From 439a225ceec5dce16ee1778993e3958aeacbbb5d Mon Sep 17 00:00:00 2001
From: chriseth <chris@ethereum.org>
Date: Thu, 9 May 2019 22:34:09 +0200
Subject: [PATCH] Simplify single-run for loops to if statements.

---
 Changelog.md                               |  1 +
 libyul/optimiser/ControlFlowSimplifier.cpp | 47 ++++++++++++++++++++--
 libyul/optimiser/ControlFlowSimplifier.h   | 10 ++++-
 3 files changed, 53 insertions(+), 5 deletions(-)

diff --git a/Changelog.md b/Changelog.md
index d182257f1..a10da4252 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -10,6 +10,7 @@ Compiler Features:
  * SMTChecker: Support ``delete``.
  * SMTChecker: Inline external function calls to ``this``.
  * Assembler: Encode the compiler version in the deployed bytecode.
+ * Yul Optimizer: Simplify single-run ``for`` loops to ``if`` statements.
 
 
 Bugfixes:
diff --git a/libyul/optimiser/ControlFlowSimplifier.cpp b/libyul/optimiser/ControlFlowSimplifier.cpp
index 7040891df..a3b2f4f37 100644
--- a/libyul/optimiser/ControlFlowSimplifier.cpp
+++ b/libyul/optimiser/ControlFlowSimplifier.cpp
@@ -115,9 +115,51 @@ void ControlFlowSimplifier::operator()(Block& _block)
 	simplify(_block.statements);
 }
 
+void ControlFlowSimplifier::visit(Statement& _st)
+{
+	if (_st.type() == typeid(ForLoop))
+	{
+		ForLoop& forLoop = boost::get<ForLoop>(_st);
+		yulAssert(forLoop.pre.statements.empty(), "");
+
+		size_t outerBreak = m_numBreakStatements;
+		size_t outerContinue = m_numContinueStatements;
+		m_numBreakStatements = 0;
+		m_numContinueStatements = 0;
+
+		ASTModifier::visit(_st);
+
+		if (!forLoop.body.statements.empty())
+		{
+			bool isTerminating = false;
+			TerminationFinder::ControlFlow controlFlow = TerminationFinder::controlFlowKind(forLoop.body.statements.back());
+			if (controlFlow == TerminationFinder::ControlFlow::Break)
+			{
+				isTerminating = true;
+				--m_numBreakStatements;
+			}
+			else if (controlFlow == TerminationFinder::ControlFlow::Terminate)
+				isTerminating = true;
+
+			if (isTerminating && m_numContinueStatements == 0 && m_numBreakStatements == 0)
+			{
+				If replacement{forLoop.location, std::move(forLoop.condition), std::move(forLoop.body)};
+				if (controlFlow == TerminationFinder::ControlFlow::Break)
+					replacement.body.statements.resize(replacement.body.statements.size() - 1);
+				_st = std::move(replacement);
+			}
+		}
+
+		m_numBreakStatements = outerBreak;
+		m_numContinueStatements = outerContinue;
+	}
+	else
+		ASTModifier::visit(_st);
+}
+
 void ControlFlowSimplifier::simplify(std::vector<yul::Statement>& _statements)
 {
-	GenericFallbackReturnsVisitor<OptionalStatements, If, Switch, ForLoop> const visitor(
+	GenericFallbackReturnsVisitor<OptionalStatements, If, Switch> const visitor(
 		[&](If& _ifStmt) -> OptionalStatements {
 			if (_ifStmt.body.statements.empty())
 			{
@@ -136,9 +178,6 @@ void ControlFlowSimplifier::simplify(std::vector<yul::Statement>& _statements)
 			else if (_switchStmt.cases.size() == 1)
 				return reduceSingleCaseSwitch(_switchStmt);
 
-			return {};
-		},
-		[&](ForLoop&) -> OptionalStatements {
 			return {};
 		}
 	);
diff --git a/libyul/optimiser/ControlFlowSimplifier.h b/libyul/optimiser/ControlFlowSimplifier.h
index 4b0b83873..a2975e5a0 100644
--- a/libyul/optimiser/ControlFlowSimplifier.h
+++ b/libyul/optimiser/ControlFlowSimplifier.h
@@ -38,7 +38,7 @@ namespace yul
  * The ControlFlowSimplifier does record the presence or absence of ``break``
  * and ``continue`` statements during its traversal.
  *
- * Prerequisite: Disambiguator, ForLoopInitRewriter.
+ * Prerequisite: Disambiguator, FunctionHoister, ForLoopInitRewriter.
  *
  * Important: Introduces EVM opcodes and thus can only be used on EVM code for now.
  */
@@ -46,9 +46,17 @@ class ControlFlowSimplifier: public ASTModifier
 {
 public:
 	using ASTModifier::operator();
+	void operator()(Break&) override { ++m_numBreakStatements; }
+	void operator()(Continue&) override { ++m_numContinueStatements; }
 	void operator()(Block& _block) override;
+
+	void visit(Statement& _st) override;
+
 private:
 	void simplify(std::vector<Statement>& _statements);
+
+	size_t m_numBreakStatements = 0;
+	size_t m_numContinueStatements = 0;
 };
 
 }