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__anon5e1735520111::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__anon5e1735520111::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__anon5e1735520111::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__anon5e1735520111::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__anon5e1735520111::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__anon5e1735520111::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__anon5e1735520111::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 using ir_control_flow_visitor::visit;
272
273 bool progress;
274
275 struct function_record function;
276 struct loop_record loop;
277 struct block_record block;
278
279 bool pull_out_jumps;
280 bool lower_continue;
281 bool lower_break;
282 bool lower_sub_return;
283 bool lower_main_return;
284
ir_lower_jumps_visitor__anon5e1735520111::ir_lower_jumps_visitor285 ir_lower_jumps_visitor()
286 : progress(false),
287 pull_out_jumps(false),
288 lower_continue(false),
289 lower_break(false),
290 lower_sub_return(false),
291 lower_main_return(false)
292 {
293 }
294
truncate_after_instruction__anon5e1735520111::ir_lower_jumps_visitor295 void truncate_after_instruction(exec_node *ir)
296 {
297 if (!ir)
298 return;
299
300 while (!ir->get_next()->is_tail_sentinel()) {
301 ((ir_instruction *)ir->get_next())->remove();
302 this->progress = true;
303 }
304 }
305
move_outer_block_inside__anon5e1735520111::ir_lower_jumps_visitor306 void move_outer_block_inside(ir_instruction *ir, exec_list *inner_block)
307 {
308 while (!ir->get_next()->is_tail_sentinel()) {
309 ir_instruction *move_ir = (ir_instruction *)ir->get_next();
310
311 move_ir->remove();
312 inner_block->push_tail(move_ir);
313 }
314 }
315
316 /**
317 * Insert the instructions necessary to lower a return statement,
318 * before the given return instruction.
319 */
insert_lowered_return__anon5e1735520111::ir_lower_jumps_visitor320 void insert_lowered_return(ir_return *ir)
321 {
322 ir_variable* return_flag = this->function.get_return_flag();
323 if(!this->function.signature->return_type->is_void()) {
324 ir_variable* return_value = this->function.get_return_value();
325 ir->insert_before(
326 new(ir) ir_assignment(
327 new (ir) ir_dereference_variable(return_value),
328 ir->value));
329 }
330 ir->insert_before(
331 new(ir) ir_assignment(
332 new (ir) ir_dereference_variable(return_flag),
333 new (ir) ir_constant(true)));
334 this->loop.may_set_return_flag = true;
335 }
336
337 /**
338 * If the given instruction is a return, lower it to instructions
339 * that store the return value (if there is one), set the return
340 * flag, and then break.
341 *
342 * It is safe to pass NULL to this function.
343 */
lower_return_unconditionally__anon5e1735520111::ir_lower_jumps_visitor344 void lower_return_unconditionally(ir_instruction *ir)
345 {
346 if (get_jump_strength(ir) != strength_return) {
347 return;
348 }
349 insert_lowered_return((ir_return*)ir);
350 ir->replace_with(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
351 }
352
353 /**
354 * Create the necessary instruction to replace a break instruction.
355 */
create_lowered_break__anon5e1735520111::ir_lower_jumps_visitor356 ir_instruction *create_lowered_break()
357 {
358 void *ctx = this->function.signature;
359 return new(ctx) ir_assignment(
360 new(ctx) ir_dereference_variable(this->loop.get_break_flag()),
361 new(ctx) ir_constant(true));
362 }
363
364 /**
365 * If the given instruction is a break, lower it to an instruction
366 * that sets the break flag, without consulting
367 * should_lower_jump().
368 *
369 * It is safe to pass NULL to this function.
370 */
lower_break_unconditionally__anon5e1735520111::ir_lower_jumps_visitor371 void lower_break_unconditionally(ir_instruction *ir)
372 {
373 if (get_jump_strength(ir) != strength_break) {
374 return;
375 }
376 ir->replace_with(create_lowered_break());
377 }
378
379 /**
380 * If the block ends in a conditional or unconditional break, lower
381 * it, even though should_lower_jump() says it needn't be lowered.
382 */
lower_final_breaks__anon5e1735520111::ir_lower_jumps_visitor383 void lower_final_breaks(exec_list *block)
384 {
385 ir_instruction *ir = (ir_instruction *) block->get_tail();
386 lower_break_unconditionally(ir);
387 ir_if *ir_if = ir->as_if();
388 if (ir_if) {
389 lower_break_unconditionally(
390 (ir_instruction *) ir_if->then_instructions.get_tail());
391 lower_break_unconditionally(
392 (ir_instruction *) ir_if->else_instructions.get_tail());
393 }
394 }
395
visit__anon5e1735520111::ir_lower_jumps_visitor396 virtual void visit(class ir_loop_jump * ir)
397 {
398 /* Eliminate all instructions after each one, since they are
399 * unreachable. This satisfies the DEAD_CODE_ELIMINATION
400 * postcondition.
401 */
402 truncate_after_instruction(ir);
403
404 /* Set this->block.min_strength based on this instruction. This
405 * satisfies the ANALYSIS postcondition. It is not necessary to
406 * update this->block.may_clear_execute_flag or
407 * this->loop.may_set_return_flag, because an unlowered jump
408 * instruction can't change any flags.
409 */
410 this->block.min_strength = ir->is_break() ? strength_break : strength_continue;
411
412 /* The CONTAINED_JUMPS_LOWERED postcondition is already
413 * satisfied, because jump statements can't contain other
414 * statements.
415 */
416 }
417
visit__anon5e1735520111::ir_lower_jumps_visitor418 virtual void visit(class ir_return * ir)
419 {
420 /* Eliminate all instructions after each one, since they are
421 * unreachable. This satisfies the DEAD_CODE_ELIMINATION
422 * postcondition.
423 */
424 truncate_after_instruction(ir);
425
426 /* Set this->block.min_strength based on this instruction. This
427 * satisfies the ANALYSIS postcondition. It is not necessary to
428 * update this->block.may_clear_execute_flag or
429 * this->loop.may_set_return_flag, because an unlowered return
430 * instruction can't change any flags.
431 */
432 this->block.min_strength = strength_return;
433
434 /* The CONTAINED_JUMPS_LOWERED postcondition is already
435 * satisfied, because jump statements can't contain other
436 * statements.
437 */
438 }
439
visit__anon5e1735520111::ir_lower_jumps_visitor440 virtual void visit(class ir_discard * ir)
441 {
442 /* Nothing needs to be done. The ANALYSIS and
443 * DEAD_CODE_ELIMINATION postconditions are already satisfied,
444 * because discard statements are ignored by this optimization
445 * pass. The CONTAINED_JUMPS_LOWERED postcondition is already
446 * satisfied, because discard statements can't contain other
447 * statements.
448 */
449 (void) ir;
450 }
451
get_jump_strength__anon5e1735520111::ir_lower_jumps_visitor452 enum jump_strength get_jump_strength(ir_instruction* ir)
453 {
454 if(!ir)
455 return strength_none;
456 else if(ir->ir_type == ir_type_loop_jump) {
457 if(((ir_loop_jump*)ir)->is_break())
458 return strength_break;
459 else
460 return strength_continue;
461 } else if(ir->ir_type == ir_type_return)
462 return strength_return;
463 else
464 return strength_none;
465 }
466
should_lower_jump__anon5e1735520111::ir_lower_jumps_visitor467 bool should_lower_jump(ir_jump* ir)
468 {
469 unsigned strength = get_jump_strength(ir);
470 bool lower;
471 switch(strength)
472 {
473 case strength_none:
474 lower = false; /* don't change this, code relies on it */
475 break;
476 case strength_continue:
477 lower = lower_continue;
478 break;
479 case strength_break:
480 assert(this->loop.loop);
481 /* never lower "canonical break" */
482 if(ir->get_next()->is_tail_sentinel() && (this->loop.nesting_depth == 0
483 || (this->loop.nesting_depth == 1 && this->loop.in_if_at_the_end_of_the_loop)))
484 lower = false;
485 else
486 lower = lower_break;
487 break;
488 case strength_return:
489 /* never lower return at the end of a this->function */
490 if(this->function.nesting_depth == 0 && ir->get_next()->is_tail_sentinel())
491 lower = false;
492 else
493 lower = this->function.lower_return;
494 break;
495 }
496 return lower;
497 }
498
visit_block__anon5e1735520111::ir_lower_jumps_visitor499 block_record visit_block(exec_list* list)
500 {
501 /* Note: since visiting a node may change that node's next
502 * pointer, we can't use visit_exec_list(), because
503 * visit_exec_list() caches the node's next pointer before
504 * visiting it. So we use foreach_in_list() instead.
505 *
506 * foreach_in_list() isn't safe if the node being visited gets
507 * removed, but fortunately this visitor doesn't do that.
508 */
509
510 block_record saved_block = this->block;
511 this->block = block_record();
512 foreach_in_list(ir_instruction, node, list) {
513 node->accept(this);
514 }
515 block_record ret = this->block;
516 this->block = saved_block;
517 return ret;
518 }
519
visit__anon5e1735520111::ir_lower_jumps_visitor520 virtual void visit(ir_if *ir)
521 {
522 if(this->loop.nesting_depth == 0 && ir->get_next()->is_tail_sentinel())
523 this->loop.in_if_at_the_end_of_the_loop = true;
524
525 ++this->function.nesting_depth;
526 ++this->loop.nesting_depth;
527
528 block_record block_records[2];
529 ir_jump* jumps[2];
530
531 /* Recursively lower nested jumps. This satisfies the
532 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
533 * unconditional jumps at the end of ir->then_instructions and
534 * ir->else_instructions, which are handled below.
535 */
536 block_records[0] = visit_block(&ir->then_instructions);
537 block_records[1] = visit_block(&ir->else_instructions);
538
539 retry: /* we get here if we put code after the if inside a branch */
540
541 /* Determine which of ir->then_instructions and
542 * ir->else_instructions end with an unconditional jump.
543 */
544 for(unsigned i = 0; i < 2; ++i) {
545 exec_list& list = i ? ir->else_instructions : ir->then_instructions;
546 jumps[i] = 0;
547 if(!list.is_empty() && get_jump_strength((ir_instruction*)list.get_tail()))
548 jumps[i] = (ir_jump*)list.get_tail();
549 }
550
551 /* Loop until we have satisfied the CONTAINED_JUMPS_LOWERED
552 * postcondition by lowering jumps in both then_instructions and
553 * else_instructions.
554 */
555 for(;;) {
556 /* Determine the types of the jumps that terminate
557 * ir->then_instructions and ir->else_instructions.
558 */
559 jump_strength jump_strengths[2];
560
561 for(unsigned i = 0; i < 2; ++i) {
562 if(jumps[i]) {
563 jump_strengths[i] = block_records[i].min_strength;
564 assert(jump_strengths[i] == get_jump_strength(jumps[i]));
565 } else
566 jump_strengths[i] = strength_none;
567 }
568
569 /* If both code paths end in a jump, and the jumps are the
570 * same, and we are pulling out jumps, replace them with a
571 * single jump that comes after the if instruction. The new
572 * jump will be visited next, and it will be lowered if
573 * necessary by the loop or conditional that encloses it.
574 */
575 if(pull_out_jumps && jump_strengths[0] == jump_strengths[1]) {
576 bool unify = true;
577 if(jump_strengths[0] == strength_continue)
578 ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_continue));
579 else if(jump_strengths[0] == strength_break)
580 ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
581 /* FINISHME: unify returns with identical expressions */
582 else if(jump_strengths[0] == strength_return && this->function.signature->return_type->is_void())
583 ir->insert_after(new(ir) ir_return(NULL));
584 else
585 unify = false;
586
587 if(unify) {
588 jumps[0]->remove();
589 jumps[1]->remove();
590 this->progress = true;
591
592 /* Update jumps[] to reflect the fact that the jumps
593 * are gone, and update block_records[] to reflect the
594 * fact that control can now flow to the next
595 * instruction.
596 */
597 jumps[0] = 0;
598 jumps[1] = 0;
599 block_records[0].min_strength = strength_none;
600 block_records[1].min_strength = strength_none;
601
602 /* The CONTAINED_JUMPS_LOWERED postcondition is now
603 * satisfied, so we can break out of the loop.
604 */
605 break;
606 }
607 }
608
609 /* lower a jump: if both need to lowered, start with the strongest one, so that
610 * we might later unify the lowered version with the other one
611 */
612 bool should_lower[2];
613 for(unsigned i = 0; i < 2; ++i)
614 should_lower[i] = should_lower_jump(jumps[i]);
615
616 int lower;
617 if(should_lower[1] && should_lower[0])
618 lower = jump_strengths[1] > jump_strengths[0];
619 else if(should_lower[0])
620 lower = 0;
621 else if(should_lower[1])
622 lower = 1;
623 else
624 /* Neither code path ends in a jump that needs to be
625 * lowered, so the CONTAINED_JUMPS_LOWERED postcondition
626 * is satisfied and we can break out of the loop.
627 */
628 break;
629
630 if(jump_strengths[lower] == strength_return) {
631 /* To lower a return, we create a return flag (if the
632 * function doesn't have one already) and add instructions
633 * that: 1. store the return value (if this function has a
634 * non-void return) and 2. set the return flag
635 */
636 insert_lowered_return((ir_return*)jumps[lower]);
637 if(this->loop.loop) {
638 /* If we are in a loop, replace the return instruction
639 * with a break instruction, and then loop so that the
640 * break instruction can be lowered if necessary.
641 */
642 ir_loop_jump* lowered = 0;
643 lowered = new(ir) ir_loop_jump(ir_loop_jump::jump_break);
644 /* Note: we must update block_records and jumps to
645 * reflect the fact that the control path has been
646 * altered from a return to a break.
647 */
648 block_records[lower].min_strength = strength_break;
649 jumps[lower]->replace_with(lowered);
650 jumps[lower] = lowered;
651 } else {
652 /* If we are not in a loop, we then proceed as we would
653 * for a continue statement (set the execute flag to
654 * false to prevent the rest of the function from
655 * executing).
656 */
657 goto lower_continue;
658 }
659 this->progress = true;
660 } else if(jump_strengths[lower] == strength_break) {
661 /* To lower a break, we create a break flag (if the loop
662 * doesn't have one already) and add an instruction that
663 * sets it.
664 *
665 * Then we proceed as we would for a continue statement
666 * (set the execute flag to false to prevent the rest of
667 * the loop body from executing).
668 *
669 * The visit() function for the loop will ensure that the
670 * break flag is checked after executing the loop body.
671 */
672 jumps[lower]->insert_before(create_lowered_break());
673 goto lower_continue;
674 } else if(jump_strengths[lower] == strength_continue) {
675 lower_continue:
676 /* To lower a continue, we create an execute flag (if the
677 * loop doesn't have one already) and replace the continue
678 * with an instruction that clears it.
679 *
680 * Note that this code path gets exercised when lowering
681 * return statements that are not inside a loop, so
682 * this->loop must be initialized even outside of loops.
683 */
684 ir_variable* execute_flag = this->loop.get_execute_flag();
685 jumps[lower]->replace_with(new(ir) ir_assignment(new (ir) ir_dereference_variable(execute_flag), new (ir) ir_constant(false)));
686 /* Note: we must update block_records and jumps to reflect
687 * the fact that the control path has been altered to an
688 * instruction that clears the execute flag.
689 */
690 jumps[lower] = 0;
691 block_records[lower].min_strength = strength_always_clears_execute_flag;
692 block_records[lower].may_clear_execute_flag = true;
693 this->progress = true;
694
695 /* Let the loop run again, in case the other branch of the
696 * if needs to be lowered too.
697 */
698 }
699 }
700
701 /* move out a jump out if possible */
702 if(pull_out_jumps) {
703 /* If one of the branches ends in a jump, and control cannot
704 * fall out the bottom of the other branch, then we can move
705 * the jump after the if.
706 *
707 * Set move_out to the branch we are moving a jump out of.
708 */
709 int move_out = -1;
710 if(jumps[0] && block_records[1].min_strength >= strength_continue)
711 move_out = 0;
712 else if(jumps[1] && block_records[0].min_strength >= strength_continue)
713 move_out = 1;
714
715 if(move_out >= 0)
716 {
717 jumps[move_out]->remove();
718 ir->insert_after(jumps[move_out]);
719 /* Note: we must update block_records and jumps to reflect
720 * the fact that the jump has been moved out of the if.
721 */
722 jumps[move_out] = 0;
723 block_records[move_out].min_strength = strength_none;
724 this->progress = true;
725 }
726 }
727
728 /* Now satisfy the ANALYSIS postcondition by setting
729 * this->block.min_strength and
730 * this->block.may_clear_execute_flag based on the
731 * characteristics of the two branches.
732 */
733 if(block_records[0].min_strength < block_records[1].min_strength)
734 this->block.min_strength = block_records[0].min_strength;
735 else
736 this->block.min_strength = block_records[1].min_strength;
737 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;
738
739 /* Now we need to clean up the instructions that follow the
740 * if.
741 *
742 * If those instructions are unreachable, then satisfy the
743 * DEAD_CODE_ELIMINATION postcondition by eliminating them.
744 * Otherwise that postcondition is already satisfied.
745 */
746 if(this->block.min_strength)
747 truncate_after_instruction(ir);
748 else if(this->block.may_clear_execute_flag)
749 {
750 /* If the "if" instruction might clear the execute flag, then
751 * we need to guard any instructions that follow so that they
752 * are only executed if the execute flag is set.
753 *
754 * If one of the branches of the "if" always clears the
755 * execute flag, and the other branch never clears it, then
756 * this is easy: just move all the instructions following the
757 * "if" into the branch that never clears it.
758 */
759 int move_into = -1;
760 if(block_records[0].min_strength && !block_records[1].may_clear_execute_flag)
761 move_into = 1;
762 else if(block_records[1].min_strength && !block_records[0].may_clear_execute_flag)
763 move_into = 0;
764
765 if(move_into >= 0) {
766 assert(!block_records[move_into].min_strength && !block_records[move_into].may_clear_execute_flag); /* otherwise, we just truncated */
767
768 exec_list* list = move_into ? &ir->else_instructions : &ir->then_instructions;
769 exec_node* next = ir->get_next();
770 if(!next->is_tail_sentinel()) {
771 move_outer_block_inside(ir, list);
772
773 /* If any instructions moved, then we need to visit
774 * them (since they are now inside the "if"). Since
775 * block_records[move_into] is in its default state
776 * (see assertion above), we can safely replace
777 * block_records[move_into] with the result of this
778 * analysis.
779 */
780 exec_list list;
781 list.head_sentinel.next = next;
782 block_records[move_into] = visit_block(&list);
783
784 /*
785 * Then we need to re-start our jump lowering, since one
786 * of the instructions we moved might be a jump that
787 * needs to be lowered.
788 */
789 this->progress = true;
790 goto retry;
791 }
792 } else {
793 /* If we get here, then the simple case didn't apply; we
794 * need to actually guard the instructions that follow.
795 *
796 * To avoid creating unnecessarily-deep nesting, first
797 * look through the instructions that follow and unwrap
798 * any instructions that that are already wrapped in the
799 * appropriate guard.
800 */
801 ir_instruction* ir_after;
802 for(ir_after = (ir_instruction*)ir->get_next(); !ir_after->is_tail_sentinel();)
803 {
804 ir_if* ir_if = ir_after->as_if();
805 if(ir_if && ir_if->else_instructions.is_empty()) {
806 ir_dereference_variable* ir_if_cond_deref = ir_if->condition->as_dereference_variable();
807 if(ir_if_cond_deref && ir_if_cond_deref->var == this->loop.execute_flag) {
808 ir_instruction* ir_next = (ir_instruction*)ir_after->get_next();
809 ir_after->insert_before(&ir_if->then_instructions);
810 ir_after->remove();
811 ir_after = ir_next;
812 continue;
813 }
814 }
815 ir_after = (ir_instruction*)ir_after->get_next();
816
817 /* only set this if we find any unprotected instruction */
818 this->progress = true;
819 }
820
821 /* Then, wrap all the instructions that follow in a single
822 * guard.
823 */
824 if(!ir->get_next()->is_tail_sentinel()) {
825 assert(this->loop.execute_flag);
826 ir_if* if_execute = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.execute_flag));
827 move_outer_block_inside(ir, &if_execute->then_instructions);
828 ir->insert_after(if_execute);
829 }
830 }
831 }
832 --this->loop.nesting_depth;
833 --this->function.nesting_depth;
834 }
835
visit__anon5e1735520111::ir_lower_jumps_visitor836 virtual void visit(ir_loop *ir)
837 {
838 /* Visit the body of the loop, with a fresh data structure in
839 * this->loop so that the analysis we do here won't bleed into
840 * enclosing loops.
841 *
842 * We assume that all code after a loop is reachable from the
843 * loop (see comments on enum jump_strength), so the
844 * DEAD_CODE_ELIMINATION postcondition is automatically
845 * satisfied, as is the block.min_strength portion of the
846 * ANALYSIS postcondition.
847 *
848 * The block.may_clear_execute_flag portion of the ANALYSIS
849 * postcondition is automatically satisfied because execute
850 * flags do not propagate outside of loops.
851 *
852 * The loop.may_set_return_flag portion of the ANALYSIS
853 * postcondition is handled below.
854 */
855 ++this->function.nesting_depth;
856 loop_record saved_loop = this->loop;
857 this->loop = loop_record(this->function.signature, ir);
858
859 /* Recursively lower nested jumps. This satisfies the
860 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
861 * an unconditional continue or return at the bottom of the
862 * loop, which are handled below.
863 */
864 block_record body = visit_block(&ir->body_instructions);
865
866 /* If the loop ends in an unconditional continue, eliminate it
867 * because it is redundant.
868 */
869 ir_instruction *ir_last
870 = (ir_instruction *) ir->body_instructions.get_tail();
871 if (get_jump_strength(ir_last) == strength_continue) {
872 ir_last->remove();
873 }
874
875 /* If the loop ends in an unconditional return, and we are
876 * lowering returns, lower it.
877 */
878 if (this->function.lower_return)
879 lower_return_unconditionally(ir_last);
880
881 if(body.min_strength >= strength_break) {
882 /* FINISHME: If the min_strength of the loop body is
883 * strength_break or strength_return, that means that it
884 * isn't a loop at all, since control flow always leaves the
885 * body of the loop via break or return. In principle the
886 * loop could be eliminated in this case. This optimization
887 * is not implemented yet.
888 */
889 }
890
891 if(this->loop.break_flag) {
892 /* We only get here if we are lowering breaks */
893 assert (lower_break);
894
895 /* If a break flag was generated while visiting the body of
896 * the loop, then at least one break was lowered, so we need
897 * to generate an if statement at the end of the loop that
898 * does a "break" if the break flag is set. The break we
899 * generate won't violate the CONTAINED_JUMPS_LOWERED
900 * postcondition, because should_lower_jump() always returns
901 * false for a break that happens at the end of a loop.
902 *
903 * However, if the loop already ends in a conditional or
904 * unconditional break, then we need to lower that break,
905 * because it won't be at the end of the loop anymore.
906 */
907 lower_final_breaks(&ir->body_instructions);
908
909 ir_if* break_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.break_flag));
910 break_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
911 ir->body_instructions.push_tail(break_if);
912 }
913
914 /* If the body of the loop may set the return flag, then at
915 * least one return was lowered to a break, so we need to ensure
916 * that the return flag is checked after the body of the loop is
917 * executed.
918 */
919 if(this->loop.may_set_return_flag) {
920 assert(this->function.return_flag);
921 /* Generate the if statement to check the return flag */
922 ir_if* return_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->function.return_flag));
923 /* Note: we also need to propagate the knowledge that the
924 * return flag may get set to the outer context. This
925 * satisfies the loop.may_set_return_flag part of the
926 * ANALYSIS postcondition.
927 */
928 saved_loop.may_set_return_flag = true;
929 if(saved_loop.loop)
930 /* If this loop is nested inside another one, then the if
931 * statement that we generated should break out of that
932 * loop if the return flag is set. Caller will lower that
933 * break statement if necessary.
934 */
935 return_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
936 else {
937 /* Otherwise, ensure that the instructions that follow are only
938 * executed if the return flag is clear. We can do that by moving
939 * those instructions into the else clause of the generated if
940 * statement.
941 */
942 move_outer_block_inside(ir, &return_if->else_instructions);
943
944 /* In case the loop is embedded inside an if add a new return to
945 * the return flag then branch and let a future pass tidy it up.
946 */
947 if (this->function.signature->return_type->is_void())
948 return_if->then_instructions.push_tail(new(ir) ir_return(NULL));
949 else {
950 assert(this->function.return_value);
951 ir_variable* return_value = this->function.return_value;
952 return_if->then_instructions.push_tail(
953 new(ir) ir_return(new(ir) ir_dereference_variable(return_value)));
954 }
955 }
956
957 ir->insert_after(return_if);
958 }
959
960 this->loop = saved_loop;
961 --this->function.nesting_depth;
962 }
963
visit__anon5e1735520111::ir_lower_jumps_visitor964 virtual void visit(ir_function_signature *ir)
965 {
966 /* these are not strictly necessary */
967 assert(!this->function.signature);
968 assert(!this->loop.loop);
969
970 bool lower_return;
971 if (strcmp(ir->function_name(), "main") == 0)
972 lower_return = lower_main_return;
973 else
974 lower_return = lower_sub_return;
975
976 function_record saved_function = this->function;
977 loop_record saved_loop = this->loop;
978 this->function = function_record(ir, lower_return);
979 this->loop = loop_record(ir);
980
981 assert(!this->loop.loop);
982
983 /* Visit the body of the function to lower any jumps that occur
984 * in it, except possibly an unconditional return statement at
985 * the end of it.
986 */
987 visit_block(&ir->body);
988
989 /* If the body ended in an unconditional return of non-void,
990 * then we don't need to lower it because it's the one canonical
991 * return.
992 *
993 * If the body ended in a return of void, eliminate it because
994 * it is redundant.
995 */
996 if (ir->return_type->is_void() &&
997 get_jump_strength((ir_instruction *) ir->body.get_tail())) {
998 ir_jump *jump = (ir_jump *) ir->body.get_tail();
999 assert (jump->ir_type == ir_type_return);
1000 jump->remove();
1001 }
1002
1003 if(this->function.return_value)
1004 ir->body.push_tail(new(ir) ir_return(new (ir) ir_dereference_variable(this->function.return_value)));
1005
1006 this->loop = saved_loop;
1007 this->function = saved_function;
1008 }
1009
visit__anon5e1735520111::ir_lower_jumps_visitor1010 virtual void visit(class ir_function * ir)
1011 {
1012 visit_block(&ir->signatures);
1013 }
1014 };
1015
1016 } /* anonymous namespace */
1017
1018 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)1019 do_lower_jumps(exec_list *instructions, bool pull_out_jumps, bool lower_sub_return, bool lower_main_return, bool lower_continue, bool lower_break)
1020 {
1021 ir_lower_jumps_visitor v;
1022 v.pull_out_jumps = pull_out_jumps;
1023 v.lower_continue = lower_continue;
1024 v.lower_break = lower_break;
1025 v.lower_sub_return = lower_sub_return;
1026 v.lower_main_return = lower_main_return;
1027
1028 bool progress_ever = false;
1029 do {
1030 v.progress = false;
1031 visit_exec_list(instructions, &v);
1032 progress_ever = v.progress || progress_ever;
1033 } while (v.progress);
1034
1035 return progress_ever;
1036 }
1037