From ff50ba561aa13a69030cd18c6d3ab2a7c5c52342 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Emmanuel=20Beno=C3=AEt?= <tseeker@nocternity.net>
Date: Sun, 17 Dec 2017 09:33:45 +0100
Subject: [PATCH] Optimizer - Dead store elimination

Works for variables and assets. Does not cover call arguments yet.
---
 TODO       |  3 +-
 c-opast.cc |  2 +-
 c-opopt.cc | 87 ++++++++++++++++++++++++++++++++++++++++++++----------
 c-opopt.hh |  3 +-
 4 files changed, 75 insertions(+), 20 deletions(-)

diff --git a/TODO b/TODO
index c94bd3e..da15ec9 100644
--- a/TODO
+++ b/TODO
@@ -17,9 +17,8 @@ Scripting:
 	* Dead code elimination
 		* Local variables
 		* Unused arguments
-		* Dead store
+		* Dead store: call arguments
 		* Unused functions
-		* Branches
 	* Common subexpressions
 	* Strength reduction
 	* Loop-invariant code motion
diff --git a/c-opast.cc b/c-opast.cc
index 788bfa9..1e3bae8 100644
--- a/c-opast.cc
+++ b/c-opast.cc
@@ -465,7 +465,7 @@ void T_InstrListNode::replaceMultiple(
 	}
 	assert( oldInstr );
 
-	if ( replacement ) {
+	if ( replacement && replacement->size( ) ) {
 		auto& rep{ *replacement };
 
 		// Move existing children to the end of the list
diff --git a/c-opopt.cc b/c-opopt.cc
index b7355c3..80cf7d4 100644
--- a/c-opopt.cc
+++ b/c-opopt.cc
@@ -1490,18 +1490,6 @@ bool opopt::PropagateConstants(
 		T_OpsParserOutput& program ,
 		T_OptData& oData ) noexcept
 {
-	// We need to follow the general execution flow of the program. This is
-	// not as straightforward as it seems.
-	//	- Handling locals is rather easy as they "die" at the end of
-	// the function in which they are defined.
-	//	- Handling variables in init and functions that are called only
-	// from init is not too hard: once a variable is set to a constant, it
-	// can be substituted until the next set instruction.
-	//	- Other variables need additional checks before propagating.
-	// For example, if a variable is set to a constant during init, but is
-	// updated at the end of the frame function, the value cannot be
-	// propagated.
-
 	oData.buildUseDefineChains( program );
 	M_LOGSTR_( "... Propagating constants" , 2 );
 
@@ -1559,6 +1547,16 @@ bool opopt::PropagateConstants(
 
 			CPReplaceWithConstant_( *oData.instructions[ use.node ].node ,
 					udc.var , *repVal , oData );
+
+			for ( auto j = 0u ; j < nRefs ; j ++ ) {
+				auto& def{ udc.defines[ use.refs[ j ] ] };
+				def.refs.remove( def.refs.indexOf( i ) );
+				for ( auto k = 0u ; k < def.refs.size( ) ; k ++ ) {
+					if ( def.refs[ k ] > i ) {
+						def.refs[ k ] --;
+					}
+				}
+			}
 			udc.uses.removeSwap( i );
 			changesMade = true;
 		}
@@ -1652,6 +1650,58 @@ bool RDCConditionals_(
 	return true;
 }
 
+/*----------------------------------------------------------------------------*/
+
+bool RDCDeadStores_(
+		T_OpsParserOutput&	program ,
+		T_OptData&		oData
+		) noexcept
+{
+	M_LOGSTR_( "...... Removing dead stores" , 3 );
+	oData.buildUseDefineChains( program );
+
+	T_AutoArray< A_InstructionNode* , 32 > deadSets;
+	for ( auto& udc : oData.varUDChains.values( ) ) {
+		const auto nDefs{ udc.defines.size( ) };
+		for ( auto i = 0u ; i < nDefs ; i ++ ) {
+			auto const& def{ udc.defines[ i ] };
+			if ( !def.refs.size( ) ) {
+				deadSets.add( oData.instructions[ def.node ].node );
+			}
+		}
+	}
+	if ( deadSets.empty( ) ) {
+		return false;
+	}
+	oData.logger( [&](){
+		T_StringBuilder sb;
+		sb << "Found " << deadSets.size( ) << " dead store candidates";
+		return sb;
+	} , 5 );
+
+	const auto nds{ deadSets.size( ) };
+	bool changesMade{ false };
+	for ( auto i = 0u ; i < nds ; i ++ ) {
+		auto* const instr{ deadSets[ i ] };
+		if ( instr->type( ) != A_Node::OP_CALL ) {
+			oData.logger( [&](){
+				T_StringBuilder sb;
+				sb << "Removing dead store at "
+					<< instr->location( );
+				return sb;
+			} , 4 );
+			( (T_InstrListNode&) instr->parent( ) ).replaceMultiple(
+					*instr , nullptr );
+			changesMade = true;
+		}
+	}
+
+	if ( changesMade ) {
+		oData.state = {};
+	}
+	return changesMade;
+}
+
 } // namespace <anon>
 
 /*----------------------------------------------------------------------------*/
@@ -1660,14 +1710,21 @@ bool opopt::RemoveDeadCode(
 		T_OpsParserOutput& program ,
 		T_OptData& oData ) noexcept
 {
-//	oData.buildUseDefineChains( program );
 	M_LOGSTR_( "... Eliminating dead code" , 2 );
 
 	bool didStuff{ false };
-//	bool tryDeadStoresFirst{ oData.state & T_OptData::E_StateItem::UDCHAINS };
+	bool tryDeadStoresFirst{ oData.state & T_OptData::E_StateItem::UDCHAINS };
 	bool didStuffThisTime;
 	do {
-		didStuffThisTime = RDCConditionals_( program , oData );
+		if ( tryDeadStoresFirst ) {
+			tryDeadStoresFirst = false;
+			didStuff = RDCDeadStores_( program , oData );
+			didStuffThisTime = true;
+			continue;
+		}
+
+		didStuffThisTime = RDCConditionals_( program , oData )
+			|| RDCDeadStores_( program , oData );
 		didStuff = didStuff || didStuffThisTime;
 	} while ( didStuffThisTime );
 
diff --git a/c-opopt.hh b/c-opopt.hh
index 9830deb..126b9f5 100644
--- a/c-opopt.hh
+++ b/c-opopt.hh
@@ -169,8 +169,7 @@ struct T_OptData
 	{
 		uint32_t node;				// Instruction that uses or sets it
 		uint32_t fnIndex;			// Function in which the use/define is located
-		ebcl::T_Set< uint32_t > refs{		// Corresponding uses/defines
-			ebcl::UseTag< ebcl::ArrayBacked< 16 > >( ) };
+		T_AutoArray< uint32_t , 16 > refs;	// Corresponding uses/defines
 	};
 
 	// Use/define chains for a variable