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__anon250c63460111::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__anon250c63460111::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__anon250c63460111::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), 0));
193 list.push_head(this->execute_flag);
194 }
195 return this->execute_flag;
196 }
197
get_break_flag__anon250c63460111::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), 0));
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__anon250c63460111::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__anon250c63460111::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), 0));
233 this->signature->body.push_head(this->return_flag);
234 }
235 return this->return_flag;
236 }
237
get_return_value__anon250c63460111::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__anon250c63460111::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__anon250c63460111::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__anon250c63460111::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__anon250c63460111::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__anon250c63460111::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__anon250c63460111::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 0);
361 }
362
363 /**
364 * If the given instruction is a break, lower it to an instruction
365 * that sets the break flag, without consulting
366 * should_lower_jump().
367 *
368 * It is safe to pass NULL to this function.
369 */
lower_break_unconditionally__anon250c63460111::ir_lower_jumps_visitor370 void lower_break_unconditionally(ir_instruction *ir)
371 {
372 if (get_jump_strength(ir) != strength_break) {
373 return;
374 }
375 ir->replace_with(create_lowered_break());
376 }
377
378 /**
379 * If the block ends in a conditional or unconditional break, lower
380 * it, even though should_lower_jump() says it needn't be lowered.
381 */
lower_final_breaks__anon250c63460111::ir_lower_jumps_visitor382 void lower_final_breaks(exec_list *block)
383 {
384 ir_instruction *ir = (ir_instruction *) block->get_tail();
385 lower_break_unconditionally(ir);
386 ir_if *ir_if = ir->as_if();
387 if (ir_if) {
388 lower_break_unconditionally(
389 (ir_instruction *) ir_if->then_instructions.get_tail());
390 lower_break_unconditionally(
391 (ir_instruction *) ir_if->else_instructions.get_tail());
392 }
393 }
394
visit__anon250c63460111::ir_lower_jumps_visitor395 virtual void visit(class ir_loop_jump * ir)
396 {
397 /* Eliminate all instructions after each one, since they are
398 * unreachable. This satisfies the DEAD_CODE_ELIMINATION
399 * postcondition.
400 */
401 truncate_after_instruction(ir);
402
403 /* Set this->block.min_strength based on this instruction. This
404 * satisfies the ANALYSIS postcondition. It is not necessary to
405 * update this->block.may_clear_execute_flag or
406 * this->loop.may_set_return_flag, because an unlowered jump
407 * instruction can't change any flags.
408 */
409 this->block.min_strength = ir->is_break() ? strength_break : strength_continue;
410
411 /* The CONTAINED_JUMPS_LOWERED postcondition is already
412 * satisfied, because jump statements can't contain other
413 * statements.
414 */
415 }
416
visit__anon250c63460111::ir_lower_jumps_visitor417 virtual void visit(class ir_return * ir)
418 {
419 /* Eliminate all instructions after each one, since they are
420 * unreachable. This satisfies the DEAD_CODE_ELIMINATION
421 * postcondition.
422 */
423 truncate_after_instruction(ir);
424
425 /* Set this->block.min_strength based on this instruction. This
426 * satisfies the ANALYSIS postcondition. It is not necessary to
427 * update this->block.may_clear_execute_flag or
428 * this->loop.may_set_return_flag, because an unlowered return
429 * instruction can't change any flags.
430 */
431 this->block.min_strength = strength_return;
432
433 /* The CONTAINED_JUMPS_LOWERED postcondition is already
434 * satisfied, because jump statements can't contain other
435 * statements.
436 */
437 }
438
visit__anon250c63460111::ir_lower_jumps_visitor439 virtual void visit(class ir_discard * ir)
440 {
441 /* Nothing needs to be done. The ANALYSIS and
442 * DEAD_CODE_ELIMINATION postconditions are already satisfied,
443 * because discard statements are ignored by this optimization
444 * pass. The CONTAINED_JUMPS_LOWERED postcondition is already
445 * satisfied, because discard statements can't contain other
446 * statements.
447 */
448 (void) ir;
449 }
450
get_jump_strength__anon250c63460111::ir_lower_jumps_visitor451 enum jump_strength get_jump_strength(ir_instruction* ir)
452 {
453 if(!ir)
454 return strength_none;
455 else if(ir->ir_type == ir_type_loop_jump) {
456 if(((ir_loop_jump*)ir)->is_break())
457 return strength_break;
458 else
459 return strength_continue;
460 } else if(ir->ir_type == ir_type_return)
461 return strength_return;
462 else
463 return strength_none;
464 }
465
should_lower_jump__anon250c63460111::ir_lower_jumps_visitor466 bool should_lower_jump(ir_jump* ir)
467 {
468 unsigned strength = get_jump_strength(ir);
469 bool lower;
470 switch(strength)
471 {
472 case strength_none:
473 lower = false; /* don't change this, code relies on it */
474 break;
475 case strength_continue:
476 lower = lower_continue;
477 break;
478 case strength_break:
479 assert(this->loop.loop);
480 /* never lower "canonical break" */
481 if(ir->get_next()->is_tail_sentinel() && (this->loop.nesting_depth == 0
482 || (this->loop.nesting_depth == 1 && this->loop.in_if_at_the_end_of_the_loop)))
483 lower = false;
484 else
485 lower = lower_break;
486 break;
487 case strength_return:
488 /* never lower return at the end of a this->function */
489 if(this->function.nesting_depth == 0 && ir->get_next()->is_tail_sentinel())
490 lower = false;
491 else
492 lower = this->function.lower_return;
493 break;
494 }
495 return lower;
496 }
497
visit_block__anon250c63460111::ir_lower_jumps_visitor498 block_record visit_block(exec_list* list)
499 {
500 /* Note: since visiting a node may change that node's next
501 * pointer, we can't use visit_exec_list(), because
502 * visit_exec_list() caches the node's next pointer before
503 * visiting it. So we use foreach_in_list() instead.
504 *
505 * foreach_in_list() isn't safe if the node being visited gets
506 * removed, but fortunately this visitor doesn't do that.
507 */
508
509 block_record saved_block = this->block;
510 this->block = block_record();
511 foreach_in_list(ir_instruction, node, list) {
512 node->accept(this);
513 }
514 block_record ret = this->block;
515 this->block = saved_block;
516 return ret;
517 }
518
visit__anon250c63460111::ir_lower_jumps_visitor519 virtual void visit(ir_if *ir)
520 {
521 if(this->loop.nesting_depth == 0 && ir->get_next()->is_tail_sentinel())
522 this->loop.in_if_at_the_end_of_the_loop = true;
523
524 ++this->function.nesting_depth;
525 ++this->loop.nesting_depth;
526
527 block_record block_records[2];
528 ir_jump* jumps[2];
529
530 /* Recursively lower nested jumps. This satisfies the
531 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
532 * unconditional jumps at the end of ir->then_instructions and
533 * ir->else_instructions, which are handled below.
534 */
535 block_records[0] = visit_block(&ir->then_instructions);
536 block_records[1] = visit_block(&ir->else_instructions);
537
538 retry: /* we get here if we put code after the if inside a branch */
539
540 /* Determine which of ir->then_instructions and
541 * ir->else_instructions end with an unconditional jump.
542 */
543 for(unsigned i = 0; i < 2; ++i) {
544 exec_list& list = i ? ir->else_instructions : ir->then_instructions;
545 jumps[i] = 0;
546 if(!list.is_empty() && get_jump_strength((ir_instruction*)list.get_tail()))
547 jumps[i] = (ir_jump*)list.get_tail();
548 }
549
550 /* Loop until we have satisfied the CONTAINED_JUMPS_LOWERED
551 * postcondition by lowering jumps in both then_instructions and
552 * else_instructions.
553 */
554 for(;;) {
555 /* Determine the types of the jumps that terminate
556 * ir->then_instructions and ir->else_instructions.
557 */
558 jump_strength jump_strengths[2];
559
560 for(unsigned i = 0; i < 2; ++i) {
561 if(jumps[i]) {
562 jump_strengths[i] = block_records[i].min_strength;
563 assert(jump_strengths[i] == get_jump_strength(jumps[i]));
564 } else
565 jump_strengths[i] = strength_none;
566 }
567
568 /* If both code paths end in a jump, and the jumps are the
569 * same, and we are pulling out jumps, replace them with a
570 * single jump that comes after the if instruction. The new
571 * jump will be visited next, and it will be lowered if
572 * necessary by the loop or conditional that encloses it.
573 */
574 if(pull_out_jumps && jump_strengths[0] == jump_strengths[1]) {
575 bool unify = true;
576 if(jump_strengths[0] == strength_continue)
577 ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_continue));
578 else if(jump_strengths[0] == strength_break)
579 ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
580 /* FINISHME: unify returns with identical expressions */
581 else if(jump_strengths[0] == strength_return && this->function.signature->return_type->is_void())
582 ir->insert_after(new(ir) ir_return(NULL));
583 else
584 unify = false;
585
586 if(unify) {
587 jumps[0]->remove();
588 jumps[1]->remove();
589 this->progress = true;
590
591 /* Update jumps[] to reflect the fact that the jumps
592 * are gone, and update block_records[] to reflect the
593 * fact that control can now flow to the next
594 * instruction.
595 */
596 jumps[0] = 0;
597 jumps[1] = 0;
598 block_records[0].min_strength = strength_none;
599 block_records[1].min_strength = strength_none;
600
601 /* The CONTAINED_JUMPS_LOWERED postcondition is now
602 * satisfied, so we can break out of the loop.
603 */
604 break;
605 }
606 }
607
608 /* lower a jump: if both need to lowered, start with the strongest one, so that
609 * we might later unify the lowered version with the other one
610 */
611 bool should_lower[2];
612 for(unsigned i = 0; i < 2; ++i)
613 should_lower[i] = should_lower_jump(jumps[i]);
614
615 int lower;
616 if(should_lower[1] && should_lower[0])
617 lower = jump_strengths[1] > jump_strengths[0];
618 else if(should_lower[0])
619 lower = 0;
620 else if(should_lower[1])
621 lower = 1;
622 else
623 /* Neither code path ends in a jump that needs to be
624 * lowered, so the CONTAINED_JUMPS_LOWERED postcondition
625 * is satisfied and we can break out of the loop.
626 */
627 break;
628
629 if(jump_strengths[lower] == strength_return) {
630 /* To lower a return, we create a return flag (if the
631 * function doesn't have one already) and add instructions
632 * that: 1. store the return value (if this function has a
633 * non-void return) and 2. set the return flag
634 */
635 insert_lowered_return((ir_return*)jumps[lower]);
636 if(this->loop.loop) {
637 /* If we are in a loop, replace the return instruction
638 * with a break instruction, and then loop so that the
639 * break instruction can be lowered if necessary.
640 */
641 ir_loop_jump* lowered = 0;
642 lowered = new(ir) ir_loop_jump(ir_loop_jump::jump_break);
643 /* Note: we must update block_records and jumps to
644 * reflect the fact that the control path has been
645 * altered from a return to a break.
646 */
647 block_records[lower].min_strength = strength_break;
648 jumps[lower]->replace_with(lowered);
649 jumps[lower] = lowered;
650 } else {
651 /* If we are not in a loop, we then proceed as we would
652 * for a continue statement (set the execute flag to
653 * false to prevent the rest of the function from
654 * executing).
655 */
656 goto lower_continue;
657 }
658 this->progress = true;
659 } else if(jump_strengths[lower] == strength_break) {
660 /* To lower a break, we create a break flag (if the loop
661 * doesn't have one already) and add an instruction that
662 * sets it.
663 *
664 * Then we proceed as we would for a continue statement
665 * (set the execute flag to false to prevent the rest of
666 * the loop body from executing).
667 *
668 * The visit() function for the loop will ensure that the
669 * break flag is checked after executing the loop body.
670 */
671 jumps[lower]->insert_before(create_lowered_break());
672 goto lower_continue;
673 } else if(jump_strengths[lower] == strength_continue) {
674 lower_continue:
675 /* To lower a continue, we create an execute flag (if the
676 * loop doesn't have one already) and replace the continue
677 * with an instruction that clears it.
678 *
679 * Note that this code path gets exercised when lowering
680 * return statements that are not inside a loop, so
681 * this->loop must be initialized even outside of loops.
682 */
683 ir_variable* execute_flag = this->loop.get_execute_flag();
684 jumps[lower]->replace_with(new(ir) ir_assignment(new (ir) ir_dereference_variable(execute_flag), new (ir) ir_constant(false), 0));
685 /* Note: we must update block_records and jumps to reflect
686 * the fact that the control path has been altered to an
687 * instruction that clears the execute flag.
688 */
689 jumps[lower] = 0;
690 block_records[lower].min_strength = strength_always_clears_execute_flag;
691 block_records[lower].may_clear_execute_flag = true;
692 this->progress = true;
693
694 /* Let the loop run again, in case the other branch of the
695 * if needs to be lowered too.
696 */
697 }
698 }
699
700 /* move out a jump out if possible */
701 if(pull_out_jumps) {
702 /* If one of the branches ends in a jump, and control cannot
703 * fall out the bottom of the other branch, then we can move
704 * the jump after the if.
705 *
706 * Set move_out to the branch we are moving a jump out of.
707 */
708 int move_out = -1;
709 if(jumps[0] && block_records[1].min_strength >= strength_continue)
710 move_out = 0;
711 else if(jumps[1] && block_records[0].min_strength >= strength_continue)
712 move_out = 1;
713
714 if(move_out >= 0)
715 {
716 jumps[move_out]->remove();
717 ir->insert_after(jumps[move_out]);
718 /* Note: we must update block_records and jumps to reflect
719 * the fact that the jump has been moved out of the if.
720 */
721 jumps[move_out] = 0;
722 block_records[move_out].min_strength = strength_none;
723 this->progress = true;
724 }
725 }
726
727 /* Now satisfy the ANALYSIS postcondition by setting
728 * this->block.min_strength and
729 * this->block.may_clear_execute_flag based on the
730 * characteristics of the two branches.
731 */
732 if(block_records[0].min_strength < block_records[1].min_strength)
733 this->block.min_strength = block_records[0].min_strength;
734 else
735 this->block.min_strength = block_records[1].min_strength;
736 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;
737
738 /* Now we need to clean up the instructions that follow the
739 * if.
740 *
741 * If those instructions are unreachable, then satisfy the
742 * DEAD_CODE_ELIMINATION postcondition by eliminating them.
743 * Otherwise that postcondition is already satisfied.
744 */
745 if(this->block.min_strength)
746 truncate_after_instruction(ir);
747 else if(this->block.may_clear_execute_flag)
748 {
749 /* If the "if" instruction might clear the execute flag, then
750 * we need to guard any instructions that follow so that they
751 * are only executed if the execute flag is set.
752 *
753 * If one of the branches of the "if" always clears the
754 * execute flag, and the other branch never clears it, then
755 * this is easy: just move all the instructions following the
756 * "if" into the branch that never clears it.
757 */
758 int move_into = -1;
759 if(block_records[0].min_strength && !block_records[1].may_clear_execute_flag)
760 move_into = 1;
761 else if(block_records[1].min_strength && !block_records[0].may_clear_execute_flag)
762 move_into = 0;
763
764 if(move_into >= 0) {
765 assert(!block_records[move_into].min_strength && !block_records[move_into].may_clear_execute_flag); /* otherwise, we just truncated */
766
767 exec_list* list = move_into ? &ir->else_instructions : &ir->then_instructions;
768 exec_node* next = ir->get_next();
769 if(!next->is_tail_sentinel()) {
770 move_outer_block_inside(ir, list);
771
772 /* If any instructions moved, then we need to visit
773 * them (since they are now inside the "if"). Since
774 * block_records[move_into] is in its default state
775 * (see assertion above), we can safely replace
776 * block_records[move_into] with the result of this
777 * analysis.
778 */
779 exec_list list;
780 list.head_sentinel.next = next;
781 block_records[move_into] = visit_block(&list);
782
783 /*
784 * Then we need to re-start our jump lowering, since one
785 * of the instructions we moved might be a jump that
786 * needs to be lowered.
787 */
788 this->progress = true;
789 goto retry;
790 }
791 } else {
792 /* If we get here, then the simple case didn't apply; we
793 * need to actually guard the instructions that follow.
794 *
795 * To avoid creating unnecessarily-deep nesting, first
796 * look through the instructions that follow and unwrap
797 * any instructions that that are already wrapped in the
798 * appropriate guard.
799 */
800 ir_instruction* ir_after;
801 for(ir_after = (ir_instruction*)ir->get_next(); !ir_after->is_tail_sentinel();)
802 {
803 ir_if* ir_if = ir_after->as_if();
804 if(ir_if && ir_if->else_instructions.is_empty()) {
805 ir_dereference_variable* ir_if_cond_deref = ir_if->condition->as_dereference_variable();
806 if(ir_if_cond_deref && ir_if_cond_deref->var == this->loop.execute_flag) {
807 ir_instruction* ir_next = (ir_instruction*)ir_after->get_next();
808 ir_after->insert_before(&ir_if->then_instructions);
809 ir_after->remove();
810 ir_after = ir_next;
811 continue;
812 }
813 }
814 ir_after = (ir_instruction*)ir_after->get_next();
815
816 /* only set this if we find any unprotected instruction */
817 this->progress = true;
818 }
819
820 /* Then, wrap all the instructions that follow in a single
821 * guard.
822 */
823 if(!ir->get_next()->is_tail_sentinel()) {
824 assert(this->loop.execute_flag);
825 ir_if* if_execute = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.execute_flag));
826 move_outer_block_inside(ir, &if_execute->then_instructions);
827 ir->insert_after(if_execute);
828 }
829 }
830 }
831 --this->loop.nesting_depth;
832 --this->function.nesting_depth;
833 }
834
visit__anon250c63460111::ir_lower_jumps_visitor835 virtual void visit(ir_loop *ir)
836 {
837 /* Visit the body of the loop, with a fresh data structure in
838 * this->loop so that the analysis we do here won't bleed into
839 * enclosing loops.
840 *
841 * We assume that all code after a loop is reachable from the
842 * loop (see comments on enum jump_strength), so the
843 * DEAD_CODE_ELIMINATION postcondition is automatically
844 * satisfied, as is the block.min_strength portion of the
845 * ANALYSIS postcondition.
846 *
847 * The block.may_clear_execute_flag portion of the ANALYSIS
848 * postcondition is automatically satisfied because execute
849 * flags do not propagate outside of loops.
850 *
851 * The loop.may_set_return_flag portion of the ANALYSIS
852 * postcondition is handled below.
853 */
854 ++this->function.nesting_depth;
855 loop_record saved_loop = this->loop;
856 this->loop = loop_record(this->function.signature, ir);
857
858 /* Recursively lower nested jumps. This satisfies the
859 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
860 * an unconditional continue or return at the bottom of the
861 * loop, which are handled below.
862 */
863 block_record body = visit_block(&ir->body_instructions);
864
865 /* If the loop ends in an unconditional continue, eliminate it
866 * because it is redundant.
867 */
868 ir_instruction *ir_last
869 = (ir_instruction *) ir->body_instructions.get_tail();
870 if (get_jump_strength(ir_last) == strength_continue) {
871 ir_last->remove();
872 }
873
874 /* If the loop ends in an unconditional return, and we are
875 * lowering returns, lower it.
876 */
877 if (this->function.lower_return)
878 lower_return_unconditionally(ir_last);
879
880 if(body.min_strength >= strength_break) {
881 /* FINISHME: If the min_strength of the loop body is
882 * strength_break or strength_return, that means that it
883 * isn't a loop at all, since control flow always leaves the
884 * body of the loop via break or return. In principle the
885 * loop could be eliminated in this case. This optimization
886 * is not implemented yet.
887 */
888 }
889
890 if(this->loop.break_flag) {
891 /* We only get here if we are lowering breaks */
892 assert (lower_break);
893
894 /* If a break flag was generated while visiting the body of
895 * the loop, then at least one break was lowered, so we need
896 * to generate an if statement at the end of the loop that
897 * does a "break" if the break flag is set. The break we
898 * generate won't violate the CONTAINED_JUMPS_LOWERED
899 * postcondition, because should_lower_jump() always returns
900 * false for a break that happens at the end of a loop.
901 *
902 * However, if the loop already ends in a conditional or
903 * unconditional break, then we need to lower that break,
904 * because it won't be at the end of the loop anymore.
905 */
906 lower_final_breaks(&ir->body_instructions);
907
908 ir_if* break_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.break_flag));
909 break_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
910 ir->body_instructions.push_tail(break_if);
911 }
912
913 /* If the body of the loop may set the return flag, then at
914 * least one return was lowered to a break, so we need to ensure
915 * that the return flag is checked after the body of the loop is
916 * executed.
917 */
918 if(this->loop.may_set_return_flag) {
919 assert(this->function.return_flag);
920 /* Generate the if statement to check the return flag */
921 ir_if* return_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->function.return_flag));
922 /* Note: we also need to propagate the knowledge that the
923 * return flag may get set to the outer context. This
924 * satisfies the loop.may_set_return_flag part of the
925 * ANALYSIS postcondition.
926 */
927 saved_loop.may_set_return_flag = true;
928 if(saved_loop.loop)
929 /* If this loop is nested inside another one, then the if
930 * statement that we generated should break out of that
931 * loop if the return flag is set. Caller will lower that
932 * break statement if necessary.
933 */
934 return_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
935 else {
936 /* Otherwise, ensure that the instructions that follow are only
937 * executed if the return flag is clear. We can do that by moving
938 * those instructions into the else clause of the generated if
939 * statement.
940 */
941 move_outer_block_inside(ir, &return_if->else_instructions);
942
943 /* In case the loop is embeded inside an if add a new return to
944 * the return flag then branch and let a future pass tidy it up.
945 */
946 if (this->function.signature->return_type->is_void())
947 return_if->then_instructions.push_tail(new(ir) ir_return(NULL));
948 }
949
950 ir->insert_after(return_if);
951 }
952
953 this->loop = saved_loop;
954 --this->function.nesting_depth;
955 }
956
visit__anon250c63460111::ir_lower_jumps_visitor957 virtual void visit(ir_function_signature *ir)
958 {
959 /* these are not strictly necessary */
960 assert(!this->function.signature);
961 assert(!this->loop.loop);
962
963 bool lower_return;
964 if (strcmp(ir->function_name(), "main") == 0)
965 lower_return = lower_main_return;
966 else
967 lower_return = lower_sub_return;
968
969 function_record saved_function = this->function;
970 loop_record saved_loop = this->loop;
971 this->function = function_record(ir, lower_return);
972 this->loop = loop_record(ir);
973
974 assert(!this->loop.loop);
975
976 /* Visit the body of the function to lower any jumps that occur
977 * in it, except possibly an unconditional return statement at
978 * the end of it.
979 */
980 visit_block(&ir->body);
981
982 /* If the body ended in an unconditional return of non-void,
983 * then we don't need to lower it because it's the one canonical
984 * return.
985 *
986 * If the body ended in a return of void, eliminate it because
987 * it is redundant.
988 */
989 if (ir->return_type->is_void() &&
990 get_jump_strength((ir_instruction *) ir->body.get_tail())) {
991 ir_jump *jump = (ir_jump *) ir->body.get_tail();
992 assert (jump->ir_type == ir_type_return);
993 jump->remove();
994 }
995
996 if(this->function.return_value)
997 ir->body.push_tail(new(ir) ir_return(new (ir) ir_dereference_variable(this->function.return_value)));
998
999 this->loop = saved_loop;
1000 this->function = saved_function;
1001 }
1002
visit__anon250c63460111::ir_lower_jumps_visitor1003 virtual void visit(class ir_function * ir)
1004 {
1005 visit_block(&ir->signatures);
1006 }
1007 };
1008
1009 } /* anonymous namespace */
1010
1011 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)1012 do_lower_jumps(exec_list *instructions, bool pull_out_jumps, bool lower_sub_return, bool lower_main_return, bool lower_continue, bool lower_break)
1013 {
1014 ir_lower_jumps_visitor v;
1015 v.pull_out_jumps = pull_out_jumps;
1016 v.lower_continue = lower_continue;
1017 v.lower_break = lower_break;
1018 v.lower_sub_return = lower_sub_return;
1019 v.lower_main_return = lower_main_return;
1020
1021 bool progress_ever = false;
1022 do {
1023 v.progress = false;
1024 visit_exec_list(instructions, &v);
1025 progress_ever = v.progress || progress_ever;
1026 } while (v.progress);
1027
1028 return progress_ever;
1029 }
1030