Partial Exectuer: Track progress of SCCs to avoid redundant visits

The parent tracks whether progress has been made while processing its SCC set.

Each visit to a child SCC returns a boolean indicating whether running that child
resulted in progress. Children are visited in reverse order.

The last child is always the starting SCC. The first child is a copy that contains
all basic blocks split into SCCs and used to detect loops. This copy is marked
reachable when, during edge registration, we detect a back-edge into the current SCC set.

If the copy is reachable, the SCC set forms a loop. After all SCCs have ben analyzed,
we use the progress flag to decide whether to visit the copy again. If progress was made,
the copy is visited, which recreates the same SCC structure with a new copy at the end.
If no progress was made, we stop recursing that set of SCC and mark them as skipped.

Author: Andres Madrid Ucha

llvm/lib/CheerpWriter/PartialExecuter.cpp

@@ -301,7 +301,7 @@ class PartialInterpreter : public llvm::Interpreter {
 		}
 		return nullptr;
 	}
-	llvm::BasicBlock* visitBasicBlock(FunctionData& data, llvm::BasicBlock& BB);
+	llvm::BasicBlock* visitBasicBlock(FunctionData& data, llvm::BasicBlock& BB, bool& BBProgress);
 	explicit PartialInterpreter(std::unique_ptr<llvm::Module> M)
 		: llvm::Interpreter(std::move(M), /*preExecute*/false)
 	{
@@ -631,7 +631,7 @@ class PartialInterpreter : public llvm::Interpreter {
 	{
 		return (++functionCounters[F] > 0x1000);
 	}
-	void visitOuter(FunctionData& data, llvm::Instruction& I);
+	void visitOuter(FunctionData& data, llvm::Instruction& I, bool& BBProgress);
 	bool replaceKnownCEs()
 	{
 		if(fullyKnownCEs.empty())
@@ -786,7 +786,7 @@ static void removeEdgeBetweenBlocks(llvm::BasicBlock* from, llvm::BasicBlock* to
 	}
 }
 
-llvm::BasicBlock* PartialInterpreter::visitBasicBlock(FunctionData& data, llvm::BasicBlock& BB)
+llvm::BasicBlock* PartialInterpreter::visitBasicBlock(FunctionData& data, llvm::BasicBlock& BB, bool& BBProgress)
 {
 	ExecutionContext& executionContext = getTopCallFrame();
 	executionContext.CurBB = &BB;
@@ -797,7 +797,7 @@ llvm::BasicBlock* PartialInterpreter::visitBasicBlock(FunctionData& data, llvm::
 		// Note that here we could also execute a Call, and that implies adding a CallFrame
 		// executing there (possibly also in depth)
 		// So getTopCallFrame() has to be called since it will possibly change
-		visitOuter(data, *getTopCallFrame().CurInst++);
+		visitOuter(data, *getTopCallFrame().CurInst++, BBProgress);
 	}
 
 	// Find (if there are enough information) the next BB to be visited
@@ -990,9 +990,10 @@ class FunctionData
 		assert(currentEE);
 		return *currentEE;
 	}
-	void registerEdge(const llvm::BasicBlock* from, const llvm::BasicBlock* to)
+	bool registerEdge(const llvm::BasicBlock* from, const llvm::BasicBlock* to)
 	{
-		visitedEdges.insert({from, to});
+		auto [it, is_inserted] = visitedEdges.insert({from, to});
+		return is_inserted;
 	}
 	bool hasNoInfo(const VectorOfArgs& arguments) const
 	{
@@ -1178,29 +1179,36 @@ class FunctionData
 	{
 		knownValues[&I].everSkipped = true;
 	}
-	void registerValueForInst(llvm::Instruction& I, const GenericValue& GV, PartialInterpreter::BitMask strongBits)
+	bool registerValueForInst(llvm::Instruction& I, const GenericValue& GV, PartialInterpreter::BitMask strongBits)
 	{
 		// Only support integers for now, to simplify the handling of GVs
 		if(!I.getType()->isIntegerTy())
-			return;
+			return false;
 		// Do not attempt tracking of partially known values
 		if(strongBits != PartialInterpreter::BitMask::ALL)
-			return markInstSkipped(I);
+		{
+			markInstSkipped(I);
+			return false;
+		}
 		auto it = knownValues.find(&I);
 		if(it == knownValues.end())
 		{
 			// Never registered any value before, save the current one
 			knownValues[&I].value = GV;
+			return true;
 		}
 		else if(it->second.everSkipped)
 		{
 			// Nothing we can do anymore
+			return false;
 		}
 		else if(it->second.value.IntVal != GV.IntVal)
 		{
 			// Different value observed, bailout
 			it->second.everSkipped = true;
+			return false;
 		}
+		return false;
 	}
 	void replaceKnownValues() const
 	{
@@ -1371,6 +1379,7 @@ class BasicBlockGroupNode
 	const DeterministicBBSet& blocks;
 	bool isMultiHead;
 	bool isReachable;
+	bool SCCProgress;
 	llvm::BasicBlock* start;
 	llvm::BasicBlock* from;
 	uint32_t currIter;
@@ -1398,7 +1407,7 @@ class BasicBlockGroupNode
 	void splitIntoSCCs(std::list<BasicBlockGroupNode>& queueToBePopulated, ReverseMapBBToGroup& blockToGroupMap);
 public:
 	BasicBlockGroupNode(FunctionData& data, BasicBlockGroupNode* parentBBGNode, const DeterministicBBSet& OWNEDblocks, llvm::BasicBlock* start = nullptr)
-		: parentNode(parentBBGNode), data(data), ownedBlocks(OWNEDblocks), blocks(ownedBlocks), isMultiHead(false), isReachable(parentNode == nullptr), start(start), from(nullptr), visitingAll(false)
+		: parentNode(parentBBGNode), data(data), ownedBlocks(OWNEDblocks), blocks(ownedBlocks), isMultiHead(false), isReachable(parentNode == nullptr), SCCProgress(false), start(start), from(nullptr), visitingAll(false)
 	{
 		if (start)
 			assert(start->getParent() == data.getFunction());
@@ -1408,7 +1417,7 @@ class BasicBlockGroupNode
 	{
 	}
 	BasicBlockGroupNode(BasicBlockGroupNode& BBGNode)
-		: parentNode(&BBGNode), data(BBGNode.data), blocks(BBGNode.blocks), isMultiHead(false), isReachable(false), start(nullptr), from(nullptr), visitingAll(false)
+		: parentNode(&BBGNode), data(BBGNode.data), blocks(BBGNode.blocks), isMultiHead(false), isReachable(false), SCCProgress(false), start(nullptr), from(nullptr), visitingAll(false)
 	{
 	}
 	void addIncomingEdge(llvm::BasicBlock* comingFrom, uint32_t currIter, llvm::BasicBlock* target)
@@ -1439,13 +1448,13 @@ class BasicBlockGroupNode
 	// Do the visit of the BB, with 'from' (possibly nullptr if unknown) as predecessor
 	// Loop backs will be directed to another BBgroup
 	// The visit will return the set of reachable BBs, to be added into visitNext
-	void runVisitBasicBlock(FunctionData& data, llvm::BasicBlock& BB, llvm::SmallVectorImpl<llvm::BasicBlock*>& visitNext)
+	void runVisitBasicBlock(FunctionData& data, llvm::BasicBlock& BB, llvm::SmallVectorImpl<llvm::BasicBlock*>& visitNext, bool& BBProgress)
 	{
 		assert(visitNext.empty());
 
 		PartialInterpreter& interpreter = data.getInterpreter();
 		interpreter.incomingBB = from;
-		BasicBlock* ret = interpreter.visitBasicBlock(data, BB);
+		BasicBlock* ret = interpreter.visitBasicBlock(data, BB, BBProgress);
 
 		if (ret)
 		{
@@ -1506,10 +1515,11 @@ class BasicBlockGroupNode
 				registerEdge(bb, succ);
 		}
 	}
-	void registerEdge(llvm::BasicBlock* from, llvm::BasicBlock* to)
+	bool registerEdge(llvm::BasicBlock* from, llvm::BasicBlock* to)
 	{
-		data.registerEdge(from, to);
+		bool is_inserted = data.registerEdge(from, to);
 		notifySuccessor(from, currIter, to);
+		return is_inserted;
 	}
 	void cleanUp(llvm::BasicBlock* block)
 	{
@@ -1522,14 +1532,14 @@ class BasicBlockGroupNode
 		}
 	}
 	// Visit the tree of BasicBlockGroupNodes, starting from the root and visiting children depth-first
-	void recursiveVisit()
+	bool recursiveVisit()
 	{
 		if (isMultiHead)
 		{
 			// There are multiple BasicBlock that are reacheable from outside
 			//  --> Mark everything as reachable
 			visitAll();
-			return;
+			return false;
 		}
 		assert(start);	//isReachable && !isMultiHead implies start being defined
 		currIter = data.getVisitCounter(start);
@@ -1540,7 +1550,7 @@ class BasicBlockGroupNode
 			// Since terminability is basically unprovable in general, we give up with the visit
 			//  --> Mark everything as reachable
 			visitAll();
-			return;
+			return false;
 		}
 		if (parentNode)
 		{
@@ -1556,20 +1566,38 @@ class BasicBlockGroupNode
 
 		llvm::SmallVector<llvm::BasicBlock*, 4> visitNext;
 
-		// Do the actual visit for start, while populating visitNext
-		runVisitBasicBlock(data, *start, visitNext);
-		for (llvm::BasicBlock* succ : visitNext)
-			registerEdge(start, succ);
+		bool BBProgress = false;
+		runVisitBasicBlock(data, *start, visitNext, BBProgress);
+		for (llvm::BasicBlock* succ : visitNext){
+			if (registerEdge(start, succ)){
+				BBProgress = true;
+			}
+		}
 
-		// First has been already done
+		// The first SCC (start) has already been visited
+		// This updates SCCProgress in case progress was made while visiting the start
+		if (BBProgress)
+			SCCProgress = true;
 		childrenNodes.pop_back();
 		while (!childrenNodes.empty())
 		{
 			auto& child = childrenNodes.back();
 			if (child.isReachable)
-				child.recursiveVisit();
+			{
+				// If childrenNodes size is 1, that means that we are visiting the copy of all basic blocks in the curret set of SCC
+				// Because the copy is reachable, it means that we are in a loop
+				// By this point we have run and visit all the SCC of a parent. If no progress was found there is no point to continue with the loop
+				if (childrenNodes.size() == 1 && !SCCProgress)
+				{
+					visitAll();
+					return false;
+				}
+				if (child.recursiveVisit())
+					SCCProgress = true;
+			}
 			childrenNodes.pop_back();
 		}
+		return BBProgress;
 	}
 };
 
@@ -1633,7 +1661,7 @@ void FunctionData::actualVisit()
 	groupData.recursiveVisit();
 }
 
-void PartialInterpreter::visitOuter(FunctionData& data, llvm::Instruction& I)
+void PartialInterpreter::visitOuter(FunctionData& data, llvm::Instruction& I, bool& BBProgress)
 {
 	if (PHINode* phi = dyn_cast<PHINode>(&I))
 	{
@@ -1647,6 +1675,7 @@ void PartialInterpreter::visitOuter(FunctionData& data, llvm::Instruction& I)
 			if (isValueComputed(incomingVal))
 			{
 				computedPhisValues.push_back({phi, incomingVal});
+				BBProgress = true;
 				return;
 			}
 		}
@@ -1721,7 +1750,7 @@ void PartialInterpreter::visitOuter(FunctionData& data, llvm::Instruction& I)
 				getTopCallFrame().CurBB = next;
 				getTopCallFrame().CurInst = getTopCallFrame().CurBB->begin();
 
-				// Also here we have set the proper state for the execution so we can return
+				BBProgress = true;
 				return;
 			}
 			skip = true;
@@ -1742,7 +1771,7 @@ void PartialInterpreter::visitOuter(FunctionData& data, llvm::Instruction& I)
 	}
 	//If we are here it means we have to actually perform the execution via Interpreter
 
-	//Iff it's a call, set up the next stack frame
+	//If it's a call, set up the next stack frame
 	if (CallInst* CI = dyn_cast<CallInst>(&I))
 		forwardArgumentsToNextFrame(*CI);
 
@@ -1752,7 +1781,10 @@ void PartialInterpreter::visitOuter(FunctionData& data, llvm::Instruction& I)
 	BitMask strongBits = computeStronglyKnownBits(I.getOpcode(), I);
 
 	if(isInitialCallFrame())
-		data.registerValueForInst(I, getOperandValue(&I), strongBits);
+	{
+		if (data.registerValueForInst(I, getOperandValue(&I), strongBits))
+			BBProgress = true;
+	}
 
 	if (!isa<CallInst>(I))
 	{