From e86b5cf14ec1fafc8d1004d0bcf0dcef52f6cc7d Mon Sep 17 00:00:00 2001
From: chriseth <chris@ethereum.org>
Date: Wed, 28 Aug 2019 17:34:31 +0200
Subject: [PATCH] Replace byte(A, X) by zero if A >= 32

---
 Changelog.md                                     |  1 +
 libevmasm/RuleList.h                             |  8 ++++++++
 .../expressionSimplifier/large_byte_access.yul   | 16 ++++++++++++++++
 3 files changed, 25 insertions(+)
 create mode 100644 test/libyul/yulOptimizerTests/expressionSimplifier/large_byte_access.yul

diff --git a/Changelog.md b/Changelog.md
index f4ee60640..bc165d4f6 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -5,6 +5,7 @@ Language Features:
 
 
 Compiler Features:
+ * Optimizer: Add rule that replaces the BYTE opcode by 0 if the first argument is larger than 31.
 
 
 Bugfixes:
diff --git a/libevmasm/RuleList.h b/libevmasm/RuleList.h
index 70f136d9d..ce00e95df 100644
--- a/libevmasm/RuleList.h
+++ b/libevmasm/RuleList.h
@@ -259,6 +259,14 @@ std::vector<SimplificationRule<Pattern>> simplificationRuleListPart5(
 		[=]() { return A.d() >= 256; }
 	});
 
+	// Replace BYTE(A, X), A >= 32 with 0
+	rules.push_back({
+		{Instruction::BYTE, {A, X}},
+		[=]() -> Pattern { return u256(0); },
+		true,
+		[=]() { return A.d() >= 32; }
+	});
+
 	for (auto const& op: std::vector<Instruction>{
 		Instruction::ADDRESS,
 		Instruction::CALLER,
diff --git a/test/libyul/yulOptimizerTests/expressionSimplifier/large_byte_access.yul b/test/libyul/yulOptimizerTests/expressionSimplifier/large_byte_access.yul
new file mode 100644
index 000000000..d5e7c0ad4
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/expressionSimplifier/large_byte_access.yul
@@ -0,0 +1,16 @@
+{
+    let a := calldataload(0)
+    let b := byte(33, a)
+    let c := byte(20, a)
+    // create cannot be removed.
+    let d := byte(33, create(0, 0, 0x20))
+}
+// ====
+// step: expressionSimplifier
+// ----
+// {
+//     let a := calldataload(0)
+//     let b := 0
+//     let c := byte(20, a)
+//     let d := byte(33, create(0, 0, 0x20))
+// }