Make DataFlowAnalyzer aware of storage / memory slot after sload / mload.

Changelog.md

@@ -9,6 +9,7 @@ Compiler Features:
  * NatSpec: Inherit tags from unique base if derived function does not provide any.
  * Commandline Interface: Prevent some incompatible commandline options from being used together.
  * NatSpec: Support NatSpec comments on events.
+ * Yul Optimizer: Store knowledge about storage / memory after ``a := sload(x)`` / ``a := mload(x)``.
 
 Bugfixes:
  * NatSpec: Do not consider ``////`` and ``/***`` as NatSpec comments.

libyul/optimiser/DataFlowAnalyzer.cpp

@@ -253,6 +253,21 @@ void DataFlowAnalyzer::handleAssignment(set<YulString> const& _variables, Expres
 		// assignment to slot contents denoted by "name"
 		m_memory.eraseValue(name);
 	}
+
+	if (_value && _variables.size() == 1)
+	{
+		YulString variable = *_variables.begin();
+		if (!movableChecker.referencedVariables().count(variable))
+		{
+			// This might erase additional knowledge about the slot.
+			// On the other hand, if we knew the value in the slot
+			// already, then the sload() / mload() would have been replaced by a variable anyway.
+			if (auto key = isSimpleLoad(evmasm::Instruction::MLOAD, *_value))
+				m_memory.set(*key, variable);
+			else if (auto key = isSimpleLoad(evmasm::Instruction::SLOAD, *_value))
+				m_storage.set(*key, variable);
+		}
+	}
 }
 
 void DataFlowAnalyzer::pushScope(bool _functionScope)
@@ -401,3 +416,25 @@ std::optional<pair<YulString, YulString>> DataFlowAnalyzer::isSimpleStore(
 	return {};
 }
 
+std::optional<YulString> DataFlowAnalyzer::isSimpleLoad(
+	evmasm::Instruction _load,
+	Expression const& _expression
+) const
+{
+	yulAssert(
+		_load == evmasm::Instruction::MLOAD ||
+		_load == evmasm::Instruction::SLOAD,
+		""
+	);
+	if (holds_alternative<FunctionCall>(_expression))
+	{
+		FunctionCall const& funCall = std::get<FunctionCall>(_expression);
+		if (EVMDialect const* dialect = dynamic_cast<EVMDialect const*>(&m_dialect))
+			if (auto const* builtin = dialect->builtin(funCall.functionName.name))
+				if (builtin->instruction == _load)
+					if (holds_alternative<Identifier>(funCall.arguments.at(0)))
+						return std::get<Identifier>(funCall.arguments.at(0)).name;
+	}
+	return {};
+}
+

libyul/optimiser/DataFlowAnalyzer.h

@@ -142,11 +142,20 @@ protected:
 	/// Returns true iff the variable is in scope.
 	bool inScope(YulString _variableName) const;
 
+	/// Checks if the statement is sstore(a, b) / mstore(a, b)
+	/// where a and b are variables and returns these variables in that case.
 	std::optional<std::pair<YulString, YulString>> isSimpleStore(
 		evmasm::Instruction _store,
 		ExpressionStatement const& _statement
 	) const;
 
+	/// Checks if the expression is sload(a) / mload(a)
+	/// where a is a variable and returns the variable in that case.
+	std::optional<YulString> isSimpleLoad(
+		evmasm::Instruction _load,
+		Expression const& _expression
+	) const;
+
 	Dialect const& m_dialect;
 	/// Side-effects of user-defined functions. Worst-case side-effects are assumed
 	/// if this is not provided or the function is not found.

test/libyul/yulOptimizerTests/loadResolver/double_mload.yul

@@ -0,0 +1,13 @@
+{
+    let x := calldataload(0)
+    let a := mload(x)
+    let b := mload(x)
+    sstore(a, b)
+}
+// ----
+// step: loadResolver
+//
+// {
+//     let a := mload(calldataload(0))
+//     sstore(a, a)
+// }

test/libyul/yulOptimizerTests/loadResolver/double_mload_with_other_reassignment.yul

@@ -0,0 +1,16 @@
+{
+    let x := calldataload(0)
+    let a := mload(x)
+    x := 7
+    let b := mload(x)
+    sstore(a, b)
+}
+// ----
+// step: loadResolver
+//
+// {
+//     let x := calldataload(0)
+//     let a := mload(x)
+//     x := 7
+//     sstore(a, mload(x))
+// }

test/libyul/yulOptimizerTests/loadResolver/double_mload_with_reassignment.yul

@@ -0,0 +1,16 @@
+{
+    let x := calldataload(0)
+    let a := mload(x)
+    a := 7
+    let b := mload(x)
+    sstore(a, b)
+}
+// ----
+// step: loadResolver
+//
+// {
+//     let x := calldataload(0)
+//     let a := mload(x)
+//     a := 7
+//     sstore(a, mload(x))
+// }

test/libyul/yulOptimizerTests/loadResolver/merge_mload_with_known_distance.yul

@@ -0,0 +1,19 @@
+{
+    let x := mload(calldataload(0))
+    if calldataload(1) {
+        mstore(add(calldataload(0), 0x20), 1)
+    }
+    let t := mload(add(calldataload(0), 0x20))
+    let q := mload(calldataload(0))
+    sstore(t, q)
+}
+// ----
+// step: loadResolver
+//
+// {
+//     let _2 := calldataload(0)
+//     let x := mload(_2)
+//     let _3 := 1
+//     if calldataload(_3) { mstore(add(_2, 0x20), _3) }
+//     sstore(mload(add(_2, 0x20)), x)
+// }

test/libyul/yulOptimizerTests/loadResolver/merge_mload_with_rewrite.yul

@@ -0,0 +1,22 @@
+{
+    let b := mload(2)
+    if calldataload(1) {
+        mstore(2, 7)
+        // Re-writing the old value, should allow to eliminate the load below.
+        mstore(2, b)
+    }
+    sstore(0, mload(2))
+}
+// ----
+// step: loadResolver
+//
+// {
+//     let _1 := 2
+//     let b := mload(_1)
+//     if calldataload(1)
+//     {
+//         mstore(_1, 7)
+//         mstore(_1, b)
+//     }
+//     sstore(0, b)
+// }

test/libyul/yulOptimizerTests/loadResolver/merge_mload_without_rewrite.yul

@@ -0,0 +1,19 @@
+{
+    let b := mload(2)
+    sstore(0, b)
+    if calldataload(1) {
+        mstore(2, 7)
+    }
+    sstore(0, mload(2))
+}
+// ----
+// step: loadResolver
+//
+// {
+//     let _1 := 2
+//     let b := mload(_1)
+//     let _2 := 0
+//     sstore(_2, b)
+//     if calldataload(1) { mstore(_1, 7) }
+//     sstore(_2, mload(_1))
+// }

test/libyul/yulOptimizerTests/loadResolver/mload_self.yul

@@ -0,0 +1,15 @@
+{
+    let x := calldataload(0)
+    x := mload(x)
+    let y := mload(x)
+    sstore(0, y)
+}
+// ----
+// step: loadResolver
+//
+// {
+//     let _1 := 0
+//     let x := calldataload(_1)
+//     x := mload(x)
+//     sstore(_1, mload(x))
+// }

test/libyul/yulOptimizerTests/loadResolver/multi_sload_loop.yul

@@ -0,0 +1,30 @@
+{
+    let x := calldataload(0)
+    let len := sload(x)
+    let sum
+    for { let i := 0} lt(i, sload(x)) { i := add(i, 1) } {
+        let p := add(x, add(i, 1))
+        if gt(p, sload(x)) { revert(0, 0) }
+        sum := add(sum, sload(p))
+    }
+    mstore(0, sum)
+    return(0, 0x20)
+}
+// ----
+// step: loadResolver
+//
+// {
+//     let _1 := 0
+//     let x := calldataload(_1)
+//     let len := sload(x)
+//     let sum
+//     let i := _1
+//     for { } lt(i, len) { i := add(i, 1) }
+//     {
+//         let p := add(add(x, i), 1)
+//         if gt(p, len) { revert(_1, _1) }
+//         sum := add(sum, sload(p))
+//     }
+//     mstore(_1, sum)
+//     return(_1, 0x20)
+// }