Replace shifts by larger than 255 with 0

Changelog.md

@@ -5,6 +5,7 @@ Language Features:
 
 Compiler Features:
  * SMTChecker: Support arithmetic compound assignment operators.
+ * Optimizer: Add rule for shifts by constants larger than 255 for Constantinople.
  * Yul: Adds break and continue keywords to for-loop syntax.
 
 

libevmasm/RuleList.h

@@ -216,7 +216,7 @@ std::vector<SimplificationRule<Pattern>> simplificationRuleListPart4(
 
 template <class Pattern>
 std::vector<SimplificationRule<Pattern>> simplificationRuleListPart5(
-	Pattern,
+	Pattern A,
 	Pattern,
 	Pattern,
 	Pattern X,
@@ -236,6 +236,22 @@ std::vector<SimplificationRule<Pattern>> simplificationRuleListPart5(
 		});
 	}
 
+	// Replace SHL >=256, X with 0
+	rules.push_back({
+		{Instruction::SHL, {A, X}},
+		[=]() -> Pattern { return u256(0); },
+		true,
+		[=]() { return A.d() >= 256; }
+	});
+
+	// Replace SHR >=256, X with 0
+	rules.push_back({
+		{Instruction::SHR, {A, X}},
+		[=]() -> Pattern { return u256(0); },
+		true,
+		[=]() { return A.d() >= 256; }
+	});
+
 	for (auto const& op: std::vector<Instruction>{
 		Instruction::ADDRESS,
 		Instruction::CALLER,

test/libevmasm/Optimiser.cpp

@@ -1250,6 +1250,48 @@ BOOST_AUTO_TEST_CASE(cse_remove_unwanted_masking_of_address)
 	});
 }
 
+BOOST_AUTO_TEST_CASE(cse_replace_too_large_shift)
+{
+	if (!dev::test::Options::get().evmVersion().hasBitwiseShifting())
+		return;
+
+	checkCSE({
+		Instruction::CALLVALUE,
+		u256(299),
+		Instruction::SHL
+	}, {
+		u256(0)
+	});
+
+	checkCSE({
+		Instruction::CALLVALUE,
+		u256(299),
+		Instruction::SHR
+	}, {
+		u256(0)
+	});
+
+	checkCSE({
+		Instruction::CALLVALUE,
+		u256(255),
+		Instruction::SHL
+	}, {
+		Instruction::CALLVALUE,
+		u256(255),
+		Instruction::SHL
+	});
+
+	checkCSE({
+		Instruction::CALLVALUE,
+		u256(255),
+		Instruction::SHR
+	}, {
+		Instruction::CALLVALUE,
+		u256(255),
+		Instruction::SHR
+	});
+}
+
 BOOST_AUTO_TEST_SUITE_END()
 
 }