVisitor::visit(block);
 
-	for(map<VariableDeclaration *, ExpressionInfo>::iterator i=expressions.begin(); i!=expressions.end(); )
+	for(map<string, VariableDeclaration *>::iterator i=block.variables.begin(); i!=block.variables.end(); ++i)
 	{
-		map<string, VariableDeclaration *>::iterator j = block.variables.find(i->first->name);
-		if(j!=block.variables.end() && j->second==i->first)
+		map<Assignment::Target, ExpressionInfo>::iterator j = expressions.lower_bound(i->second);
+		for(; (j!=expressions.end() && j->first.declaration==i->second); )
 		{
-			if(i->second.expression && i->second.inline_point)
-				inline_expression(*i->second.expression, *i->second.inline_point, i->second.outer_oper, i->second.inner_oper, i->second.inline_on_rhs);
+			if(j->second.expression && j->second.inline_point)
+				inline_expression(*j->second.expression, *j->second.inline_point, j->second.outer_oper, j->second.inner_oper, j->second.inline_on_rhs);
 
-			expressions.erase(i++);
-		}
-		else
-		{
-			/* The expression was assigned in this block and may depend on local
-			variables of the block.  If this is a conditionally executed block,
-			the assignment might not always happen.  Mark the expression as not
-			available to any outer blocks. */
-			if(i->second.assign_scope==&block)
-				i->second.available = false;
-
-			++i;
+			expressions.erase(j++);
 		}
 	}
+
+	/* Expressions assigned in this block may depend on local variables of the
+	block.  If this is a conditionally executed block, the assignments might not
+	always happen.  Mark the expressions as not available to any outer blocks. */
+	for(map<Assignment::Target, ExpressionInfo>::iterator i=expressions.begin(); i!=expressions.end(); ++i)
+		if(i->second.assign_scope==&block)
+			i->second.available = false;
+}
+
+void ExpressionInliner::visit(RefPtr<Expression> &expr)
+{
+	visit_and_record(expr, 0, false);
 }
 
 void ExpressionInliner::visit(VariableReference &var)
 {
 	if(var.declaration)
 	{
-		map<VariableDeclaration *, ExpressionInfo>::iterator i = expressions.find(var.declaration);
+		map<Assignment::Target, ExpressionInfo>::iterator i = expressions.find(var.declaration);
 		if(i!=expressions.end())
 		{
 			/* If a non-trivial expression is referenced multiple times, don't
@@ -439,6 +401,13 @@ void ExpressionInliner::visit(MemberAccess &memacc)
 	r_trivial = false;
 }
 
+void ExpressionInliner::visit(Swizzle &swizzle)
+{
+	visit_and_record(swizzle.left, swizzle.oper, false);
+	r_oper = swizzle.oper;
+	r_trivial = false;
+}
+
 void ExpressionInliner::visit(UnaryExpression &unary)
 {
 	SetFlag set_target(mutating, mutating || unary.oper->token[1]=='+' || unary.oper->token[1]=='-');
@@ -467,19 +436,16 @@ void ExpressionInliner::visit(Assignment &assign)
 	r_oper = 0;
 	visit_and_record(assign.right, assign.oper, true);
 
-	if(assign.target_declaration)
+	map<Assignment::Target, ExpressionInfo>::iterator i = expressions.find(assign.target);
+	if(i!=expressions.end())
 	{
-		map<VariableDeclaration *, ExpressionInfo>::iterator i = expressions.find(assign.target_declaration);
-		if(i!=expressions.end())
-		{
-			/* Self-referencing assignments can't be inlined without additional
-			work.  Just clear any previous expression. */
-			i->second.expression = (assign.self_referencing ? 0 : assign.right.get());
-			i->second.assign_scope = current_block;
-			i->second.inline_point = 0;
-			i->second.inner_oper = r_oper;
-			i->second.available = true;
-		}
+		/* Self-referencing assignments can't be inlined without additional
+		work.  Just clear any previous expression. */
+		i->second.expression = (assign.self_referencing ? 0 : assign.right.get());
+		i->second.assign_scope = current_block;
+		i->second.inline_point = 0;
+		i->second.inner_oper = r_oper;
+		i->second.available = true;
 	}
 
 	r_oper = assign.oper;
@@ -488,8 +454,7 @@ void ExpressionInliner::visit(Assignment &assign)
 
 void ExpressionInliner::visit(FunctionCall &call)
 {
-	for(NodeArray<Expression>::iterator i=call.arguments.begin(); i!=call.arguments.end(); ++i)
-		visit_and_record(*i, 0, false);
+	TraversingVisitor::visit(call);
 	r_oper = 0;
 	r_trivial = false;
 }
@@ -498,8 +463,7 @@ void ExpressionInliner::visit(VariableDeclaration &var)
 {
 	r_oper = 0;
 	r_trivial = true;
-	if(var.init_expression)
-		visit_and_record(var.init_expression, 0, false);
+	TraversingVisitor::visit(var);
 
 	bool constant = var.constant;
 	if(constant && var.layout)
@@ -523,12 +487,6 @@ void ExpressionInliner::visit(VariableDeclaration &var)
 	}
 }
 
-void ExpressionInliner::visit(Conditional &cond)
-{
-	visit_and_record(cond.condition, 0, false);
-	cond.body.visit(*this);
-}
-
 void ExpressionInliner::visit(Iteration &iter)
 {
 	SetForScope<Block *> set_block(current_block, &iter.body);
@@ -540,16 +498,10 @@ void ExpressionInliner::visit(Iteration &iter)
 
 	SetForScope<Block *> set_body(iteration_body, &iter.body);
 	if(iter.condition)
-		iter.condition->visit(*this);
+		visit(iter.condition);
 	iter.body.visit(*this);
 	if(iter.loop_expression)
-		iter.loop_expression->visit(*this);
-}
-
-void ExpressionInliner::visit(Return &ret)
-{
-	if(ret.expression)
-		visit_and_record(ret.expression, 0, false);
+		visit(iter.loop_expression);
 }
 
 
@@ -571,15 +523,14 @@ void ConstantConditionEliminator::visit(Block &block)
 
 void ConstantConditionEliminator::visit(Conditional &cond)
 {
-	ExpressionEvaluator eval;
-	cond.condition->visit(eval);
-	if(eval.is_result_valid())
-	{
-		Block &block = (eval.get_result() ? cond.body : cond.else_body);
-		current_block->body.splice(insert_point, block.body);
-		nodes_to_remove.insert(&cond);
-		return;
-	}
+	if(Literal *literal = dynamic_cast<Literal *>(cond.condition.get()))
+		if(literal->value.check_type<bool>())
+		{
+			Block &block = (literal->value.value<bool>() ? cond.body : cond.else_body);
+			current_block->body.splice(insert_point, block.body);
+			nodes_to_remove.insert(&cond);
+			return;
+		}
 
 	TraversingVisitor::visit(cond);
 }
@@ -606,13 +557,6 @@ void ConstantConditionEliminator::visit(Iteration &iter)
 }
 
 
-UnusedVariableRemover::VariableInfo::VariableInfo():
-	local(false),
-	conditionally_assigned(false),
-	referenced(false)
-{ }
-
-
 bool UnusedTypeRemover::apply(Stage &stage)
 {
 	stage.content.visit(*this);
@@ -681,75 +625,71 @@ void UnusedTypeRemover::visit(FunctionDeclaration &func)
 
 
 UnusedVariableRemover::UnusedVariableRemover():
-	aggregate(0),
+	stage(0),
+	interface_block(0),
 	r_assignment(0),
 	assignment_target(false),
-	r_assign_to_subfield(false),
 	r_side_effects(false)
 { }
 
-bool UnusedVariableRemover::apply(Stage &stage)
+bool UnusedVariableRemover::apply(Stage &s)
 {
-	variables.push_back(BlockVariableMap());
-	stage.content.visit(*this);
-	BlockVariableMap &global_variables = variables.back();
-	for(BlockVariableMap::iterator i=global_variables.begin(); i!=global_variables.end(); ++i)
-	{
-		string interface = i->first->interface;
-		bool linked = i->first->linked_declaration;
-		map<VariableDeclaration *, Node *>::iterator j = aggregates.find(i->first);
-		if(j!=aggregates.end())
-			if(InterfaceBlock *iface = dynamic_cast<InterfaceBlock *>(j->second))
-			{
-				interface = iface->interface;
-				linked = iface->linked_block;
-			}
-
-		/* Don't remove output variables which are used by the next stage or the
-		graphics API. */
-		if(interface=="out" && (stage.type==Stage::FRAGMENT || linked || !i->first->name.compare(0, 3, "gl_")))
-			continue;
-
-		// Mark other unreferenced global variables as unused.
-		if(!i->second.referenced)
+	stage = &s;
+	s.content.visit(*this);
+
+	for(list<AssignmentInfo>::const_iterator i=assignments.begin(); i!=assignments.end(); ++i)
+		if(i->used_by.empty())
+			unused_nodes.insert(i->node);
+
+	for(map<string, InterfaceBlock *>::const_iterator i=s.interface_blocks.begin(); i!=s.interface_blocks.end(); ++i)
+		if(i->second->instance_name.empty())
+			unused_nodes.insert(i->second);
+
+	for(BlockVariableMap::const_iterator i=variables.begin(); i!=variables.end(); ++i)
+	{
+		if(i->second.output)
+		{
+			/* The last visible assignments of output variables are used by the
+			next stage or the API. */
+			for(vector<AssignmentInfo *>::const_iterator j=i->second.assignments.begin(); j!=i->second.assignments.end(); ++j)
+				unused_nodes.erase((*j)->node);
+		}
+
+		if(!i->second.output && !i->second.referenced)
 		{
-			unused_nodes.insert(i->first);
-			clear_assignments(i->second, true);
+			// Don't remove variables from inside interface blocks.
+			if(!i->second.interface_block)
+				unused_nodes.insert(i->first);
 		}
+		else if(i->second.interface_block)
+			// Interface blocks are kept if even one member is used.
+			unused_nodes.erase(i->second.interface_block);
 	}
-	variables.pop_back();
 
-	NodeRemover().apply(stage, unused_nodes);
+	NodeRemover().apply(s, unused_nodes);
 
 	return !unused_nodes.empty();
 }
 
-void UnusedVariableRemover::visit(VariableReference &var)
+void UnusedVariableRemover::referenced(const Assignment::Target &target, Node &node)
 {
-	map<VariableDeclaration *, Node *>::iterator i = aggregates.find(var.declaration);
-	if(i!=aggregates.end())
-		unused_nodes.erase(i->second);
-
-	if(var.declaration && !assignment_target)
+	VariableInfo &var_info = variables[target.declaration];
+	var_info.referenced = true;
+	if(!assignment_target)
 	{
-		VariableInfo &var_info = variables.back()[var.declaration];
-		// Previous assignments are used by this reference.
-		clear_assignments(var_info, false);
-		var_info.referenced = true;
+		for(vector<AssignmentInfo *>::const_iterator i=var_info.assignments.begin(); i!=var_info.assignments.end(); ++i)
+			(*i)->used_by.push_back(&node);
 	}
 }
 
-void UnusedVariableRemover::visit(InterfaceBlockReference &iface)
+void UnusedVariableRemover::visit(VariableReference &var)
 {
-	unused_nodes.erase(iface.declaration);
+	referenced(var.declaration, var);
 }
 
-void UnusedVariableRemover::visit(MemberAccess &memacc)
+void UnusedVariableRemover::visit(InterfaceBlockReference &iface)
 {
-	if(assignment_target)
-		r_assign_to_subfield = true;
-	TraversingVisitor::visit(memacc);
-	unused_nodes.erase(memacc.declaration);
+	referenced(iface.declaration, iface);
 }
 
 void UnusedVariableRemover::visit(UnaryExpression &unary)
@@ -763,8 +703,6 @@ void UnusedVariableRemover::visit(BinaryExpression &binary)
 {
 	if(binary.oper->token[0]=='[')
 	{
-		if(assignment_target)
-			r_assign_to_subfield = true;
 		binary.left->visit(*this);
 		SetFlag set(assignment_target, false);
 		binary.right->visit(*this);
@@ -776,7 +714,7 @@ void UnusedVariableRemover::visit(BinaryExpression &binary)
 void UnusedVariableRemover::visit(Assignment &assign)
 {
 	{
-		SetFlag set(assignment_target, !assign.self_referencing);
+		SetFlag set(assignment_target, (assign.oper->token[0]=='='));
 		assign.left->visit(*this);
 	}
 	assign.right->visit(*this);
@@ -790,166 +728,156 @@ void UnusedVariableRemover::visit(FunctionCall &call)
 	/* Treat function calls as having side effects so expression statements
 	consisting of nothing but a function call won't be optimized away. */
 	r_side_effects = true;
-}
 
-void UnusedVariableRemover::record_assignment(VariableDeclaration &var, Node &node, bool chained)
-{
-	VariableInfo &var_info = variables.back()[&var];
-	/* An assignment which completely replaces the value of the variable causes
-	any previous unreferenced assignments to be unused. */
-	if(!chained)
-		clear_assignments(var_info, true);
-	var_info.assignments.push_back(&node);
-	var_info.conditionally_assigned = false;
+	if(stage->type==Stage::GEOMETRY && call.name=="EmitVertex")
+	{
+		for(map<Statement *, VariableInfo>::const_iterator i=variables.begin(); i!=variables.end(); ++i)
+			if(i->second.output)
+				referenced(i->first, call);
+	}
 }
 
-void UnusedVariableRemover::clear_assignments(VariableInfo &var_info, bool mark_unused)
+void UnusedVariableRemover::record_assignment(const Assignment::Target &target, Node &node)
 {
-	if(mark_unused)
+	assignments.push_back(AssignmentInfo());
+	AssignmentInfo &assign_info = assignments.back();
+	assign_info.node = &node;
+	assign_info.target = target;
+
+	/* An assignment to the target hides any assignments to the same target or
+	its subfields. */
+	VariableInfo &var_info = variables[target.declaration];
+	for(unsigned i=0; i<var_info.assignments.size(); ++i)
 	{
-		for(vector<Node *>::iterator i=var_info.assignments.begin(); i!=var_info.assignments.end(); ++i)
-			unused_nodes.insert(*i);
+		const Assignment::Target &t = var_info.assignments[i]->target;
+
+		bool subfield = (t.chain_len>=target.chain_len);
+		for(unsigned j=0; (subfield && j<target.chain_len); ++j)
+			subfield = (t.chain[j]==target.chain[j]);
+
+		if(subfield)
+			var_info.assignments.erase(var_info.assignments.begin()+i);
+		else
+			++i;
 	}
-	var_info.assignments.clear();
+
+	var_info.assignments.push_back(&assign_info);
 }
 
 void UnusedVariableRemover::visit(ExpressionStatement &expr)
 {
 	r_assignment = 0;
-	r_assign_to_subfield = false;
 	r_side_effects = false;
 	TraversingVisitor::visit(expr);
-	if(r_assignment && r_assignment->target_declaration)
-		record_assignment(*r_assignment->target_declaration, expr, (r_assignment->self_referencing || r_assign_to_subfield));
+	if(r_assignment && r_assignment->target.declaration)
+		record_assignment(r_assignment->target, expr);
 	if(!r_side_effects)
 		unused_nodes.insert(&expr);
 }
 
-void UnusedVariableRemover::visit(StructDeclaration &strct)
-{
-	SetForScope<Node *> set(aggregate, &strct);
-	TraversingVisitor::visit(strct);
-}
-
 void UnusedVariableRemover::visit(VariableDeclaration &var)
 {
-	if(aggregate)
-		aggregates[&var] = aggregate;
+	VariableInfo &var_info = variables[&var];
+	var_info.interface_block = interface_block;
+
+	/* Mark variables as output if they're used by the next stage or the
+	graphics API. */
+	if(interface_block)
+		var_info.output = (interface_block->interface=="out" && (interface_block->linked_block || !interface_block->name.compare(0, 3, "gl_")));
 	else
+		var_info.output = (var.interface=="out" && (stage->type==Stage::FRAGMENT || var.linked_declaration || !var.name.compare(0, 3, "gl_")));
+
+	if(var.init_expression)
 	{
-		variables.back()[&var].local = true;
-		if(var.init_expression)
-			record_assignment(var, *var.init_expression, false);
+		var_info.initialized = true;
+		record_assignment(&var, *var.init_expression);
 	}
 	TraversingVisitor::visit(var);
 }
 
 void UnusedVariableRemover::visit(InterfaceBlock &iface)
 {
-	SetForScope<Node *> set(aggregate, &iface);
-	unused_nodes.insert(&iface);
-	iface.struct_declaration->members.visit(*this);
+	if(iface.instance_name.empty())
+	{
+		SetForScope<InterfaceBlock *> set_block(interface_block, &iface);
+		iface.struct_declaration->members.visit(*this);
+	}
+	else
+	{
+		VariableInfo &var_info = variables[&iface];
+		var_info.output = (iface.interface=="out" && (iface.linked_block || !iface.name.compare(0, 3, "gl_")));
+	}
 }
 
-void UnusedVariableRemover::visit(FunctionDeclaration &func)
+void UnusedVariableRemover::merge_variables(const BlockVariableMap &other_vars)
 {
-	variables.push_back(BlockVariableMap());
-
-	for(NodeArray<VariableDeclaration>::iterator i=func.parameters.begin(); i!=func.parameters.end(); ++i)
-		(*i)->visit(*this);
-	func.body.visit(*this);
+	for(BlockVariableMap::const_iterator i=other_vars.begin(); i!=other_vars.end(); ++i)
+	{
+		BlockVariableMap::iterator j = variables.find(i->first);
+		if(j!=variables.end())
+		{
+			/* The merged blocks started as copies of each other so any common
+			assignments must be in the beginning. */
+			unsigned k = 0;
+			for(; (k<i->second.assignments.size() && k<j->second.assignments.size()); ++k)
+				if(i->second.assignments[k]!=j->second.assignments[k])
+					break;
+
+			// Remaining assignments are unique to each block; merge them.
+			j->second.assignments.insert(j->second.assignments.end(), i->second.assignments.begin()+k, i->second.assignments.end());
+			j->second.referenced |= i->second.referenced;
+		}
+		else
+			variables.insert(*i);
+	}
+}
 
-	BlockVariableMap &block_variables = variables.back();
+void UnusedVariableRemover::visit(FunctionDeclaration &func)
+{
+	if(func.body.body.empty())
+		return;
 
-	/* Mark global variables as conditionally assigned so assignments in other
-	functions won't be removed. */
-	for(BlockVariableMap::iterator i=block_variables.begin(); i!=block_variables.end(); ++i)
-		if(!i->second.local)
-			i->second.conditionally_assigned = true;
+	BlockVariableMap saved_vars = variables;
+	// Assignments from other functions should not be visible.
+	for(BlockVariableMap::iterator i=variables.begin(); i!=variables.end(); ++i)
+		i->second.assignments.resize(i->second.initialized);
+	TraversingVisitor::visit(func);
+	swap(variables, saved_vars);
+	merge_variables(saved_vars);
 
 	/* Always treat function parameters as referenced.  Removing unused
 	parameters is not currently supported. */
 	for(NodeArray<VariableDeclaration>::iterator i=func.parameters.begin(); i!=func.parameters.end(); ++i)
-		block_variables[i->get()].referenced = true;
-
-	merge_down_variables();
-}
-
-void UnusedVariableRemover::merge_down_variables()
-{
-	BlockVariableMap &parent_variables = variables[variables.size()-2];
-	BlockVariableMap &block_variables = variables.back();
-	for(BlockVariableMap::iterator i=block_variables.begin(); i!=block_variables.end(); ++i)
 	{
-		if(i->second.local)
-		{
-			if(!i->second.referenced)
-				unused_nodes.insert(i->first);
-			/* Any unreferenced assignments when a variable runs out of scope
-			become unused. */
-			clear_assignments(i->second, true);
-			continue;
-		}
-
-		BlockVariableMap::iterator j = parent_variables.find(i->first);
-		if(j==parent_variables.end())
-			parent_variables.insert(*i);
-		else
-		{
-			// Merge a non-local variable's state into the parent scope.
-			if(i->second.referenced || !i->second.conditionally_assigned)
-				clear_assignments(j->second, !i->second.referenced);
-			j->second.conditionally_assigned = i->second.conditionally_assigned;
-			j->second.referenced |= i->second.referenced;
-			j->second.assignments.insert(j->second.assignments.end(), i->second.assignments.begin(), i->second.assignments.end());
-		}
+		BlockVariableMap::iterator j = variables.find(i->get());
+		if(j!=variables.end())
+			j->second.referenced = true;
 	}
-	variables.pop_back();
 }
 
 void UnusedVariableRemover::visit(Conditional &cond)
 {
 	cond.condition->visit(*this);
-	variables.push_back(BlockVariableMap());
+	BlockVariableMap saved_vars = variables;
 	cond.body.visit(*this);
-
-	BlockVariableMap if_variables;
-	swap(variables.back(), if_variables);
+	swap(saved_vars, variables);
 	cond.else_body.visit(*this);
 
-	// Combine variables from both branches.
-	BlockVariableMap &else_variables = variables.back();
-	for(BlockVariableMap::iterator i=else_variables.begin(); i!=else_variables.end(); ++i)
-	{
-		BlockVariableMap::iterator j = if_variables.find(i->first);
-		if(j!=if_variables.end())
-		{
-			// The variable was found in both branches.
-			i->second.assignments.insert(i->second.assignments.end(), j->second.assignments.begin(), j->second.assignments.end());
-			i->second.conditionally_assigned |= j->second.conditionally_assigned;
-			if_variables.erase(j);
-		}
-		else
-			// Mark variables found in only one branch as conditionally assigned.
-			i->second.conditionally_assigned = true;
-	}
-
-	/* Move variables which were only used in the if block into the combined
-	block. */
-	for(BlockVariableMap::iterator i=if_variables.begin(); i!=if_variables.end(); ++i)
-	{
-		i->second.conditionally_assigned = true;
-		else_variables.insert(*i);
-	}
-
-	merge_down_variables();
+	/* Visible assignments after the conditional is the union of those visible
+	at the end of the if and else blocks.  If there was no else block, then it's
+	the union of the if block and the state before it. */
+	merge_variables(saved_vars);
 }
 
 void UnusedVariableRemover::visit(Iteration &iter)
 {
-	variables.push_back(BlockVariableMap());
+	BlockVariableMap saved_vars = variables;
 	TraversingVisitor::visit(iter);
-	merge_down_variables();
+
+	/* Merge assignments from the iteration, without clearing previous state.
+	Further analysis is needed to determine which parts of the iteration body
+	are always executed, if any. */
+	merge_variables(saved_vars);
 }