Merge pull request #1104 from ethereum/fixmemcosts

Fix memory resize costs during call

3 files changed, 42 insertions, 4 deletions

diff --git a/Changelog.md b/Changelog.md
index e18d8507..c438b914 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -2,6 +2,8 @@
 
  * Code Generator: Fix library functions being called from payable functions.
  * Type Checker: Fixed a crash about invalid array types.
+ * Code Generator: Fixed a call gas bug that became visible after
+   version 0.4.0 for calls where the output is larger than the input.
 
 ### 0.4.1 (2016-09-09)
 
diff --git a/libsolidity/codegen/ExpressionCompiler.cpp b/libsolidity/codegen/ExpressionCompiler.cpp
index 96ca4296..26acd8a4 100644
--- a/libsolidity/codegen/ExpressionCompiler.cpp
+++ b/libsolidity/codegen/ExpressionCompiler.cpp
@@ -1476,6 +1476,18 @@ void ExpressionCompiler::appendExternalFunctionCall(
 		utils().storeFreeMemoryPointer();
 	}
 
+	// Touch the end of the output area so that we do not pay for memory resize during the call
+	// (which we would have to subtract from the gas left)
+	// We could also just use MLOAD; POP right before the gas calculation, but the optimizer
+	// would remove that, so we use MSTORE here.
+	if (!_functionType.gasSet() && retSize > 0)
+	{
+		m_context << u256(0);
+		utils().fetchFreeMemoryPointer();
+		// This touches too much, but that way we save some rounding arithmetics
+		m_context << u256(retSize) << Instruction::ADD << Instruction::MSTORE;
+	}
+
 	// Copy function identifier to memory.
 	utils().fetchFreeMemoryPointer();
 	if (!_functionType.isBareCall() || manualFunctionId)
@@ -1551,10 +1563,7 @@ void ExpressionCompiler::appendExternalFunctionCall(
 			gasNeededByCaller += eth::GasCosts::callValueTransferGas;
 		if (!isCallCode && !isDelegateCall && !existenceChecked)
 			gasNeededByCaller += eth::GasCosts::callNewAccountGas; // we never know
-		m_context <<
-			gasNeededByCaller <<
-			Instruction::GAS <<
-			Instruction::SUB;
+		m_context << gasNeededByCaller << Instruction::GAS << Instruction::SUB;
 	}
 	if (isDelegateCall)
 		m_context << Instruction::DELEGATECALL;
diff --git a/test/libsolidity/SolidityEndToEndTest.cpp b/test/libsolidity/SolidityEndToEndTest.cpp
index 3c949e9a..23bd2abc 100644
--- a/test/libsolidity/SolidityEndToEndTest.cpp
+++ b/test/libsolidity/SolidityEndToEndTest.cpp
@@ -7203,6 +7203,33 @@ BOOST_AUTO_TEST_CASE(no_nonpayable_circumvention_by_modifier)
 	BOOST_CHECK_EQUAL(balanceAt(m_contractAddress), 0);
 }
 
+BOOST_AUTO_TEST_CASE(mem_resize_is_not_paid_at_call)
+{
+	// This tests that memory resize for return values is not paid during the call, which would
+	// make the gas calculation overly complex. We access the end of the output area before
+	// the call is made.
+	// Tests that this also survives the optimizer.
+	char const* sourceCode = R"(
+		contract C {
+			function f() returns (uint[200]) {}
+		}
+		contract D {
+			function f(C c) returns (uint) { c.f(); return 7; }
+		}
+	)";
+
+	compileAndRun(sourceCode, 0, "C");
+	u160 cAddr = m_contractAddress;
+	compileAndRun(sourceCode, 0, "D");
+	BOOST_CHECK(callContractFunction("f(address)", cAddr) == encodeArgs(u256(7)));
+
+	m_optimize = true;
+
+	compileAndRun(sourceCode, 0, "C");
+	u160 cAddrOpt = m_contractAddress;
+	compileAndRun(sourceCode, 0, "D");
+	BOOST_CHECK(callContractFunction("f(address)", cAddrOpt) == encodeArgs(u256(7)));
+}
 
 BOOST_AUTO_TEST_SUITE_END()