fernflower/src/org/jetbrains/java/decompiler/modules/code/DeadCodeHelper.java

/*
 *    Fernflower - The Analytical Java Decompiler
 *    http://www.reversed-java.com
 *
 *    (C) 2008 - 2010, Stiver
 *
 *    This software is NEITHER public domain NOR free software 
 *    as per GNU License. See license.txt for more details.
 *
 *    This software is distributed WITHOUT ANY WARRANTY; without 
 *    even the implied warranty of MERCHANTABILITY or FITNESS FOR 
 *    A PARTICULAR PURPOSE. 
 */

package org.jetbrains.java.decompiler.modules.code;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;

import org.jetbrains.java.decompiler.code.CodeConstants;
import org.jetbrains.java.decompiler.code.Instruction;
import org.jetbrains.java.decompiler.code.InstructionSequence;
import org.jetbrains.java.decompiler.code.cfg.BasicBlock;
import org.jetbrains.java.decompiler.code.cfg.ControlFlowGraph;
import org.jetbrains.java.decompiler.code.cfg.ExceptionRangeCFG;
import org.jetbrains.java.decompiler.main.DecompilerContext;
import org.jetbrains.java.decompiler.main.extern.IFernflowerPreferences;

public class DeadCodeHelper {

	public static void removeDeadBlocks(ControlFlowGraph graph) {
		
		LinkedList<BasicBlock> stack = new LinkedList<BasicBlock>();
		HashSet<BasicBlock> setStacked = new HashSet<BasicBlock>();
		
		stack.add(graph.getFirst());
		setStacked.add(graph.getFirst());

		while(!stack.isEmpty()) {
			BasicBlock block = stack.removeFirst();
		
			List<BasicBlock> lstSuccs = new ArrayList<BasicBlock>(block.getSuccs());
			lstSuccs.addAll(block.getSuccExceptions());
			
			for(BasicBlock succ : lstSuccs) {
				if(!setStacked.contains(succ)) {
					stack.add(succ);
					setStacked.add(succ);
				}
			}
		}
		
		HashSet<BasicBlock> setAllBlocks = new HashSet<BasicBlock>(graph.getBlocks());
		setAllBlocks.removeAll(setStacked);
		
		for(BasicBlock block : setAllBlocks) {
			graph.removeBlock(block);
		}
	}
	
	public static void removeEmptyBlocks(ControlFlowGraph graph) {

		List<BasicBlock> blocks = graph.getBlocks(); 
		
		boolean cont;
		do {
			cont = false;
			
			for(int i=blocks.size()-1;i>=0;i--) {
				BasicBlock block = (BasicBlock)blocks.get(i);
				
				if(removeEmptyBlock(graph, block, false)) {
					cont = true;
					break;
				}
			}
			
		} while(cont);
	}
	
	private static boolean removeEmptyBlock(ControlFlowGraph graph, BasicBlock block, boolean merging) {
		
		boolean deletedRanges = false;
		
		if(block.getSeq().isEmpty()) {
			
			if(block.getSuccs().size() > 1) {
				if(block.getPreds().size()>1) {
					// ambiguous block
					throw new RuntimeException("ERROR: empty block with multiple predecessors and successors found"); 
				} else if(!merging) {
					throw new RuntimeException("ERROR: empty block with multiple successors found"); 
				}
			}
			
			HashSet<BasicBlock> setExits = new HashSet<BasicBlock>(graph.getLast().getPreds());

			if(block.getPredExceptions().isEmpty() && 
					(!setExits.contains(block) || block.getPreds().size() == 1)) {
				
				if(setExits.contains(block)) {
					BasicBlock pred = block.getPreds().get(0);
					
					// FIXME: flag in the basic block
					if(pred.getSuccs().size() != 1 || (!pred.getSeq().isEmpty()
							&& pred.getSeq().getLastInstr().group == CodeConstants.GROUP_SWITCH)) {
						return false;
					}
				}
				
				HashSet<BasicBlock> setPreds = new HashSet<BasicBlock>(block.getPreds());
				HashSet<BasicBlock> setSuccs = new HashSet<BasicBlock>(block.getSuccs());

				// collect common exception ranges of predecessors and successors
				HashSet<BasicBlock> setCommonExceptionHandlers = null; 
				for(int i = 0; i < 2; ++i) {
					for(BasicBlock pred : i == 0 ? setPreds : setSuccs) {
						if(setCommonExceptionHandlers == null) {
							setCommonExceptionHandlers = new HashSet<BasicBlock>(pred.getSuccExceptions());
						} else {
							setCommonExceptionHandlers.retainAll(pred.getSuccExceptions());
						}
					}
				}

				// check the block to be in each of the common ranges
				if(setCommonExceptionHandlers != null && !setCommonExceptionHandlers.isEmpty()) {
					for(BasicBlock handler : setCommonExceptionHandlers) {
						if(!block.getSuccExceptions().contains(handler)) {
							return false;
						}
					}
				}
				
				// remove ranges consisting of this one block
				List<ExceptionRangeCFG> lstRanges = graph.getExceptions();
				for(int i=lstRanges.size()-1;i>=0;i--) {
					ExceptionRangeCFG range = lstRanges.get(i);
					List<BasicBlock> lst = range.getProtectedRange();

					if(lst.size() == 1 && lst.get(0) == block) {
						if(DecompilerContext.getOption(IFernflowerPreferences.REMOVE_EMPTY_RANGES)) {
							block.removeSuccessorException(range.getHandler());
							lstRanges.remove(i);
							
							deletedRanges = true;
						} else {
							return false;
						}
					}
				}

				
				// connect remaining nodes
				if(merging) {
					BasicBlock pred = block.getPreds().get(0);
					pred.removeSuccessor(block);
					
					List<BasicBlock> lstSuccs = new ArrayList<BasicBlock>(block.getSuccs());
					for(BasicBlock succ : lstSuccs) {
						block.removeSuccessor(succ);
						pred.addSuccessor(succ);
					}
					
				} else {
					for(BasicBlock pred : setPreds) {
						for(BasicBlock succ : setSuccs) {
							pred.replaceSuccessor(block, succ);
						}
					}
				}

				// finally exit edges
				HashSet<BasicBlock> setFinallyExits = graph.getFinallyExits(); 
				if(setFinallyExits.contains(block)) {
					setFinallyExits.remove(block);
					setFinallyExits.add(setPreds.iterator().next());
				}
				
				// replace first if necessary
				if(graph.getFirst() == block) {
					if(setSuccs.size() != 1) {
						throw new RuntimeException("multiple or no entry blocks!");
					} else {
						graph.setFirst(setSuccs.iterator().next());
					}
				}

				// remove this block
				graph.removeBlock(block);
				
				if(deletedRanges) {
					DeadCodeHelper.removeDeadBlocks(graph);
				}
			}
		}
		
		return deletedRanges;
	}
	
	
	public static boolean isDominator(ControlFlowGraph graph, BasicBlock block, BasicBlock dom) {
		
		HashSet<BasicBlock> marked = new HashSet<BasicBlock>();
		
		if(block == dom) {
			return true;
		}
		
		LinkedList<BasicBlock> lstNodes = new LinkedList<BasicBlock>();
		lstNodes.add(block);

		while(!lstNodes.isEmpty()) {
			
			BasicBlock node = (BasicBlock)lstNodes.remove(0);
			if(marked.contains(node)) {
				continue;
			} else {
				marked.add(node);
			}
			
			if(node == graph.getFirst()) {
				return false;
			}

			for(int i=0;i<node.getPreds().size();i++) {
				BasicBlock pred = (BasicBlock)node.getPreds().get(i);
				if(!marked.contains(pred) && pred != dom) {
					lstNodes.add(pred);
				}
			}
			
			for(int i=0;i<node.getPredExceptions().size();i++) {
				BasicBlock pred = (BasicBlock)node.getPredExceptions().get(i);
				if(!marked.contains(pred) && pred != dom) {
					lstNodes.add(pred);
				}
			}
		}
		
		return true;
	}
	
	public static void removeGotos(ControlFlowGraph graph) {
		
		for(BasicBlock block : graph.getBlocks()) {
			Instruction instr = block.getLastInstruction();
			
			if(instr!=null && instr.opcode == CodeConstants.opc_goto) {
				block.getSeq().removeInstruction(block.getSeq().length()-1);
			}
		}

		DeadCodeHelper.removeEmptyBlocks(graph);
	}
	
	public static void connectDummyExitBlock(ControlFlowGraph graph) {
		
		BasicBlock exit = graph.getLast();
		for(BasicBlock block : new HashSet<BasicBlock>(exit.getPreds())) {
			exit.removePredecessor(block);
			block.addSuccessor(exit);
		}
	}
	
	public static void incorporateValueReturns(ControlFlowGraph graph) {

		for(BasicBlock block: graph.getBlocks()) {
			InstructionSequence seq = block.getSeq();
			
			int len = seq.length();
			if(len > 0 && len < 3) {
				
				boolean ok = false;
				
				if(seq.getLastInstr().opcode >= CodeConstants.opc_ireturn && seq.getLastInstr().opcode <= CodeConstants.opc_return) {
					if(len == 1) {
						ok = true;
					} else if(seq.getLastInstr().opcode != CodeConstants.opc_return){
						switch(seq.getInstr(0).opcode) {
						case CodeConstants.opc_iload:		
						case CodeConstants.opc_lload:		
						case CodeConstants.opc_fload:		
						case CodeConstants.opc_dload:		
						case CodeConstants.opc_aload:		
						case CodeConstants.opc_aconst_null:
						case CodeConstants.opc_bipush:
						case CodeConstants.opc_sipush:
						case CodeConstants.opc_lconst_0:
						case CodeConstants.opc_lconst_1:
						case CodeConstants.opc_fconst_0:    
						case CodeConstants.opc_fconst_1:    
						case CodeConstants.opc_fconst_2:
						case CodeConstants.opc_dconst_0:    
						case CodeConstants.opc_dconst_1:
						case CodeConstants.opc_ldc:		
						case CodeConstants.opc_ldc_w:
						case CodeConstants.opc_ldc2_w:
							ok = true;
						}
					}
				}
				
				if(ok) {
					
					if(!block.getPreds().isEmpty()) {
						
						HashSet<BasicBlock> setPredHandlersUnion = new HashSet<BasicBlock>(); 
						HashSet<BasicBlock> setPredHandlersIntersection = new HashSet<BasicBlock>(); 
						
						boolean firstpred = true;
						for(BasicBlock pred : block.getPreds()) {
							if(firstpred) {
								setPredHandlersIntersection.addAll(pred.getSuccExceptions());
								firstpred = false;
							} else {
								setPredHandlersIntersection.retainAll(pred.getSuccExceptions());
							}

							setPredHandlersUnion.addAll(pred.getSuccExceptions());
						}
						
						// add exception ranges from predecessors
						setPredHandlersIntersection.removeAll(block.getSuccExceptions());
						BasicBlock predecessor = block.getPreds().get(0);
						
						for(BasicBlock handler : setPredHandlersIntersection) {
							ExceptionRangeCFG range = graph.getExceptionRange(handler, predecessor);

							range.getProtectedRange().add(block);
							block.addSuccessorException(handler);
						}

						// remove redundant ranges
						HashSet<BasicBlock> setRangesToBeRemoved = new HashSet<BasicBlock>(block.getSuccExceptions());
						setRangesToBeRemoved.removeAll(setPredHandlersUnion);
						
						for(BasicBlock handler : setRangesToBeRemoved) {
							ExceptionRangeCFG range = graph.getExceptionRange(handler, block);

							if(range.getProtectedRange().size() > 1) {
								range.getProtectedRange().remove(block);
								block.removeSuccessorException(handler);
							}
						}
					}
					
					
					if(block.getPreds().size() == 1 && block.getPredExceptions().isEmpty()) {
						
						BasicBlock bpred = block.getPreds().get(0);
						if(bpred.getSuccs().size() == 1) {
							
							// add exception ranges of predecessor
							for(BasicBlock succ : bpred.getSuccExceptions()) {
								if(!block.getSuccExceptions().contains(succ)) {
									ExceptionRangeCFG range = graph.getExceptionRange(succ, bpred);
									
									range.getProtectedRange().add(block);
									block.addSuccessorException(succ);
								}
							}

							// remove superfluous ranges from successors
							for(BasicBlock succ : new HashSet<BasicBlock>(block.getSuccExceptions())) {
								if(!bpred.getSuccExceptions().contains(succ)) {
									ExceptionRangeCFG range = graph.getExceptionRange(succ, block);
									
									if(range.getProtectedRange().size() > 1) {
										range.getProtectedRange().remove(block);
										block.removeSuccessorException(succ);
									}
								}
							}
						}
					}
					
				}
			}
			
		}
		
	}
	
	
	public static void mergeBasicBlocks(ControlFlowGraph graph) {
		
		for(;;) {
			
			boolean merged = false;
			
			for(BasicBlock block: graph.getBlocks()) {
	
				InstructionSequence seq = block.getSeq();
				
				if(block.getSuccs().size() == 1) {
					BasicBlock next = block.getSuccs().get(0);

					if(next != graph.getLast() && (seq.isEmpty() || seq.getLastInstr().group != CodeConstants.GROUP_SWITCH)) {
						
						if(next.getPreds().size() == 1 && next.getPredExceptions().isEmpty()
								&& next != graph.getFirst()) {
							// TODO: implement a dummy start block
							boolean sameRanges = true; 
							for(ExceptionRangeCFG range : graph.getExceptions()) {
								if(range.getProtectedRange().contains(block) ^
										range.getProtectedRange().contains(next)) {
									sameRanges = false;
									break;
								}
							}
							
							if(sameRanges) {
								seq.addSequence(next.getSeq());
								next.getSeq().clear();
								
								removeEmptyBlock(graph, next, true);
								
								merged = true;
								break;
							}
						}
					}
				}
				
			}
			
			if(!merged) {
				break;
			}
		}
		
	}
	
	
}