Merge pull request #9895 from ethereum/exp-negative-one

Optimize exp when base is -1

Changelog.md

@@ -2,6 +2,7 @@
 
 Compiler Features:
  * SMTChecker: Support ``addmod`` and ``mulmod``.
+ * Optimizer: Optimize ``exp`` when base is -1.
 
 
 ### 0.7.2 (2020-09-28)

libevmasm/RuleList.h

@@ -643,6 +643,16 @@ std::vector<SimplificationRule<Pattern>> evmRuleList(
 			Builtins::EXP(2, X),
 			[=]() -> Pattern { return Builtins::SHL(X, 1); }
 		);
+	rules.emplace_back(
+		Builtins::EXP(Word(-1), X),
+		[=]() -> Pattern
+		{
+			return Builtins::SUB(
+				Builtins::ISZERO(Builtins::AND(X, Word(1))),
+				Builtins::AND(X, Word(1))
+			);
+		}
+	);
 
 	return rules;
 }

test/formal/exp_neg_one.py

@@ -0,0 +1,16 @@
+from rule import Rule
+from opcodes import *
+from util import *
+
+"""
+Checking conversion of exp(-1, X) to sub(isZero(and(X, 1)), and(X, 1))
+"""
+
+rule = Rule()
+n_bits = 256
+
+X = BitVec('X', n_bits)
+
+exp_neg_one = If(MOD(X, 2) == 0, BitVecVal(1, n_bits), BVUnsignedMax(n_bits, n_bits))
+
+rule.check(SUB(ISZERO(AND(X, 1)), AND(X, 1)), exp_neg_one)

test/libsolidity/gasTests/exp.sol

@@ -0,0 +1,29 @@
+pragma experimental ABIEncoderV2;
+
+contract C {
+	function exp_neg_one(uint exponent) public returns(int) {
+		return (-1)**exponent;
+	}
+	function exp_two(uint exponent) public returns(uint) {
+		return 2**exponent;
+	}
+	function exp_zero(uint exponent) public returns(uint) {
+		return 0**exponent;
+	}
+	function exp_one(uint exponent) public returns(uint) {
+		return 1**exponent;
+	}
+}
+// ====
+// optimize: false
+// optimize-yul: false
+// ----
+// creation:
+//   codeDepositCost: 119800
+//   executionCost: 165
+//   totalCost: 119965
+// external:
+//   exp_neg_one(uint256): 2259
+//   exp_one(uint256): infinite
+//   exp_two(uint256): infinite
+//   exp_zero(uint256): infinite

test/libsolidity/gasTests/exp_optimized.sol

@@ -0,0 +1,29 @@
+pragma experimental ABIEncoderV2;
+
+contract C {
+	function exp_neg_one(uint exponent) public returns(int) {
+		return (-1)**exponent;
+	}
+	function exp_two(uint exponent) public returns(uint) {
+		return 2**exponent;
+	}
+	function exp_zero(uint exponent) public returns(uint) {
+		return 0**exponent;
+	}
+	function exp_one(uint exponent) public returns(uint) {
+		return 1**exponent;
+	}
+}
+// ====
+// optimize: true
+// optimize-yul: true
+// ----
+// creation:
+//   codeDepositCost: 53200
+//   executionCost: 105
+//   totalCost: 53305
+// external:
+//   exp_neg_one(uint256): 1962
+//   exp_one(uint256): 1915
+//   exp_two(uint256): 1893
+//   exp_zero(uint256): 1937

test/libyul/yulOptimizerTests/expressionSimplifier/exp_simplifications.yul

@@ -5,6 +5,7 @@
     sstore(2, exp(2, t))
     // The following should not be simplified
     sstore(3, exp(8, t))
+    sstore(4, exp(115792089237316195423570985008687907853269984665640564039457584007913129639935, t))
 }
 // ====
 // EVMVersion: >=constantinople
@@ -19,4 +20,5 @@
 //     let _8 := 2
 //     sstore(_8, shl(t, 1))
 //     sstore(3, exp(8, t))
+//     sstore(4, sub(iszero(and(t, 1)), and(t, 1)))
 // }