From 684fff34a9c1a714fafe7171292c6b08c4faa51d Mon Sep 17 00:00:00 2001
From: chriseth <chris@ethereum.org>
Date: Tue, 28 Jul 2020 20:26:22 +0200
Subject: [PATCH] Replace exp by shl.

---
 Changelog.md                                  |  2 +-
 libevmasm/RuleList.h                          | 17 +++++++++++++-
 .../exp_simplifications.yul                   | 22 +++++++++++++++++++
 3 files changed, 39 insertions(+), 2 deletions(-)
 create mode 100644 test/libyul/yulOptimizerTests/expressionSimplifier/exp_simplifications.yul

diff --git a/Changelog.md b/Changelog.md
index 33f2c2ef5..ba393c0ee 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -14,7 +14,7 @@ Compiler Features:
  * Yul Optimizer: Inline into functions further down in the call graph first.
  * Yul Optimizer: Try to simplify function names.
  * Yul IR Generator: Report source locations related to unimplemented features.
-
+ * Optimizer: Optimize ``exp`` when base is 0, 1 or 2.
 
 Bugfixes:
  * Code generator: Fix internal error on stripping dynamic types from return parameters on EVM versions without ``RETURNDATACOPY``.
diff --git a/libevmasm/RuleList.h b/libevmasm/RuleList.h
index 4adeb34c9..883de4c9f 100644
--- a/libevmasm/RuleList.h
+++ b/libevmasm/RuleList.h
@@ -615,12 +615,13 @@ std::vector<SimplificationRule<Pattern>> evmRuleList(
 	Pattern,
 	Pattern,
 	Pattern,
-	Pattern,
+	Pattern X,
 	Pattern,
 	Pattern
 )
 {
 	using Builtins = typename Pattern::Builtins;
+	using Word = typename Pattern::Word;
 	std::vector<SimplificationRule<Pattern>> rules;
 
 	if (_evmVersion.hasSelfBalance())
@@ -629,6 +630,20 @@ std::vector<SimplificationRule<Pattern>> evmRuleList(
 			[]() -> Pattern { return Instruction::SELFBALANCE; }
 		});
 
+	rules.emplace_back(
+		Builtins::EXP(0, X),
+		[=]() -> Pattern { return Builtins::ISZERO(X); }
+	);
+	rules.emplace_back(
+		Builtins::EXP(1, X),
+		[=]() -> Pattern { return Word(1); }
+	);
+	if (_evmVersion.hasBitwiseShifting())
+		rules.emplace_back(
+			Builtins::EXP(2, X),
+			[=]() -> Pattern { return Builtins::SHL(X, 1); }
+		);
+
 	return rules;
 }
 
diff --git a/test/libyul/yulOptimizerTests/expressionSimplifier/exp_simplifications.yul b/test/libyul/yulOptimizerTests/expressionSimplifier/exp_simplifications.yul
new file mode 100644
index 000000000..879b05883
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/expressionSimplifier/exp_simplifications.yul
@@ -0,0 +1,22 @@
+{
+    let t := calldataload(0)
+    sstore(0, exp(0, t))
+    sstore(1, exp(1, t))
+    sstore(2, exp(2, t))
+    // The following should not be simplified
+    sstore(3, exp(8, t))
+}
+// ====
+// EVMVersion: >=constantinople
+// ----
+// step: expressionSimplifier
+//
+// {
+//     let _1 := 0
+//     let t := calldataload(_1)
+//     sstore(_1, iszero(t))
+//     sstore(1, 1)
+//     let _8 := 2
+//     sstore(_8, shl(t, 1))
+//     sstore(3, exp(8, t))
+// }