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23 
24 
25 package org.graalvm.compiler.lir;
26 
27 import static org.graalvm.compiler.lir.LIR.verifyBlocks;
28 
29 import java.util.ArrayList;
30 
31 import org.graalvm.compiler.core.common.cfg.AbstractBlockBase;
32 import org.graalvm.compiler.debug.CounterKey;
33 import org.graalvm.compiler.debug.DebugContext;
34 import org.graalvm.compiler.lir.gen.LIRGenerationResult;
35 import org.graalvm.compiler.lir.phases.PostAllocationOptimizationPhase;
36 
37 import jdk.vm.ci.code.TargetDescription;
38 
39 /**
40  * This class performs basic optimizations on the control flow graph after LIR generation.
41  */
42 public final class ControlFlowOptimizer extends PostAllocationOptimizationPhase {
43 
44     /**
45      * Performs control flow optimizations on the given LIR graph.
46      */
47     @Override
run(TargetDescription target, LIRGenerationResult lirGenRes, PostAllocationOptimizationContext context)48     protected void run(TargetDescription target, LIRGenerationResult lirGenRes, PostAllocationOptimizationContext context) {
49         LIR lir = lirGenRes.getLIR();
50         new Optimizer(lir).deleteEmptyBlocks(lir.codeEmittingOrder());
51     }
52 
53     private static final class Optimizer {
54 
55         private final LIR lir;
56 
Optimizer(LIR lir)57         private Optimizer(LIR lir) {
58             this.lir = lir;
59         }
60 
61         private static final CounterKey BLOCKS_DELETED = DebugContext.counter("BlocksDeleted");
62 
63         /**
64          * Checks whether a block can be deleted. Only blocks with exactly one successor and an
65          * unconditional branch to this successor are eligable.
66          *
67          * @param block the block checked for deletion
68          * @return whether the block can be deleted
69          */
canDeleteBlock(AbstractBlockBase<?> block)70         private boolean canDeleteBlock(AbstractBlockBase<?> block) {
71             if (block == null || block.getSuccessorCount() != 1 || block.getPredecessorCount() == 0 || block.getSuccessors()[0] == block) {
72                 return false;
73             }
74 
75             ArrayList<LIRInstruction> instructions = lir.getLIRforBlock(block);
76 
77             assert instructions.size() >= 2 : "block must have label and branch";
78             assert instructions.get(0) instanceof StandardOp.LabelOp : "first instruction must always be a label";
79             assert instructions.get(instructions.size() - 1) instanceof StandardOp.JumpOp : "last instruction must always be a branch";
80             assert ((StandardOp.JumpOp) instructions.get(instructions.size() - 1)).destination().label() == ((StandardOp.LabelOp) lir.getLIRforBlock(block.getSuccessors()[0]).get(
81                             0)).getLabel() : "branch target must be the successor";
82 
83             // Block must have exactly one successor.
84             return instructions.size() == 2 && !instructions.get(instructions.size() - 1).hasState() && !block.isExceptionEntry();
85         }
86 
alignBlock(AbstractBlockBase<?> block)87         private void alignBlock(AbstractBlockBase<?> block) {
88             if (!block.isAligned()) {
89                 block.setAlign(true);
90                 ArrayList<LIRInstruction> instructions = lir.getLIRforBlock(block);
91                 assert instructions.get(0) instanceof StandardOp.LabelOp : "first instruction must always be a label";
92                 StandardOp.LabelOp label = (StandardOp.LabelOp) instructions.get(0);
93                 instructions.set(0, new StandardOp.LabelOp(label.getLabel(), true));
94             }
95         }
96 
deleteEmptyBlocks(AbstractBlockBase<?>[] blocks)97         private void deleteEmptyBlocks(AbstractBlockBase<?>[] blocks) {
98             assert verifyBlocks(lir, blocks);
99             for (int i = 0; i < blocks.length; i++) {
100                 AbstractBlockBase<?> block = blocks[i];
101                 if (canDeleteBlock(block)) {
102 
103                     block.delete();
104                     // adjust successor and predecessor lists
105                     AbstractBlockBase<?> other = block.getSuccessors()[0];
106                     if (block.isAligned()) {
107                         alignBlock(other);
108                     }
109 
110                     BLOCKS_DELETED.increment(lir.getDebug());
111                     blocks[i] = null;
112                 }
113             }
114             assert verifyBlocks(lir, blocks);
115         }
116     }
117 }
118