Add Peephole Optimization for Function Selector Guard

Changelog.md

@@ -17,6 +17,7 @@ Language Features:
 * Type Checker: Warn about deprecation of `send` and `transfer` functions on instances of `address`.
 * Type Checker: Warn about deprecation of comparisons between variables of contract types.
 * Yul: Introduce builtin `clz(x)` for counting the number of leading zero bits in a 256-bit word.
+* peepholeoptimiser: Peephole pattern FunctionSelectorGuard added.
 
 
 Compiler Features:

libevmasm/PeepholeOptimiser.cpp

@@ -489,7 +489,7 @@ struct JumpToNext: SimplePeepholeOptimizerMethod<JumpToNext>
 
 struct RJumpToNext: SimplePeepholeOptimizerMethod<RJumpToNext>
 {
-	static size_t applySimple(
+	static bool applySimple(
 		AssemblyItem const& _rjump,
 		AssemblyItem const& _tag,
 		std::back_insert_iterator<AssemblyItems> _out
@@ -696,6 +696,52 @@ struct DeduplicateNextTagSize1 : SimplePeepholeOptimizerMethod<DeduplicateNextTa
 	}
 };
 
+struct FunctionSelectorGuard: SimplePeepholeOptimizerMethod<FunctionSelectorGuard>
+{
+	static bool apply(OptimiserState& _state)
+	{
+		size_t window = 6;
+		if (_state.i + window > _state.items.size())
+			return false;
+
+		if (_state.i == 0)
+			return false;
+		if (_state.items[_state.i - 1].type() != Tag)
+			return false;
+
+		auto& a = _state.items[_state.i + 0];
+		auto& b = _state.items[_state.i + 1];
+		auto& c = _state.items[_state.i + 2];
+		auto& d = _state.items[_state.i + 3];
+		auto& e = _state.items[_state.i + 4];
+		auto& f = _state.items[_state.i + 5];
+
+		if (a.type() != Push || a.data() != u256(4))
+			return false;
+
+		if (!(b == Instruction::CALLDATASIZE))
+			return false;
+		if (!(c == Instruction::LT))
+			return false;
+		if (!(d == Instruction::ISZERO))
+			return false;
+
+		if (e.type() != PushTag)
+			return false;
+		if (!(f == Instruction::JUMPI))
+			return false;
+
+		*_state.out = AssemblyItem(u256(3), a.debugData());
+		*_state.out = AssemblyItem(Instruction::CALLDATASIZE, b.debugData());
+		*_state.out = AssemblyItem(Instruction::GT, c.debugData());
+		*_state.out = e;
+		*_state.out = f;
+
+		_state.i += window;
+		return true;
+	}
+};
+
 template <typename... Method>
 void applyMethods(OptimiserState& _state)
 {
@@ -746,6 +792,7 @@ bool PeepholeOptimiser::optimise()
 			DeduplicateNextTagSize2,
 			DeduplicateNextTagSize1,
 			TagConjunctions,
+			FunctionSelectorGuard,
 			TruthyAnd,
 			Identity
 		>(state);

test/libevmasm/Optimiser.cpp

@@ -1479,6 +1479,44 @@ BOOST_AUTO_TEST_CASE(peephole_iszero_iszero_jumpi)
 	);
 }
 
+BOOST_AUTO_TEST_CASE(peephole_function_selector_guard)
+{
+    AssemblyItem tag0 = AssemblyItem(Tag, 0);
+    AssemblyItem tag1 = AssemblyItem(Tag, 1);
+    AssemblyItem pushTag1 = AssemblyItem(PushTag, 1);
+
+    AssemblyItems items{
+        tag0,
+        u256(4),
+        Instruction::CALLDATASIZE,
+        Instruction::LT,
+        Instruction::ISZERO,
+        pushTag1,
+        Instruction::JUMPI,
+        tag1,
+        Instruction::STOP
+    };
+
+    AssemblyItems expectation{
+        tag0,
+        u256(3),
+        Instruction::CALLDATASIZE,
+        Instruction::GT,
+        pushTag1,
+        Instruction::JUMPI,
+        tag1,
+        Instruction::STOP
+    };
+
+    PeepholeOptimiser peepOpt(items, solidity::test::CommonOptions::get().evmVersion());
+    BOOST_REQUIRE(peepOpt.optimise());
+
+    BOOST_CHECK_EQUAL_COLLECTIONS(
+        items.begin(), items.end(),
+        expectation.begin(), expectation.end()
+    );
+}
+
 BOOST_AUTO_TEST_CASE(jumpdest_removal)
 {
 	AssemblyItems items{