1 /*
2 * Copyright © 2010 Luca Barbieri
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining a
5 * copy of this software and associated documentation files (the "Software"),
6 * to deal in the Software without restriction, including without limitation
7 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
8 * and/or sell copies of the Software, and to permit persons to whom the
9 * Software is furnished to do so, subject to the following conditions:
10 *
11 * The above copyright notice and this permission notice (including the next
12 * paragraph) shall be included in all copies or substantial portions of the
13 * Software.
14 *
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
18 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
20 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
21 * DEALINGS IN THE SOFTWARE.
22 */
23
24 /**
25 * \file lower_jumps.cpp
26 *
27 * This pass lowers jumps (break, continue, and return) to if/else structures.
28 *
29 * It can be asked to:
30 * 1. Pull jumps out of ifs where possible
31 * 2. Remove all "continue"s, replacing them with an "execute flag"
32 * 3. Replace all "break" with a single conditional one at the end of the loop
33 * 4. Replace all "return"s with a single return at the end of the function,
34 * for the main function and/or other functions
35 *
36 * Applying this pass gives several benefits:
37 * 1. All functions can be inlined.
38 * 2. nv40 and other pre-DX10 chips without "continue" can be supported
39 * 3. nv30 and other pre-DX10 chips with no control flow at all are better
40 * supported
41 *
42 * Continues are lowered by adding a per-loop "execute flag", initialized to
43 * true, that when cleared inhibits all execution until the end of the loop.
44 *
45 * Breaks are lowered to continues, plus setting a "break flag" that is checked
46 * at the end of the loop, and trigger the unique "break".
47 *
48 * Returns are lowered to breaks/continues, plus adding a "return flag" that
49 * causes loops to break again out of their enclosing loops until all the
50 * loops are exited: then the "execute flag" logic will ignore everything
51 * until the end of the function.
52 *
53 * Note that "continue" and "return" can also be implemented by adding
54 * a dummy loop and using break.
55 * However, this is bad for hardware with limited nesting depth, and
56 * prevents further optimization, and thus is not currently performed.
57 */
58
59 #include "compiler/glsl_types.h"
60 #include <string.h>
61 #include "ir.h"
62
63 /**
64 * Enum recording the result of analyzing how control flow might exit
65 * an IR node.
66 *
67 * Each possible value of jump_strength indicates a strictly stronger
68 * guarantee on control flow than the previous value.
69 *
70 * The ordering of strengths roughly reflects the way jumps are
71 * lowered: jumps with higher strength tend to be lowered to jumps of
72 * lower strength. Accordingly, strength is used as a heuristic to
73 * determine which lowering to perform first.
74 *
75 * This enum is also used by get_jump_strength() to categorize
76 * instructions as either break, continue, return, or other. When
77 * used in this fashion, strength_always_clears_execute_flag is not
78 * used.
79 *
80 * The control flow analysis made by this optimization pass makes two
81 * simplifying assumptions:
82 *
83 * - It ignores discard instructions, since they are lowered by a
84 * separate pass (lower_discard.cpp).
85 *
86 * - It assumes it is always possible for control to flow from a loop
87 * to the instruction immediately following it. Technically, this
88 * is not true (since all execution paths through the loop might
89 * jump back to the top, or return from the function).
90 *
91 * Both of these simplifying assumtions are safe, since they can never
92 * cause reachable code to be incorrectly classified as unreachable;
93 * they can only do the opposite.
94 */
95 enum jump_strength
96 {
97 /**
98 * Analysis has produced no guarantee on how control flow might
99 * exit this IR node. It might fall out the bottom (with or
100 * without clearing the execute flag, if present), or it might
101 * continue to the top of the innermost enclosing loop, break out
102 * of it, or return from the function.
103 */
104 strength_none,
105
106 /**
107 * The only way control can fall out the bottom of this node is
108 * through a code path that clears the execute flag. It might also
109 * continue to the top of the innermost enclosing loop, break out
110 * of it, or return from the function.
111 */
112 strength_always_clears_execute_flag,
113
114 /**
115 * Control cannot fall out the bottom of this node. It might
116 * continue to the top of the innermost enclosing loop, break out
117 * of it, or return from the function.
118 */
119 strength_continue,
120
121 /**
122 * Control cannot fall out the bottom of this node, or continue the
123 * top of the innermost enclosing loop. It can only break out of
124 * it or return from the function.
125 */
126 strength_break,
127
128 /**
129 * Control cannot fall out the bottom of this node, continue to the
130 * top of the innermost enclosing loop, or break out of it. It can
131 * only return from the function.
132 */
133 strength_return
134 };
135
136 namespace {
137
138 struct block_record
139 {
140 /* minimum jump strength (of lowered IR, not pre-lowering IR)
141 *
142 * If the block ends with a jump, must be the strength of the jump.
143 * Otherwise, the jump would be dead and have been deleted before)
144 *
145 * If the block doesn't end with a jump, it can be different than strength_none if all paths before it lead to some jump
146 * (e.g. an if with a return in one branch, and a break in the other, while not lowering them)
147 * Note that identical jumps are usually unified though.
148 */
149 jump_strength min_strength;
150
151 /* can anything clear the execute flag? */
152 bool may_clear_execute_flag;
153
block_record__anon14be2fd00111::block_record154 block_record()
155 {
156 this->min_strength = strength_none;
157 this->may_clear_execute_flag = false;
158 }
159 };
160
161 struct loop_record
162 {
163 ir_function_signature* signature;
164 ir_loop* loop;
165
166 /* used to avoid lowering the break used to represent lowered breaks */
167 unsigned nesting_depth;
168 bool in_if_at_the_end_of_the_loop;
169
170 bool may_set_return_flag;
171
172 ir_variable* break_flag;
173 ir_variable* execute_flag; /* cleared to emulate continue */
174
loop_record__anon14be2fd00111::loop_record175 loop_record(ir_function_signature* p_signature = 0, ir_loop* p_loop = 0)
176 {
177 this->signature = p_signature;
178 this->loop = p_loop;
179 this->nesting_depth = 0;
180 this->in_if_at_the_end_of_the_loop = false;
181 this->may_set_return_flag = false;
182 this->break_flag = 0;
183 this->execute_flag = 0;
184 }
185
get_execute_flag__anon14be2fd00111::loop_record186 ir_variable* get_execute_flag()
187 {
188 /* also supported for the "function loop" */
189 if(!this->execute_flag) {
190 exec_list& list = this->loop ? this->loop->body_instructions : signature->body;
191 this->execute_flag = new(this->signature) ir_variable(glsl_type::bool_type, "execute_flag", ir_var_temporary);
192 list.push_head(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(execute_flag), new(this->signature) ir_constant(true)));
193 list.push_head(this->execute_flag);
194 }
195 return this->execute_flag;
196 }
197
get_break_flag__anon14be2fd00111::loop_record198 ir_variable* get_break_flag()
199 {
200 assert(this->loop);
201 if(!this->break_flag) {
202 this->break_flag = new(this->signature) ir_variable(glsl_type::bool_type, "break_flag", ir_var_temporary);
203 this->loop->insert_before(this->break_flag);
204 this->loop->insert_before(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(break_flag), new(this->signature) ir_constant(false)));
205 }
206 return this->break_flag;
207 }
208 };
209
210 struct function_record
211 {
212 ir_function_signature* signature;
213 ir_variable* return_flag; /* used to break out of all loops and then jump to the return instruction */
214 ir_variable* return_value;
215 bool lower_return;
216 unsigned nesting_depth;
217
function_record__anon14be2fd00111::function_record218 function_record(ir_function_signature* p_signature = 0,
219 bool lower_return = false)
220 {
221 this->signature = p_signature;
222 this->return_flag = 0;
223 this->return_value = 0;
224 this->nesting_depth = 0;
225 this->lower_return = lower_return;
226 }
227
get_return_flag__anon14be2fd00111::function_record228 ir_variable* get_return_flag()
229 {
230 if(!this->return_flag) {
231 this->return_flag = new(this->signature) ir_variable(glsl_type::bool_type, "return_flag", ir_var_temporary);
232 this->signature->body.push_head(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(return_flag), new(this->signature) ir_constant(false)));
233 this->signature->body.push_head(this->return_flag);
234 }
235 return this->return_flag;
236 }
237
get_return_value__anon14be2fd00111::function_record238 ir_variable* get_return_value()
239 {
240 if(!this->return_value) {
241 assert(!this->signature->return_type->is_void());
242 return_value = new(this->signature) ir_variable(this->signature->return_type, "return_value", ir_var_temporary);
243 this->signature->body.push_head(this->return_value);
244 }
245 return this->return_value;
246 }
247 };
248
249 struct ir_lower_jumps_visitor : public ir_control_flow_visitor {
250 /* Postconditions: on exit of any visit() function:
251 *
252 * ANALYSIS: this->block.min_strength,
253 * this->block.may_clear_execute_flag, and
254 * this->loop.may_set_return_flag are updated to reflect the
255 * characteristics of the visited statement.
256 *
257 * DEAD_CODE_ELIMINATION: If this->block.min_strength is not
258 * strength_none, the visited node is at the end of its exec_list.
259 * In other words, any unreachable statements that follow the
260 * visited statement in its exec_list have been removed.
261 *
262 * CONTAINED_JUMPS_LOWERED: If the visited statement contains other
263 * statements, then should_lower_jump() is false for all of the
264 * return, break, or continue statements it contains.
265 *
266 * Note that visiting a jump does not lower it. That is the
267 * responsibility of the statement (or function signature) that
268 * contains the jump.
269 */
270
271 bool progress;
272
273 struct function_record function;
274 struct loop_record loop;
275 struct block_record block;
276
277 bool pull_out_jumps;
278 bool lower_continue;
279 bool lower_break;
280 bool lower_sub_return;
281 bool lower_main_return;
282
ir_lower_jumps_visitor__anon14be2fd00111::ir_lower_jumps_visitor283 ir_lower_jumps_visitor()
284 : progress(false),
285 pull_out_jumps(false),
286 lower_continue(false),
287 lower_break(false),
288 lower_sub_return(false),
289 lower_main_return(false)
290 {
291 }
292
truncate_after_instruction__anon14be2fd00111::ir_lower_jumps_visitor293 void truncate_after_instruction(exec_node *ir)
294 {
295 if (!ir)
296 return;
297
298 while (!ir->get_next()->is_tail_sentinel()) {
299 ((ir_instruction *)ir->get_next())->remove();
300 this->progress = true;
301 }
302 }
303
move_outer_block_inside__anon14be2fd00111::ir_lower_jumps_visitor304 void move_outer_block_inside(ir_instruction *ir, exec_list *inner_block)
305 {
306 while (!ir->get_next()->is_tail_sentinel()) {
307 ir_instruction *move_ir = (ir_instruction *)ir->get_next();
308
309 move_ir->remove();
310 inner_block->push_tail(move_ir);
311 }
312 }
313
314 /**
315 * Insert the instructions necessary to lower a return statement,
316 * before the given return instruction.
317 */
insert_lowered_return__anon14be2fd00111::ir_lower_jumps_visitor318 void insert_lowered_return(ir_return *ir)
319 {
320 ir_variable* return_flag = this->function.get_return_flag();
321 if(!this->function.signature->return_type->is_void()) {
322 ir_variable* return_value = this->function.get_return_value();
323 ir->insert_before(
324 new(ir) ir_assignment(
325 new (ir) ir_dereference_variable(return_value),
326 ir->value));
327 }
328 ir->insert_before(
329 new(ir) ir_assignment(
330 new (ir) ir_dereference_variable(return_flag),
331 new (ir) ir_constant(true)));
332 this->loop.may_set_return_flag = true;
333 }
334
335 /**
336 * If the given instruction is a return, lower it to instructions
337 * that store the return value (if there is one), set the return
338 * flag, and then break.
339 *
340 * It is safe to pass NULL to this function.
341 */
lower_return_unconditionally__anon14be2fd00111::ir_lower_jumps_visitor342 void lower_return_unconditionally(ir_instruction *ir)
343 {
344 if (get_jump_strength(ir) != strength_return) {
345 return;
346 }
347 insert_lowered_return((ir_return*)ir);
348 ir->replace_with(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
349 }
350
351 /**
352 * Create the necessary instruction to replace a break instruction.
353 */
create_lowered_break__anon14be2fd00111::ir_lower_jumps_visitor354 ir_instruction *create_lowered_break()
355 {
356 void *ctx = this->function.signature;
357 return new(ctx) ir_assignment(
358 new(ctx) ir_dereference_variable(this->loop.get_break_flag()),
359 new(ctx) ir_constant(true));
360 }
361
362 /**
363 * If the given instruction is a break, lower it to an instruction
364 * that sets the break flag, without consulting
365 * should_lower_jump().
366 *
367 * It is safe to pass NULL to this function.
368 */
lower_break_unconditionally__anon14be2fd00111::ir_lower_jumps_visitor369 void lower_break_unconditionally(ir_instruction *ir)
370 {
371 if (get_jump_strength(ir) != strength_break) {
372 return;
373 }
374 ir->replace_with(create_lowered_break());
375 }
376
377 /**
378 * If the block ends in a conditional or unconditional break, lower
379 * it, even though should_lower_jump() says it needn't be lowered.
380 */
lower_final_breaks__anon14be2fd00111::ir_lower_jumps_visitor381 void lower_final_breaks(exec_list *block)
382 {
383 ir_instruction *ir = (ir_instruction *) block->get_tail();
384 lower_break_unconditionally(ir);
385 ir_if *ir_if = ir->as_if();
386 if (ir_if) {
387 lower_break_unconditionally(
388 (ir_instruction *) ir_if->then_instructions.get_tail());
389 lower_break_unconditionally(
390 (ir_instruction *) ir_if->else_instructions.get_tail());
391 }
392 }
393
visit__anon14be2fd00111::ir_lower_jumps_visitor394 virtual void visit(class ir_loop_jump * ir)
395 {
396 /* Eliminate all instructions after each one, since they are
397 * unreachable. This satisfies the DEAD_CODE_ELIMINATION
398 * postcondition.
399 */
400 truncate_after_instruction(ir);
401
402 /* Set this->block.min_strength based on this instruction. This
403 * satisfies the ANALYSIS postcondition. It is not necessary to
404 * update this->block.may_clear_execute_flag or
405 * this->loop.may_set_return_flag, because an unlowered jump
406 * instruction can't change any flags.
407 */
408 this->block.min_strength = ir->is_break() ? strength_break : strength_continue;
409
410 /* The CONTAINED_JUMPS_LOWERED postcondition is already
411 * satisfied, because jump statements can't contain other
412 * statements.
413 */
414 }
415
visit__anon14be2fd00111::ir_lower_jumps_visitor416 virtual void visit(class ir_return * ir)
417 {
418 /* Eliminate all instructions after each one, since they are
419 * unreachable. This satisfies the DEAD_CODE_ELIMINATION
420 * postcondition.
421 */
422 truncate_after_instruction(ir);
423
424 /* Set this->block.min_strength based on this instruction. This
425 * satisfies the ANALYSIS postcondition. It is not necessary to
426 * update this->block.may_clear_execute_flag or
427 * this->loop.may_set_return_flag, because an unlowered return
428 * instruction can't change any flags.
429 */
430 this->block.min_strength = strength_return;
431
432 /* The CONTAINED_JUMPS_LOWERED postcondition is already
433 * satisfied, because jump statements can't contain other
434 * statements.
435 */
436 }
437
visit__anon14be2fd00111::ir_lower_jumps_visitor438 virtual void visit(class ir_discard * ir)
439 {
440 /* Nothing needs to be done. The ANALYSIS and
441 * DEAD_CODE_ELIMINATION postconditions are already satisfied,
442 * because discard statements are ignored by this optimization
443 * pass. The CONTAINED_JUMPS_LOWERED postcondition is already
444 * satisfied, because discard statements can't contain other
445 * statements.
446 */
447 (void) ir;
448 }
449
get_jump_strength__anon14be2fd00111::ir_lower_jumps_visitor450 enum jump_strength get_jump_strength(ir_instruction* ir)
451 {
452 if(!ir)
453 return strength_none;
454 else if(ir->ir_type == ir_type_loop_jump) {
455 if(((ir_loop_jump*)ir)->is_break())
456 return strength_break;
457 else
458 return strength_continue;
459 } else if(ir->ir_type == ir_type_return)
460 return strength_return;
461 else
462 return strength_none;
463 }
464
should_lower_jump__anon14be2fd00111::ir_lower_jumps_visitor465 bool should_lower_jump(ir_jump* ir)
466 {
467 unsigned strength = get_jump_strength(ir);
468 bool lower;
469 switch(strength)
470 {
471 case strength_none:
472 lower = false; /* don't change this, code relies on it */
473 break;
474 case strength_continue:
475 lower = lower_continue;
476 break;
477 case strength_break:
478 assert(this->loop.loop);
479 /* never lower "canonical break" */
480 if(ir->get_next()->is_tail_sentinel() && (this->loop.nesting_depth == 0
481 || (this->loop.nesting_depth == 1 && this->loop.in_if_at_the_end_of_the_loop)))
482 lower = false;
483 else
484 lower = lower_break;
485 break;
486 case strength_return:
487 /* never lower return at the end of a this->function */
488 if(this->function.nesting_depth == 0 && ir->get_next()->is_tail_sentinel())
489 lower = false;
490 else
491 lower = this->function.lower_return;
492 break;
493 }
494 return lower;
495 }
496
visit_block__anon14be2fd00111::ir_lower_jumps_visitor497 block_record visit_block(exec_list* list)
498 {
499 /* Note: since visiting a node may change that node's next
500 * pointer, we can't use visit_exec_list(), because
501 * visit_exec_list() caches the node's next pointer before
502 * visiting it. So we use foreach_in_list() instead.
503 *
504 * foreach_in_list() isn't safe if the node being visited gets
505 * removed, but fortunately this visitor doesn't do that.
506 */
507
508 block_record saved_block = this->block;
509 this->block = block_record();
510 foreach_in_list(ir_instruction, node, list) {
511 node->accept(this);
512 }
513 block_record ret = this->block;
514 this->block = saved_block;
515 return ret;
516 }
517
visit__anon14be2fd00111::ir_lower_jumps_visitor518 virtual void visit(ir_if *ir)
519 {
520 if(this->loop.nesting_depth == 0 && ir->get_next()->is_tail_sentinel())
521 this->loop.in_if_at_the_end_of_the_loop = true;
522
523 ++this->function.nesting_depth;
524 ++this->loop.nesting_depth;
525
526 block_record block_records[2];
527 ir_jump* jumps[2];
528
529 /* Recursively lower nested jumps. This satisfies the
530 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
531 * unconditional jumps at the end of ir->then_instructions and
532 * ir->else_instructions, which are handled below.
533 */
534 block_records[0] = visit_block(&ir->then_instructions);
535 block_records[1] = visit_block(&ir->else_instructions);
536
537 retry: /* we get here if we put code after the if inside a branch */
538
539 /* Determine which of ir->then_instructions and
540 * ir->else_instructions end with an unconditional jump.
541 */
542 for(unsigned i = 0; i < 2; ++i) {
543 exec_list& list = i ? ir->else_instructions : ir->then_instructions;
544 jumps[i] = 0;
545 if(!list.is_empty() && get_jump_strength((ir_instruction*)list.get_tail()))
546 jumps[i] = (ir_jump*)list.get_tail();
547 }
548
549 /* Loop until we have satisfied the CONTAINED_JUMPS_LOWERED
550 * postcondition by lowering jumps in both then_instructions and
551 * else_instructions.
552 */
553 for(;;) {
554 /* Determine the types of the jumps that terminate
555 * ir->then_instructions and ir->else_instructions.
556 */
557 jump_strength jump_strengths[2];
558
559 for(unsigned i = 0; i < 2; ++i) {
560 if(jumps[i]) {
561 jump_strengths[i] = block_records[i].min_strength;
562 assert(jump_strengths[i] == get_jump_strength(jumps[i]));
563 } else
564 jump_strengths[i] = strength_none;
565 }
566
567 /* If both code paths end in a jump, and the jumps are the
568 * same, and we are pulling out jumps, replace them with a
569 * single jump that comes after the if instruction. The new
570 * jump will be visited next, and it will be lowered if
571 * necessary by the loop or conditional that encloses it.
572 */
573 if(pull_out_jumps && jump_strengths[0] == jump_strengths[1]) {
574 bool unify = true;
575 if(jump_strengths[0] == strength_continue)
576 ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_continue));
577 else if(jump_strengths[0] == strength_break)
578 ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
579 /* FINISHME: unify returns with identical expressions */
580 else if(jump_strengths[0] == strength_return && this->function.signature->return_type->is_void())
581 ir->insert_after(new(ir) ir_return(NULL));
582 else
583 unify = false;
584
585 if(unify) {
586 jumps[0]->remove();
587 jumps[1]->remove();
588 this->progress = true;
589
590 /* Update jumps[] to reflect the fact that the jumps
591 * are gone, and update block_records[] to reflect the
592 * fact that control can now flow to the next
593 * instruction.
594 */
595 jumps[0] = 0;
596 jumps[1] = 0;
597 block_records[0].min_strength = strength_none;
598 block_records[1].min_strength = strength_none;
599
600 /* The CONTAINED_JUMPS_LOWERED postcondition is now
601 * satisfied, so we can break out of the loop.
602 */
603 break;
604 }
605 }
606
607 /* lower a jump: if both need to lowered, start with the strongest one, so that
608 * we might later unify the lowered version with the other one
609 */
610 bool should_lower[2];
611 for(unsigned i = 0; i < 2; ++i)
612 should_lower[i] = should_lower_jump(jumps[i]);
613
614 int lower;
615 if(should_lower[1] && should_lower[0])
616 lower = jump_strengths[1] > jump_strengths[0];
617 else if(should_lower[0])
618 lower = 0;
619 else if(should_lower[1])
620 lower = 1;
621 else
622 /* Neither code path ends in a jump that needs to be
623 * lowered, so the CONTAINED_JUMPS_LOWERED postcondition
624 * is satisfied and we can break out of the loop.
625 */
626 break;
627
628 if(jump_strengths[lower] == strength_return) {
629 /* To lower a return, we create a return flag (if the
630 * function doesn't have one already) and add instructions
631 * that: 1. store the return value (if this function has a
632 * non-void return) and 2. set the return flag
633 */
634 insert_lowered_return((ir_return*)jumps[lower]);
635 if(this->loop.loop) {
636 /* If we are in a loop, replace the return instruction
637 * with a break instruction, and then loop so that the
638 * break instruction can be lowered if necessary.
639 */
640 ir_loop_jump* lowered = 0;
641 lowered = new(ir) ir_loop_jump(ir_loop_jump::jump_break);
642 /* Note: we must update block_records and jumps to
643 * reflect the fact that the control path has been
644 * altered from a return to a break.
645 */
646 block_records[lower].min_strength = strength_break;
647 jumps[lower]->replace_with(lowered);
648 jumps[lower] = lowered;
649 } else {
650 /* If we are not in a loop, we then proceed as we would
651 * for a continue statement (set the execute flag to
652 * false to prevent the rest of the function from
653 * executing).
654 */
655 goto lower_continue;
656 }
657 this->progress = true;
658 } else if(jump_strengths[lower] == strength_break) {
659 /* To lower a break, we create a break flag (if the loop
660 * doesn't have one already) and add an instruction that
661 * sets it.
662 *
663 * Then we proceed as we would for a continue statement
664 * (set the execute flag to false to prevent the rest of
665 * the loop body from executing).
666 *
667 * The visit() function for the loop will ensure that the
668 * break flag is checked after executing the loop body.
669 */
670 jumps[lower]->insert_before(create_lowered_break());
671 goto lower_continue;
672 } else if(jump_strengths[lower] == strength_continue) {
673 lower_continue:
674 /* To lower a continue, we create an execute flag (if the
675 * loop doesn't have one already) and replace the continue
676 * with an instruction that clears it.
677 *
678 * Note that this code path gets exercised when lowering
679 * return statements that are not inside a loop, so
680 * this->loop must be initialized even outside of loops.
681 */
682 ir_variable* execute_flag = this->loop.get_execute_flag();
683 jumps[lower]->replace_with(new(ir) ir_assignment(new (ir) ir_dereference_variable(execute_flag), new (ir) ir_constant(false)));
684 /* Note: we must update block_records and jumps to reflect
685 * the fact that the control path has been altered to an
686 * instruction that clears the execute flag.
687 */
688 jumps[lower] = 0;
689 block_records[lower].min_strength = strength_always_clears_execute_flag;
690 block_records[lower].may_clear_execute_flag = true;
691 this->progress = true;
692
693 /* Let the loop run again, in case the other branch of the
694 * if needs to be lowered too.
695 */
696 }
697 }
698
699 /* move out a jump out if possible */
700 if(pull_out_jumps) {
701 /* If one of the branches ends in a jump, and control cannot
702 * fall out the bottom of the other branch, then we can move
703 * the jump after the if.
704 *
705 * Set move_out to the branch we are moving a jump out of.
706 */
707 int move_out = -1;
708 if(jumps[0] && block_records[1].min_strength >= strength_continue)
709 move_out = 0;
710 else if(jumps[1] && block_records[0].min_strength >= strength_continue)
711 move_out = 1;
712
713 if(move_out >= 0)
714 {
715 jumps[move_out]->remove();
716 ir->insert_after(jumps[move_out]);
717 /* Note: we must update block_records and jumps to reflect
718 * the fact that the jump has been moved out of the if.
719 */
720 jumps[move_out] = 0;
721 block_records[move_out].min_strength = strength_none;
722 this->progress = true;
723 }
724 }
725
726 /* Now satisfy the ANALYSIS postcondition by setting
727 * this->block.min_strength and
728 * this->block.may_clear_execute_flag based on the
729 * characteristics of the two branches.
730 */
731 if(block_records[0].min_strength < block_records[1].min_strength)
732 this->block.min_strength = block_records[0].min_strength;
733 else
734 this->block.min_strength = block_records[1].min_strength;
735 this->block.may_clear_execute_flag = this->block.may_clear_execute_flag || block_records[0].may_clear_execute_flag || block_records[1].may_clear_execute_flag;
736
737 /* Now we need to clean up the instructions that follow the
738 * if.
739 *
740 * If those instructions are unreachable, then satisfy the
741 * DEAD_CODE_ELIMINATION postcondition by eliminating them.
742 * Otherwise that postcondition is already satisfied.
743 */
744 if(this->block.min_strength)
745 truncate_after_instruction(ir);
746 else if(this->block.may_clear_execute_flag)
747 {
748 /* If the "if" instruction might clear the execute flag, then
749 * we need to guard any instructions that follow so that they
750 * are only executed if the execute flag is set.
751 *
752 * If one of the branches of the "if" always clears the
753 * execute flag, and the other branch never clears it, then
754 * this is easy: just move all the instructions following the
755 * "if" into the branch that never clears it.
756 */
757 int move_into = -1;
758 if(block_records[0].min_strength && !block_records[1].may_clear_execute_flag)
759 move_into = 1;
760 else if(block_records[1].min_strength && !block_records[0].may_clear_execute_flag)
761 move_into = 0;
762
763 if(move_into >= 0) {
764 assert(!block_records[move_into].min_strength && !block_records[move_into].may_clear_execute_flag); /* otherwise, we just truncated */
765
766 exec_list* list = move_into ? &ir->else_instructions : &ir->then_instructions;
767 exec_node* next = ir->get_next();
768 if(!next->is_tail_sentinel()) {
769 move_outer_block_inside(ir, list);
770
771 /* If any instructions moved, then we need to visit
772 * them (since they are now inside the "if"). Since
773 * block_records[move_into] is in its default state
774 * (see assertion above), we can safely replace
775 * block_records[move_into] with the result of this
776 * analysis.
777 */
778 exec_list list;
779 list.head_sentinel.next = next;
780 block_records[move_into] = visit_block(&list);
781
782 /*
783 * Then we need to re-start our jump lowering, since one
784 * of the instructions we moved might be a jump that
785 * needs to be lowered.
786 */
787 this->progress = true;
788 goto retry;
789 }
790 } else {
791 /* If we get here, then the simple case didn't apply; we
792 * need to actually guard the instructions that follow.
793 *
794 * To avoid creating unnecessarily-deep nesting, first
795 * look through the instructions that follow and unwrap
796 * any instructions that that are already wrapped in the
797 * appropriate guard.
798 */
799 ir_instruction* ir_after;
800 for(ir_after = (ir_instruction*)ir->get_next(); !ir_after->is_tail_sentinel();)
801 {
802 ir_if* ir_if = ir_after->as_if();
803 if(ir_if && ir_if->else_instructions.is_empty()) {
804 ir_dereference_variable* ir_if_cond_deref = ir_if->condition->as_dereference_variable();
805 if(ir_if_cond_deref && ir_if_cond_deref->var == this->loop.execute_flag) {
806 ir_instruction* ir_next = (ir_instruction*)ir_after->get_next();
807 ir_after->insert_before(&ir_if->then_instructions);
808 ir_after->remove();
809 ir_after = ir_next;
810 continue;
811 }
812 }
813 ir_after = (ir_instruction*)ir_after->get_next();
814
815 /* only set this if we find any unprotected instruction */
816 this->progress = true;
817 }
818
819 /* Then, wrap all the instructions that follow in a single
820 * guard.
821 */
822 if(!ir->get_next()->is_tail_sentinel()) {
823 assert(this->loop.execute_flag);
824 ir_if* if_execute = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.execute_flag));
825 move_outer_block_inside(ir, &if_execute->then_instructions);
826 ir->insert_after(if_execute);
827 }
828 }
829 }
830 --this->loop.nesting_depth;
831 --this->function.nesting_depth;
832 }
833
visit__anon14be2fd00111::ir_lower_jumps_visitor834 virtual void visit(ir_loop *ir)
835 {
836 /* Visit the body of the loop, with a fresh data structure in
837 * this->loop so that the analysis we do here won't bleed into
838 * enclosing loops.
839 *
840 * We assume that all code after a loop is reachable from the
841 * loop (see comments on enum jump_strength), so the
842 * DEAD_CODE_ELIMINATION postcondition is automatically
843 * satisfied, as is the block.min_strength portion of the
844 * ANALYSIS postcondition.
845 *
846 * The block.may_clear_execute_flag portion of the ANALYSIS
847 * postcondition is automatically satisfied because execute
848 * flags do not propagate outside of loops.
849 *
850 * The loop.may_set_return_flag portion of the ANALYSIS
851 * postcondition is handled below.
852 */
853 ++this->function.nesting_depth;
854 loop_record saved_loop = this->loop;
855 this->loop = loop_record(this->function.signature, ir);
856
857 /* Recursively lower nested jumps. This satisfies the
858 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
859 * an unconditional continue or return at the bottom of the
860 * loop, which are handled below.
861 */
862 block_record body = visit_block(&ir->body_instructions);
863
864 /* If the loop ends in an unconditional continue, eliminate it
865 * because it is redundant.
866 */
867 ir_instruction *ir_last
868 = (ir_instruction *) ir->body_instructions.get_tail();
869 if (get_jump_strength(ir_last) == strength_continue) {
870 ir_last->remove();
871 }
872
873 /* If the loop ends in an unconditional return, and we are
874 * lowering returns, lower it.
875 */
876 if (this->function.lower_return)
877 lower_return_unconditionally(ir_last);
878
879 if(body.min_strength >= strength_break) {
880 /* FINISHME: If the min_strength of the loop body is
881 * strength_break or strength_return, that means that it
882 * isn't a loop at all, since control flow always leaves the
883 * body of the loop via break or return. In principle the
884 * loop could be eliminated in this case. This optimization
885 * is not implemented yet.
886 */
887 }
888
889 if(this->loop.break_flag) {
890 /* We only get here if we are lowering breaks */
891 assert (lower_break);
892
893 /* If a break flag was generated while visiting the body of
894 * the loop, then at least one break was lowered, so we need
895 * to generate an if statement at the end of the loop that
896 * does a "break" if the break flag is set. The break we
897 * generate won't violate the CONTAINED_JUMPS_LOWERED
898 * postcondition, because should_lower_jump() always returns
899 * false for a break that happens at the end of a loop.
900 *
901 * However, if the loop already ends in a conditional or
902 * unconditional break, then we need to lower that break,
903 * because it won't be at the end of the loop anymore.
904 */
905 lower_final_breaks(&ir->body_instructions);
906
907 ir_if* break_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.break_flag));
908 break_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
909 ir->body_instructions.push_tail(break_if);
910 }
911
912 /* If the body of the loop may set the return flag, then at
913 * least one return was lowered to a break, so we need to ensure
914 * that the return flag is checked after the body of the loop is
915 * executed.
916 */
917 if(this->loop.may_set_return_flag) {
918 assert(this->function.return_flag);
919 /* Generate the if statement to check the return flag */
920 ir_if* return_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->function.return_flag));
921 /* Note: we also need to propagate the knowledge that the
922 * return flag may get set to the outer context. This
923 * satisfies the loop.may_set_return_flag part of the
924 * ANALYSIS postcondition.
925 */
926 saved_loop.may_set_return_flag = true;
927 if(saved_loop.loop)
928 /* If this loop is nested inside another one, then the if
929 * statement that we generated should break out of that
930 * loop if the return flag is set. Caller will lower that
931 * break statement if necessary.
932 */
933 return_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
934 else {
935 /* Otherwise, ensure that the instructions that follow are only
936 * executed if the return flag is clear. We can do that by moving
937 * those instructions into the else clause of the generated if
938 * statement.
939 */
940 move_outer_block_inside(ir, &return_if->else_instructions);
941
942 /* In case the loop is embedded inside an if add a new return to
943 * the return flag then branch and let a future pass tidy it up.
944 */
945 if (this->function.signature->return_type->is_void())
946 return_if->then_instructions.push_tail(new(ir) ir_return(NULL));
947 else {
948 assert(this->function.return_value);
949 ir_variable* return_value = this->function.return_value;
950 return_if->then_instructions.push_tail(
951 new(ir) ir_return(new(ir) ir_dereference_variable(return_value)));
952 }
953 }
954
955 ir->insert_after(return_if);
956 }
957
958 this->loop = saved_loop;
959 --this->function.nesting_depth;
960 }
961
visit__anon14be2fd00111::ir_lower_jumps_visitor962 virtual void visit(ir_function_signature *ir)
963 {
964 /* these are not strictly necessary */
965 assert(!this->function.signature);
966 assert(!this->loop.loop);
967
968 bool lower_return;
969 if (strcmp(ir->function_name(), "main") == 0)
970 lower_return = lower_main_return;
971 else
972 lower_return = lower_sub_return;
973
974 function_record saved_function = this->function;
975 loop_record saved_loop = this->loop;
976 this->function = function_record(ir, lower_return);
977 this->loop = loop_record(ir);
978
979 assert(!this->loop.loop);
980
981 /* Visit the body of the function to lower any jumps that occur
982 * in it, except possibly an unconditional return statement at
983 * the end of it.
984 */
985 visit_block(&ir->body);
986
987 /* If the body ended in an unconditional return of non-void,
988 * then we don't need to lower it because it's the one canonical
989 * return.
990 *
991 * If the body ended in a return of void, eliminate it because
992 * it is redundant.
993 */
994 if (ir->return_type->is_void() &&
995 get_jump_strength((ir_instruction *) ir->body.get_tail())) {
996 ir_jump *jump = (ir_jump *) ir->body.get_tail();
997 assert (jump->ir_type == ir_type_return);
998 jump->remove();
999 }
1000
1001 if(this->function.return_value)
1002 ir->body.push_tail(new(ir) ir_return(new (ir) ir_dereference_variable(this->function.return_value)));
1003
1004 this->loop = saved_loop;
1005 this->function = saved_function;
1006 }
1007
visit__anon14be2fd00111::ir_lower_jumps_visitor1008 virtual void visit(class ir_function * ir)
1009 {
1010 visit_block(&ir->signatures);
1011 }
1012 };
1013
1014 } /* anonymous namespace */
1015
1016 bool
do_lower_jumps(exec_list * instructions,bool pull_out_jumps,bool lower_sub_return,bool lower_main_return,bool lower_continue,bool lower_break)1017 do_lower_jumps(exec_list *instructions, bool pull_out_jumps, bool lower_sub_return, bool lower_main_return, bool lower_continue, bool lower_break)
1018 {
1019 ir_lower_jumps_visitor v;
1020 v.pull_out_jumps = pull_out_jumps;
1021 v.lower_continue = lower_continue;
1022 v.lower_break = lower_break;
1023 v.lower_sub_return = lower_sub_return;
1024 v.lower_main_return = lower_main_return;
1025
1026 bool progress_ever = false;
1027 do {
1028 v.progress = false;
1029 visit_exec_list(instructions, &v);
1030 progress_ever = v.progress || progress_ever;
1031 } while (v.progress);
1032
1033 return progress_ever;
1034 }
1035