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 */ 95enum 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 136namespace { 137 138struct 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; 153 154 block_record() 155 { 156 this->min_strength = strength_none; 157 this->may_clear_execute_flag = false; 158 } 159}; 160 161struct 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 */ 174 175 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 } 185 186 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 } 197 198 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 210struct 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; 217 218 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 } 227 228 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 } 237 238 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 249struct 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; 282 283 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 } 292 293 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 } 303 304 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 */ 318 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 */ 342 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 */ 354 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 */ 370 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 */ 382 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 } 394 395 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 } 416 417 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 } 438 439 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 } 450 451 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 } 465 466 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 } 497 498 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 } 518 519 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 538retry: /* 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) { 674lower_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 } 834 835 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 } 956 957 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 1003 virtual void visit(class ir_function * ir) 1004 { 1005 visit_block(&ir->signatures); 1006 } 1007}; 1008 1009} /* anonymous namespace */ 1010 1011bool 1012do_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