Variables are free with regards to code size.

libyul/optimiser/Metrics.cpp

@@ -25,6 +25,9 @@
 
 #include <libevmasm/Instruction.h>
 
+#include <libdevcore/Visitor.h>
+
+using namespace std;
 using namespace dev;
 using namespace yul;
 
@@ -45,7 +48,7 @@ size_t CodeSize::codeSize(Expression const& _expression)
 size_t CodeSize::codeSize(Block const& _block)
 {
 	CodeSize cs;
-	cs(_block);
+	cs.visit(_block);
 	return cs.m_size;
 }
 
@@ -53,14 +56,21 @@ void CodeSize::visit(Statement const& _statement)
 {
 	if (_statement.type() == typeid(FunctionDefinition))
 		return;
+	else if (!(
+		_statement.type() == typeid(Block) ||
+		_statement.type() == typeid(ExpressionStatement) ||
+		_statement.type() == typeid(Assignment) ||
+		_statement.type() == typeid(VariableDeclaration)
+	))
+		++m_size;
 
-	++m_size;
 	ASTWalker::visit(_statement);
 }
 
 void CodeSize::visit(Expression const& _expression)
 {
-	++m_size;
+	if (_expression.type() != typeid(Identifier))
+		++m_size;
 	ASTWalker::visit(_expression);
 }
 

libyul/optimiser/Metrics.h

@@ -30,6 +30,13 @@ namespace yul
  * More specifically, the number of AST nodes.
  * Ignores function definitions while traversing the AST.
  * If you want to know the size of a function, you have to invoke this on its body.
+ *
+ * As an exception, the following AST elements have a cost of zero:
+ *  - expression statement (only the expression inside has a cost)
+ *  - block (only the statements inside have a cost)
+ *  - variable references
+ *  - variable declarations (only the right hand side has a cost)
+ *  - assignments (only the value has a cost)
  */
 class CodeSize: public ASTWalker
 {
@@ -39,6 +46,8 @@ public:
 	static size_t codeSize(Block const& _block);
 
 private:
+	CodeSize() {}
+
 	void visit(Statement const& _statement) override;
 	void visit(Expression const& _expression) override;
 

test/libyul/Metrics.cpp

@@ -0,0 +1,116 @@
+/*
+    This file is part of solidity.
+
+    solidity is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    solidity is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+/**
+ * Unit tests for the code metrics.
+ */
+
+#include <test/Options.h>
+
+#include <test/libyul/Common.h>
+
+#include <libyul/optimiser/Metrics.h>
+#include <libyul/AsmData.h>
+
+
+using namespace std;
+using namespace langutil;
+
+namespace yul
+{
+namespace test
+{
+
+namespace
+{
+
+size_t codeSize(string const& _source)
+{
+	shared_ptr<Block> ast = parse(_source, false).first;
+	BOOST_REQUIRE(ast);
+	return CodeSize::codeSize(*ast);
+}
+
+}
+
+BOOST_AUTO_TEST_SUITE(YulCodeSize)
+
+BOOST_AUTO_TEST_CASE(empty_code)
+{
+	BOOST_CHECK_EQUAL(codeSize("{}"), 0);
+}
+
+BOOST_AUTO_TEST_CASE(nested_blocks)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ {} {} {{ }} }"), 0);
+}
+
+BOOST_AUTO_TEST_CASE(instruction)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ pop(calldatasize()) }"), 2);
+}
+
+BOOST_AUTO_TEST_CASE(variables_are_free)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ let x let y let a, b, c }"), 0);
+}
+
+BOOST_AUTO_TEST_CASE(constants_cost_one)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ let x := 3 }"), 1);
+}
+
+BOOST_AUTO_TEST_CASE(functions_are_skipped)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ function f(x) -> r { r := mload(x) } }"), 0);
+}
+
+BOOST_AUTO_TEST_CASE(function_with_arguments)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ function f(x) { sstore(x, 2) } f(2) }"), 2);
+}
+
+BOOST_AUTO_TEST_CASE(function_with_variables_as_arguments)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ function f(x) { sstore(x, 2) } let y f(y) }"), 1);
+}
+
+BOOST_AUTO_TEST_CASE(function_with_variables_and_constants_as_arguments)
+{
+	BOOST_CHECK_EQUAL(codeSize(
+		"{ function f(x, r) -> z { sstore(x, r) z := r } let y let t := f(y, 2) }"
+	), 2);
+}
+
+BOOST_AUTO_TEST_CASE(assignment)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ let a a := 3 }"), 1);
+}
+
+BOOST_AUTO_TEST_CASE(assignments_between_vars_are_free)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ let a let b := a a := b }"), 0);
+}
+
+BOOST_AUTO_TEST_CASE(assignment_complex)
+{
+	BOOST_CHECK_EQUAL(codeSize("{ let a let x := mload(a) a := sload(x) }"), 2);
+}
+
+BOOST_AUTO_TEST_SUITE_END()
+
+}
+}