jdk.graal.compiler.lir.EdgeMoveOptimizer Maven / Gradle / Ivy
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package jdk.graal.compiler.lir;
import java.util.ArrayList;
import java.util.List;
import jdk.graal.compiler.core.common.cfg.BasicBlock;
import jdk.graal.compiler.core.common.util.CompilationAlarm;
import jdk.graal.compiler.lir.StandardOp.LoadConstantOp;
import jdk.graal.compiler.lir.StandardOp.MoveOp;
import jdk.graal.compiler.lir.StandardOp.ValueMoveOp;
import jdk.graal.compiler.lir.gen.LIRGenerationResult;
import jdk.graal.compiler.lir.phases.PostAllocationOptimizationPhase;
import jdk.vm.ci.code.TargetDescription;
/**
* This class optimizes moves, particularly those that result from eliminating SSA form.
*
* When a block has more than one predecessor, and all predecessors end with the
* {@linkplain Optimizer#same(LIRInstruction, LIRInstruction) same} sequence of {@linkplain MoveOp
* move} instructions, then these sequences can be replaced with a single copy of the sequence at
* the beginning of the block.
*
* Similarly, when a block has more than one successor, then same sequences of moves at the
* beginning of the successors can be placed once at the end of the block. But because the moves
* must be inserted before all branch instructions, this works only when there is exactly one
* conditional branch at the end of the block (because the moves must be inserted before all
* branches, but after all compares).
*
* This optimization affects all kind of moves (reg->reg, reg->stack and stack->reg).
* Because this optimization works best when a block contains only a few moves, it has a huge impact
* on the number of blocks that are totally empty.
*/
public final class EdgeMoveOptimizer extends PostAllocationOptimizationPhase {
@Override
protected void run(TargetDescription target, LIRGenerationResult lirGenRes, PostAllocationOptimizationContext context) {
LIR ir = lirGenRes.getLIR();
Optimizer optimizer = new Optimizer(ir);
int[] blockIds = ir.linearScanOrder();
// ignore the first block in the list (index 0 is not processed)
for (int blockId = blockIds.length - 1; blockId >= 1; blockId--) {
BasicBlock block = ir.getBlockById(blockIds[blockId]);
if (block.getPredecessorCount() > 1) {
optimizer.optimizeMovesAtBlockEnd(block, ir);
}
if (block.getSuccessorCount() == 2) {
optimizer.optimizeMovesAtBlockBegin(block, ir);
}
}
}
private static final class Optimizer {
private final List> edgeInstructionSeqences;
private LIR ir;
Optimizer(LIR ir) {
this.ir = ir;
edgeInstructionSeqences = new ArrayList<>(4);
}
/**
* Determines if two operations are both {@linkplain MoveOp moves} that have the same source
* and {@linkplain MoveOp#getResult() destination} operands.
*
* @param op1 the first instruction to compare
* @param op2 the second instruction to compare
* @return {@code true} if {@code op1} and {@code op2} are the same by the above algorithm
*/
private static boolean same(LIRInstruction op1, LIRInstruction op2) {
assert op1 != null;
assert op2 != null;
if (ValueMoveOp.isValueMoveOp(op1) && ValueMoveOp.isValueMoveOp(op2)) {
ValueMoveOp move1 = ValueMoveOp.asValueMoveOp(op1);
ValueMoveOp move2 = ValueMoveOp.asValueMoveOp(op2);
if (move1.getInput().equals(move2.getInput()) && move1.getResult().equals(move2.getResult())) {
// these moves are exactly equal and can be optimized
return true;
}
} else if (LoadConstantOp.isLoadConstantOp(op1) && LoadConstantOp.isLoadConstantOp(op2)) {
LoadConstantOp move1 = LoadConstantOp.asLoadConstantOp(op1);
LoadConstantOp move2 = LoadConstantOp.asLoadConstantOp(op2);
if (move1.getConstant().equals(move2.getConstant()) && move1.getResult().equals(move2.getResult())) {
// these moves are exactly equal and can be optimized
return true;
}
}
return false;
}
/**
* Moves the longest {@linkplain #same common} subsequence at the end all predecessors of
* {@code block} to the start of {@code block}.
*/
private void optimizeMovesAtBlockEnd(BasicBlock block, LIR lir) {
for (int i = 0; i < block.getPredecessorCount(); i++) {
BasicBlock pred = block.getPredecessorAt(i);
if (pred == block) {
// currently we can't handle this correctly.
return;
}
}
// clear all internal data structures
edgeInstructionSeqences.clear();
int numPreds = block.getPredecessorCount();
assert numPreds > 1 : "do not call otherwise";
// setup a list with the LIR instructions of all predecessors
for (int i = 0; i < block.getPredecessorCount(); i++) {
BasicBlock pred = block.getPredecessorAt(i);
assert pred != null;
assert ir.getLIRforBlock(pred) != null;
ArrayList predInstructions = ir.getLIRforBlock(pred);
if (pred.getSuccessorCount() != 1) {
// this can happen with switch-statements where multiple edges are between
// the same blocks.
return;
}
assert pred.getSuccessorAt(0) == block : "invalid control flow";
assert predInstructions.get(predInstructions.size() - 1) instanceof StandardOp.JumpOp : "block must end with unconditional jump";
if (predInstructions.get(predInstructions.size() - 1).hasState()) {
// can not optimize instructions that have debug info
return;
}
// ignore the unconditional branch at the end of the block
List seq = predInstructions.subList(0, predInstructions.size() - 1);
edgeInstructionSeqences.add(seq);
}
// process lir-instructions while all predecessors end with the same instruction
while (true) { // TERMINATION ARGUMENT: processing edge instructions of a fixed set of
// predecessor blocks
CompilationAlarm.checkProgress(lir.getOptions(), lir);
List seq = edgeInstructionSeqences.get(0);
if (seq.isEmpty()) {
return;
}
LIRInstruction op = last(seq);
for (int i = 1; i < numPreds; ++i) {
List otherSeq = edgeInstructionSeqences.get(i);
if (otherSeq.isEmpty() || !same(op, last(otherSeq))) {
return;
}
}
// insert the instruction at the beginning of the current block
ir.getLIRforBlock(block).add(1, op);
// delete the instruction at the end of all predecessors
for (int i = 0; i < numPreds; i++) {
seq = edgeInstructionSeqences.get(i);
removeLast(seq);
}
}
}
/**
* Moves the longest {@linkplain #same common} subsequence at the start of all successors of
* {@code block} to the end of {@code block} just prior to the branch instruction ending
* {@code block}.
*/
private void optimizeMovesAtBlockBegin(BasicBlock block, LIR lir) {
edgeInstructionSeqences.clear();
int numSux = block.getSuccessorCount();
ArrayList instructions = ir.getLIRforBlock(block);
assert numSux == 2 : "method should not be called otherwise";
LIRInstruction lastInstruction = instructions.get(instructions.size() - 1);
if (lastInstruction.hasState()) {
// cannot optimize instructions when debug info is needed
return;
}
LIRInstruction branch = lastInstruction;
if (!(branch instanceof StandardOp.BranchOp) || branch.hasOperands()) {
// Only blocks that end with a conditional branch are optimized.
// In addition, a conditional branch with operands (including state) cannot
// be optimized. Moving a successor instruction before such a branch may
// interfere with the operands of the branch. For example, a successive move
// instruction may redefine an input operand of the branch.
return;
}
// Now it is guaranteed that the block ends with a conditional branch.
// The instructions are inserted at the end of the block before the branch.
int insertIdx = instructions.size() - 1;
// setup a list with the lir-instructions of all successors
for (int i = 0; i < block.getSuccessorCount(); i++) {
BasicBlock sux = block.getSuccessorAt(i);
ArrayList suxInstructions = ir.getLIRforBlock(sux);
assert suxInstructions.get(0) instanceof StandardOp.LabelOp : "block must start with label";
if (sux.getPredecessorCount() != 1) {
// this can happen with switch-statements where multiple edges are between
// the same blocks.
return;
}
assert sux.getPredecessorAt(0) == block : "invalid control flow";
// ignore the label at the beginning of the block
List seq = suxInstructions.subList(1, suxInstructions.size());
edgeInstructionSeqences.add(seq);
}
// process LIR instructions while all successors begin with the same instruction
while (true) { // TERMINATION ARGUMENT: processing edge instructions for a fixed set of
// predecessor blocks
CompilationAlarm.checkProgress(lir.getOptions(), lir);
List seq = edgeInstructionSeqences.get(0);
if (seq.isEmpty()) {
return;
}
LIRInstruction op = first(seq);
for (int i = 1; i < numSux; i++) {
List otherSeq = edgeInstructionSeqences.get(i);
if (otherSeq.isEmpty() || !same(op, first(otherSeq))) {
// these instructions are different and cannot be optimized .
// no further optimization possible
return;
}
}
// insert instruction at end of current block
ir.getLIRforBlock(block).add(insertIdx, op);
insertIdx++;
// delete the instructions at the beginning of all successors
for (int i = 0; i < numSux; i++) {
seq = edgeInstructionSeqences.get(i);
removeFirst(seq);
}
}
}
/**
* Gets the first element from a LIR instruction sequence.
*/
private static LIRInstruction first(List seq) {
return seq.get(0);
}
/**
* Gets the last element from a LIR instruction sequence.
*/
private static LIRInstruction last(List seq) {
return seq.get(seq.size() - 1);
}
/**
* Removes the first element from a LIR instruction sequence.
*/
private static void removeFirst(List seq) {
seq.remove(0);
}
/**
* Removes the last element from a LIR instruction sequence.
*/
private static void removeLast(List seq) {
seq.remove(seq.size() - 1);
}
}
}