1 /*
2 * Copyright (c) 2009, 2015, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 */
23 package org.graalvm.compiler.lir;
24
25 import java.util.ArrayList;
26 import java.util.List;
27
28 import org.graalvm.compiler.core.common.cfg.AbstractBlockBase;
29 import org.graalvm.compiler.lir.StandardOp.LoadConstantOp;
30 import org.graalvm.compiler.lir.StandardOp.MoveOp;
31 import org.graalvm.compiler.lir.StandardOp.ValueMoveOp;
32 import org.graalvm.compiler.lir.gen.LIRGenerationResult;
33 import org.graalvm.compiler.lir.phases.PostAllocationOptimizationPhase;
34
35 import jdk.vm.ci.code.TargetDescription;
36
37 /**
38 * This class optimizes moves, particularly those that result from eliminating SSA form.
39 * <p>
40 * When a block has more than one predecessor, and all predecessors end with the
41 * {@linkplain Optimizer#same(LIRInstruction, LIRInstruction) same} sequence of {@linkplain MoveOp
42 * move} instructions, then these sequences can be replaced with a single copy of the sequence at
43 * the beginning of the block.
44 * <p>
45 * Similarly, when a block has more than one successor, then same sequences of moves at the
46 * beginning of the successors can be placed once at the end of the block. But because the moves
47 * must be inserted before all branch instructions, this works only when there is exactly one
48 * conditional branch at the end of the block (because the moves must be inserted before all
49 * branches, but after all compares).
50 * <p>
51 * This optimization affects all kind of moves (reg->reg, reg->stack and stack->reg).
52 * Because this optimization works best when a block contains only a few moves, it has a huge impact
53 * on the number of blocks that are totally empty.
54 */
55 public final class EdgeMoveOptimizer extends PostAllocationOptimizationPhase {
56
57 @Override
58 protected void run(TargetDescription target, LIRGenerationResult lirGenRes, PostAllocationOptimizationContext context) {
59 LIR ir = lirGenRes.getLIR();
60 Optimizer optimizer = new Optimizer(ir);
61
62 AbstractBlockBase<?>[] blockList = ir.linearScanOrder();
63 // ignore the first block in the list (index 0 is not processed)
64 for (int i = blockList.length - 1; i >= 1; i--) {
65 AbstractBlockBase<?> block = blockList[i];
66
67 if (block.getPredecessorCount() > 1) {
68 optimizer.optimizeMovesAtBlockEnd(block);
69 }
70 if (block.getSuccessorCount() == 2) {
71 optimizer.optimizeMovesAtBlockBegin(block);
72 }
73 }
74 }
75
76 private static final class Optimizer {
77 private final List<List<LIRInstruction>> edgeInstructionSeqences;
78 private LIR ir;
79
80 Optimizer(LIR ir) {
81 this.ir = ir;
82 edgeInstructionSeqences = new ArrayList<>(4);
83 }
84
85 /**
86 * Determines if two operations are both {@linkplain MoveOp moves} that have the same source
87 * and {@linkplain MoveOp#getResult() destination} operands.
88 *
89 * @param op1 the first instruction to compare
90 * @param op2 the second instruction to compare
91 * @return {@code true} if {@code op1} and {@code op2} are the same by the above algorithm
92 */
93 private static boolean same(LIRInstruction op1, LIRInstruction op2) {
94 assert op1 != null;
95 assert op2 != null;
96
97 if (op1 instanceof ValueMoveOp && op2 instanceof ValueMoveOp) {
98 ValueMoveOp move1 = (ValueMoveOp) op1;
99 ValueMoveOp move2 = (ValueMoveOp) op2;
100 if (move1.getInput().equals(move2.getInput()) && move1.getResult().equals(move2.getResult())) {
101 // these moves are exactly equal and can be optimized
102 return true;
103 }
104 } else if (op1 instanceof LoadConstantOp && op2 instanceof LoadConstantOp) {
105 LoadConstantOp move1 = (LoadConstantOp) op1;
106 LoadConstantOp move2 = (LoadConstantOp) op2;
107 if (move1.getConstant().equals(move2.getConstant()) && move1.getResult().equals(move2.getResult())) {
108 // these moves are exactly equal and can be optimized
109 return true;
110 }
111 }
112 return false;
113 }
114
115 /**
116 * Moves the longest {@linkplain #same common} subsequence at the end all predecessors of
117 * {@code block} to the start of {@code block}.
118 */
119 private void optimizeMovesAtBlockEnd(AbstractBlockBase<?> block) {
120 for (AbstractBlockBase<?> pred : block.getPredecessors()) {
121 if (pred == block) {
122 // currently we can't handle this correctly.
123 return;
124 }
125 }
126
127 // clear all internal data structures
128 edgeInstructionSeqences.clear();
129
130 int numPreds = block.getPredecessorCount();
131 assert numPreds > 1 : "do not call otherwise";
132
133 // setup a list with the LIR instructions of all predecessors
134 for (AbstractBlockBase<?> pred : block.getPredecessors()) {
135 assert pred != null;
136 assert ir.getLIRforBlock(pred) != null;
137 List<LIRInstruction> predInstructions = ir.getLIRforBlock(pred);
138
139 if (pred.getSuccessorCount() != 1) {
140 // this can happen with switch-statements where multiple edges are between
141 // the same blocks.
142 return;
143 }
144
145 assert pred.getSuccessors()[0] == block : "invalid control flow";
146 assert predInstructions.get(predInstructions.size() - 1) instanceof StandardOp.JumpOp : "block must end with unconditional jump";
147
148 if (predInstructions.get(predInstructions.size() - 1).hasState()) {
149 // can not optimize instructions that have debug info
150 return;
151 }
152
153 // ignore the unconditional branch at the end of the block
154 List<LIRInstruction> seq = predInstructions.subList(0, predInstructions.size() - 1);
155 edgeInstructionSeqences.add(seq);
156 }
157
158 // process lir-instructions while all predecessors end with the same instruction
159 while (true) {
160 List<LIRInstruction> seq = edgeInstructionSeqences.get(0);
161 if (seq.isEmpty()) {
162 return;
163 }
164
165 LIRInstruction op = last(seq);
166 for (int i = 1; i < numPreds; ++i) {
167 List<LIRInstruction> otherSeq = edgeInstructionSeqences.get(i);
168 if (otherSeq.isEmpty() || !same(op, last(otherSeq))) {
169 return;
170 }
171 }
172
173 // insert the instruction at the beginning of the current block
174 ir.getLIRforBlock(block).add(1, op);
175
176 // delete the instruction at the end of all predecessors
177 for (int i = 0; i < numPreds; i++) {
178 seq = edgeInstructionSeqences.get(i);
179 removeLast(seq);
180 }
181 }
182 }
183
184 /**
185 * Moves the longest {@linkplain #same common} subsequence at the start of all successors of
186 * {@code block} to the end of {@code block} just prior to the branch instruction ending
187 * {@code block}.
188 */
189 private void optimizeMovesAtBlockBegin(AbstractBlockBase<?> block) {
190
191 edgeInstructionSeqences.clear();
192 int numSux = block.getSuccessorCount();
193
194 List<LIRInstruction> instructions = ir.getLIRforBlock(block);
195
196 assert numSux == 2 : "method should not be called otherwise";
197
198 LIRInstruction lastInstruction = instructions.get(instructions.size() - 1);
199 if (lastInstruction.hasState()) {
200 // cannot optimize instructions when debug info is needed
201 return;
202 }
203
204 LIRInstruction branch = lastInstruction;
205 if (!(branch instanceof StandardOp.BranchOp) || branch.hasOperands()) {
206 // Only blocks that end with a conditional branch are optimized.
207 // In addition, a conditional branch with operands (including state) cannot
208 // be optimized. Moving a successor instruction before such a branch may
209 // interfere with the operands of the branch. For example, a successive move
210 // instruction may redefine an input operand of the branch.
211 return;
212 }
213
214 // Now it is guaranteed that the block ends with a conditional branch.
215 // The instructions are inserted at the end of the block before the branch.
216 int insertIdx = instructions.size() - 1;
217
218 // setup a list with the lir-instructions of all successors
219 for (AbstractBlockBase<?> sux : block.getSuccessors()) {
220 List<LIRInstruction> suxInstructions = ir.getLIRforBlock(sux);
221
222 assert suxInstructions.get(0) instanceof StandardOp.LabelOp : "block must start with label";
223
224 if (sux.getPredecessorCount() != 1) {
225 // this can happen with switch-statements where multiple edges are between
226 // the same blocks.
227 return;
228 }
229 assert sux.getPredecessors()[0] == block : "invalid control flow";
230
231 // ignore the label at the beginning of the block
232 List<LIRInstruction> seq = suxInstructions.subList(1, suxInstructions.size());
233 edgeInstructionSeqences.add(seq);
234 }
235
236 // process LIR instructions while all successors begin with the same instruction
237 while (true) {
238 List<LIRInstruction> seq = edgeInstructionSeqences.get(0);
239 if (seq.isEmpty()) {
240 return;
241 }
242
243 LIRInstruction op = first(seq);
244 for (int i = 1; i < numSux; i++) {
245 List<LIRInstruction> otherSeq = edgeInstructionSeqences.get(i);
246 if (otherSeq.isEmpty() || !same(op, first(otherSeq))) {
247 // these instructions are different and cannot be optimized .
248 // no further optimization possible
249 return;
250 }
251 }
252
253 // insert instruction at end of current block
254 ir.getLIRforBlock(block).add(insertIdx, op);
255 insertIdx++;
256
257 // delete the instructions at the beginning of all successors
258 for (int i = 0; i < numSux; i++) {
259 seq = edgeInstructionSeqences.get(i);
260 removeFirst(seq);
261 }
262 }
263 }
264
265 /**
266 * Gets the first element from a LIR instruction sequence.
267 */
268 private static LIRInstruction first(List<LIRInstruction> seq) {
269 return seq.get(0);
270 }
271
272 /**
273 * Gets the last element from a LIR instruction sequence.
274 */
275 private static LIRInstruction last(List<LIRInstruction> seq) {
276 return seq.get(seq.size() - 1);
277 }
278
279 /**
280 * Removes the first element from a LIR instruction sequence.
281 */
282 private static void removeFirst(List<LIRInstruction> seq) {
283 seq.remove(0);
284 }
285
286 /**
287 * Removes the last element from a LIR instruction sequence.
288 */
289 private static void removeLast(List<LIRInstruction> seq) {
290 seq.remove(seq.size() - 1);
291 }
292
293 }
294 }