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package org.graalvm.compiler.lir;

import static org.graalvm.compiler.lir.LIR.verifyBlocks;

import java.util.ArrayList;

import org.graalvm.compiler.core.common.cfg.AbstractBlockBase;
import org.graalvm.compiler.debug.CounterKey;
import org.graalvm.compiler.debug.DebugContext;
import org.graalvm.compiler.lir.gen.LIRGenerationResult;
import org.graalvm.compiler.lir.phases.PostAllocationOptimizationPhase;

import jdk.vm.ci.code.TargetDescription;

This class performs basic optimizations on the control flow graph after LIR generation.
/** * This class performs basic optimizations on the control flow graph after LIR generation. */
public final class ControlFlowOptimizer extends PostAllocationOptimizationPhase {
Performs control flow optimizations on the given LIR graph.
/** * Performs control flow optimizations on the given LIR graph. */
@Override protected void run(TargetDescription target, LIRGenerationResult lirGenRes, PostAllocationOptimizationContext context) { LIR lir = lirGenRes.getLIR(); new Optimizer(lir).deleteEmptyBlocks(lir.codeEmittingOrder()); } private static final class Optimizer { private final LIR lir; private Optimizer(LIR lir) { this.lir = lir; } private static final CounterKey BLOCKS_DELETED = DebugContext.counter("BlocksDeleted");
Checks whether a block can be deleted. Only blocks with exactly one successor and an unconditional branch to this successor are eligable.
Params:
  • block – the block checked for deletion
Returns:whether the block can be deleted
/** * Checks whether a block can be deleted. Only blocks with exactly one successor and an * unconditional branch to this successor are eligable. * * @param block the block checked for deletion * @return whether the block can be deleted */
private boolean canDeleteBlock(AbstractBlockBase<?> block) { if (block == null || block.getSuccessorCount() != 1 || block.getPredecessorCount() == 0 || block.getSuccessors()[0] == block) { return false; } ArrayList<LIRInstruction> instructions = lir.getLIRforBlock(block); assert instructions.size() >= 2 : "block must have label and branch"; assert instructions.get(0) instanceof StandardOp.LabelOp : "first instruction must always be a label"; assert instructions.get(instructions.size() - 1) instanceof StandardOp.JumpOp : "last instruction must always be a branch"; assert ((StandardOp.JumpOp) instructions.get(instructions.size() - 1)).destination().label() == ((StandardOp.LabelOp) lir.getLIRforBlock(block.getSuccessors()[0]).get( 0)).getLabel() : "branch target must be the successor"; // Block must have exactly one successor. return instructions.size() == 2 && !instructions.get(instructions.size() - 1).hasState() && !block.isExceptionEntry(); } private void alignBlock(AbstractBlockBase<?> block) { if (!block.isAligned()) { block.setAlign(true); ArrayList<LIRInstruction> instructions = lir.getLIRforBlock(block); assert instructions.get(0) instanceof StandardOp.LabelOp : "first instruction must always be a label"; StandardOp.LabelOp label = (StandardOp.LabelOp) instructions.get(0); instructions.set(0, new StandardOp.LabelOp(label.getLabel(), true)); } } private void deleteEmptyBlocks(AbstractBlockBase<?>[] blocks) { assert verifyBlocks(lir, blocks); for (int i = 0; i < blocks.length; i++) { AbstractBlockBase<?> block = blocks[i]; if (canDeleteBlock(block)) { block.delete(); // adjust successor and predecessor lists AbstractBlockBase<?> other = block.getSuccessors()[0]; if (block.isAligned()) { alignBlock(other); } BLOCKS_DELETED.increment(lir.getDebug()); blocks[i] = null; } } assert verifyBlocks(lir, blocks); } } }