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 "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 struct block_record 137 { 138 /* minimum jump strength (of lowered IR, not pre-lowering IR) 139 * 140 * If the block ends with a jump, must be the strength of the jump. 141 * Otherwise, the jump would be dead and have been deleted before) 142 * 143 * 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 144 * (e.g. an if with a return in one branch, and a break in the other, while not lowering them) 145 * Note that identical jumps are usually unified though. 146 */ 147 jump_strength min_strength; 148 149 /* can anything clear the execute flag? */ 150 bool may_clear_execute_flag; 151 152 block_record() 153 { 154 this->min_strength = strength_none; 155 this->may_clear_execute_flag = false; 156 } 157 }; 158 159 struct loop_record 160 { 161 ir_function_signature* signature; 162 ir_loop* loop; 163 164 /* used to avoid lowering the break used to represent lowered breaks */ 165 unsigned nesting_depth; 166 bool in_if_at_the_end_of_the_loop; 167 168 bool may_set_return_flag; 169 170 ir_variable* break_flag; 171 ir_variable* execute_flag; /* cleared to emulate continue */ 172 173 loop_record(ir_function_signature* p_signature = 0, ir_loop* p_loop = 0) 174 { 175 this->signature = p_signature; 176 this->loop = p_loop; 177 this->nesting_depth = 0; 178 this->in_if_at_the_end_of_the_loop = false; 179 this->may_set_return_flag = false; 180 this->break_flag = 0; 181 this->execute_flag = 0; 182 } 183 184 ir_variable* get_execute_flag() 185 { 186 /* also supported for the "function loop" */ 187 if(!this->execute_flag) { 188 exec_list& list = this->loop ? this->loop->body_instructions : signature->body; 189 this->execute_flag = new(this->signature) ir_variable(glsl_type::bool_type, "execute_flag", ir_var_temporary); 190 list.push_head(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(execute_flag), new(this->signature) ir_constant(true), 0)); 191 list.push_head(this->execute_flag); 192 } 193 return this->execute_flag; 194 } 195 196 ir_variable* get_break_flag() 197 { 198 assert(this->loop); 199 if(!this->break_flag) { 200 this->break_flag = new(this->signature) ir_variable(glsl_type::bool_type, "break_flag", ir_var_temporary); 201 this->loop->insert_before(this->break_flag); 202 this->loop->insert_before(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(break_flag), new(this->signature) ir_constant(false), 0)); 203 } 204 return this->break_flag; 205 } 206 }; 207 208 struct function_record 209 { 210 ir_function_signature* signature; 211 ir_variable* return_flag; /* used to break out of all loops and then jump to the return instruction */ 212 ir_variable* return_value; 213 bool lower_return; 214 unsigned nesting_depth; 215 216 function_record(ir_function_signature* p_signature = 0, 217 bool lower_return = false) 218 { 219 this->signature = p_signature; 220 this->return_flag = 0; 221 this->return_value = 0; 222 this->nesting_depth = 0; 223 this->lower_return = lower_return; 224 } 225 226 ir_variable* get_return_flag() 227 { 228 if(!this->return_flag) { 229 this->return_flag = new(this->signature) ir_variable(glsl_type::bool_type, "return_flag", ir_var_temporary); 230 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)); 231 this->signature->body.push_head(this->return_flag); 232 } 233 return this->return_flag; 234 } 235 236 ir_variable* get_return_value() 237 { 238 if(!this->return_value) { 239 assert(!this->signature->return_type->is_void()); 240 return_value = new(this->signature) ir_variable(this->signature->return_type, "return_value", ir_var_temporary); 241 this->signature->body.push_head(this->return_value); 242 } 243 return this->return_value; 244 } 245 }; 246 247 struct ir_lower_jumps_visitor : public ir_control_flow_visitor { 248 /* Postconditions: on exit of any visit() function: 249 * 250 * ANALYSIS: this->block.min_strength, 251 * this->block.may_clear_execute_flag, and 252 * this->loop.may_set_return_flag are updated to reflect the 253 * characteristics of the visited statement. 254 * 255 * DEAD_CODE_ELIMINATION: If this->block.min_strength is not 256 * strength_none, the visited node is at the end of its exec_list. 257 * In other words, any unreachable statements that follow the 258 * visited statement in its exec_list have been removed. 259 * 260 * CONTAINED_JUMPS_LOWERED: If the visited statement contains other 261 * statements, then should_lower_jump() is false for all of the 262 * return, break, or continue statements it contains. 263 * 264 * Note that visiting a jump does not lower it. That is the 265 * responsibility of the statement (or function signature) that 266 * contains the jump. 267 */ 268 269 bool progress; 270 271 struct function_record function; 272 struct loop_record loop; 273 struct block_record block; 274 275 bool pull_out_jumps; 276 bool lower_continue; 277 bool lower_break; 278 bool lower_sub_return; 279 bool lower_main_return; 280 281 ir_lower_jumps_visitor() 282 { 283 this->progress = false; 284 } 285 286 void truncate_after_instruction(exec_node *ir) 287 { 288 if (!ir) 289 return; 290 291 while (!ir->get_next()->is_tail_sentinel()) { 292 ((ir_instruction *)ir->get_next())->remove(); 293 this->progress = true; 294 } 295 } 296 297 void move_outer_block_inside(ir_instruction *ir, exec_list *inner_block) 298 { 299 while (!ir->get_next()->is_tail_sentinel()) { 300 ir_instruction *move_ir = (ir_instruction *)ir->get_next(); 301 302 move_ir->remove(); 303 inner_block->push_tail(move_ir); 304 } 305 } 306 307 /** 308 * Insert the instructions necessary to lower a return statement, 309 * before the given return instruction. 310 */ 311 void insert_lowered_return(ir_return *ir) 312 { 313 ir_variable* return_flag = this->function.get_return_flag(); 314 if(!this->function.signature->return_type->is_void()) { 315 ir_variable* return_value = this->function.get_return_value(); 316 ir->insert_before( 317 new(ir) ir_assignment( 318 new (ir) ir_dereference_variable(return_value), 319 ir->value)); 320 } 321 ir->insert_before( 322 new(ir) ir_assignment( 323 new (ir) ir_dereference_variable(return_flag), 324 new (ir) ir_constant(true))); 325 this->loop.may_set_return_flag = true; 326 } 327 328 /** 329 * If the given instruction is a return, lower it to instructions 330 * that store the return value (if there is one), set the return 331 * flag, and then break. 332 * 333 * It is safe to pass NULL to this function. 334 */ 335 void lower_return_unconditionally(ir_instruction *ir) 336 { 337 if (get_jump_strength(ir) != strength_return) { 338 return; 339 } 340 insert_lowered_return((ir_return*)ir); 341 ir->replace_with(new(ir) ir_loop_jump(ir_loop_jump::jump_break)); 342 } 343 344 /** 345 * Create the necessary instruction to replace a break instruction. 346 */ 347 ir_instruction *create_lowered_break() 348 { 349 void *ctx = this->function.signature; 350 return new(ctx) ir_assignment( 351 new(ctx) ir_dereference_variable(this->loop.get_break_flag()), 352 new(ctx) ir_constant(true), 353 0); 354 } 355 356 /** 357 * If the given instruction is a break, lower it to an instruction 358 * that sets the break flag, without consulting 359 * should_lower_jump(). 360 * 361 * It is safe to pass NULL to this function. 362 */ 363 void lower_break_unconditionally(ir_instruction *ir) 364 { 365 if (get_jump_strength(ir) != strength_break) { 366 return; 367 } 368 ir->replace_with(create_lowered_break()); 369 } 370 371 /** 372 * If the block ends in a conditional or unconditional break, lower 373 * it, even though should_lower_jump() says it needn't be lowered. 374 */ 375 void lower_final_breaks(exec_list *block) 376 { 377 ir_instruction *ir = (ir_instruction *) block->get_tail(); 378 lower_break_unconditionally(ir); 379 ir_if *ir_if = ir->as_if(); 380 if (ir_if) { 381 lower_break_unconditionally( 382 (ir_instruction *) ir_if->then_instructions.get_tail()); 383 lower_break_unconditionally( 384 (ir_instruction *) ir_if->else_instructions.get_tail()); 385 } 386 } 387 388 virtual void visit(class ir_loop_jump * ir) 389 { 390 /* Eliminate all instructions after each one, since they are 391 * unreachable. This satisfies the DEAD_CODE_ELIMINATION 392 * postcondition. 393 */ 394 truncate_after_instruction(ir); 395 396 /* Set this->block.min_strength based on this instruction. This 397 * satisfies the ANALYSIS postcondition. It is not necessary to 398 * update this->block.may_clear_execute_flag or 399 * this->loop.may_set_return_flag, because an unlowered jump 400 * instruction can't change any flags. 401 */ 402 this->block.min_strength = ir->is_break() ? strength_break : strength_continue; 403 404 /* The CONTAINED_JUMPS_LOWERED postcondition is already 405 * satisfied, because jump statements can't contain other 406 * statements. 407 */ 408 } 409 410 virtual void visit(class ir_return * ir) 411 { 412 /* Eliminate all instructions after each one, since they are 413 * unreachable. This satisfies the DEAD_CODE_ELIMINATION 414 * postcondition. 415 */ 416 truncate_after_instruction(ir); 417 418 /* Set this->block.min_strength based on this instruction. This 419 * satisfies the ANALYSIS postcondition. It is not necessary to 420 * update this->block.may_clear_execute_flag or 421 * this->loop.may_set_return_flag, because an unlowered return 422 * instruction can't change any flags. 423 */ 424 this->block.min_strength = strength_return; 425 426 /* The CONTAINED_JUMPS_LOWERED postcondition is already 427 * satisfied, because jump statements can't contain other 428 * statements. 429 */ 430 } 431 432 virtual void visit(class ir_discard * ir) 433 { 434 /* Nothing needs to be done. The ANALYSIS and 435 * DEAD_CODE_ELIMINATION postconditions are already satisfied, 436 * because discard statements are ignored by this optimization 437 * pass. The CONTAINED_JUMPS_LOWERED postcondition is already 438 * satisfied, because discard statements can't contain other 439 * statements. 440 */ 441 (void) ir; 442 } 443 444 enum jump_strength get_jump_strength(ir_instruction* ir) 445 { 446 if(!ir) 447 return strength_none; 448 else if(ir->ir_type == ir_type_loop_jump) { 449 if(((ir_loop_jump*)ir)->is_break()) 450 return strength_break; 451 else 452 return strength_continue; 453 } else if(ir->ir_type == ir_type_return) 454 return strength_return; 455 else 456 return strength_none; 457 } 458 459 bool should_lower_jump(ir_jump* ir) 460 { 461 unsigned strength = get_jump_strength(ir); 462 bool lower; 463 switch(strength) 464 { 465 case strength_none: 466 lower = false; /* don't change this, code relies on it */ 467 break; 468 case strength_continue: 469 lower = lower_continue; 470 break; 471 case strength_break: 472 assert(this->loop.loop); 473 /* never lower "canonical break" */ 474 if(ir->get_next()->is_tail_sentinel() && (this->loop.nesting_depth == 0 475 || (this->loop.nesting_depth == 1 && this->loop.in_if_at_the_end_of_the_loop))) 476 lower = false; 477 else 478 lower = lower_break; 479 break; 480 case strength_return: 481 /* never lower return at the end of a this->function */ 482 if(this->function.nesting_depth == 0 && ir->get_next()->is_tail_sentinel()) 483 lower = false; 484 else 485 lower = this->function.lower_return; 486 break; 487 } 488 return lower; 489 } 490 491 block_record visit_block(exec_list* list) 492 { 493 /* Note: since visiting a node may change that node's next 494 * pointer, we can't use visit_exec_list(), because 495 * visit_exec_list() caches the node's next pointer before 496 * visiting it. So we use foreach_list() instead. 497 * 498 * foreach_list() isn't safe if the node being visited gets 499 * removed, but fortunately this visitor doesn't do that. 500 */ 501 502 block_record saved_block = this->block; 503 this->block = block_record(); 504 foreach_list(node, list) { 505 ((ir_instruction *) node)->accept(this); 506 } 507 block_record ret = this->block; 508 this->block = saved_block; 509 return ret; 510 } 511 512 virtual void visit(ir_if *ir) 513 { 514 if(this->loop.nesting_depth == 0 && ir->get_next()->is_tail_sentinel()) 515 this->loop.in_if_at_the_end_of_the_loop = true; 516 517 ++this->function.nesting_depth; 518 ++this->loop.nesting_depth; 519 520 block_record block_records[2]; 521 ir_jump* jumps[2]; 522 523 /* Recursively lower nested jumps. This satisfies the 524 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of 525 * unconditional jumps at the end of ir->then_instructions and 526 * ir->else_instructions, which are handled below. 527 */ 528 block_records[0] = visit_block(&ir->then_instructions); 529 block_records[1] = visit_block(&ir->else_instructions); 530 531 retry: /* we get here if we put code after the if inside a branch */ 532 533 /* Determine which of ir->then_instructions and 534 * ir->else_instructions end with an unconditional jump. 535 */ 536 for(unsigned i = 0; i < 2; ++i) { 537 exec_list& list = i ? ir->else_instructions : ir->then_instructions; 538 jumps[i] = 0; 539 if(!list.is_empty() && get_jump_strength((ir_instruction*)list.get_tail())) 540 jumps[i] = (ir_jump*)list.get_tail(); 541 } 542 543 /* Loop until we have satisfied the CONTAINED_JUMPS_LOWERED 544 * postcondition by lowering jumps in both then_instructions and 545 * else_instructions. 546 */ 547 for(;;) { 548 /* Determine the types of the jumps that terminate 549 * ir->then_instructions and ir->else_instructions. 550 */ 551 jump_strength jump_strengths[2]; 552 553 for(unsigned i = 0; i < 2; ++i) { 554 if(jumps[i]) { 555 jump_strengths[i] = block_records[i].min_strength; 556 assert(jump_strengths[i] == get_jump_strength(jumps[i])); 557 } else 558 jump_strengths[i] = strength_none; 559 } 560 561 /* If both code paths end in a jump, and the jumps are the 562 * same, and we are pulling out jumps, replace them with a 563 * single jump that comes after the if instruction. The new 564 * jump will be visited next, and it will be lowered if 565 * necessary by the loop or conditional that encloses it. 566 */ 567 if(pull_out_jumps && jump_strengths[0] == jump_strengths[1]) { 568 bool unify = true; 569 if(jump_strengths[0] == strength_continue) 570 ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_continue)); 571 else if(jump_strengths[0] == strength_break) 572 ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_break)); 573 /* FINISHME: unify returns with identical expressions */ 574 else if(jump_strengths[0] == strength_return && this->function.signature->return_type->is_void()) 575 ir->insert_after(new(ir) ir_return(NULL)); 576 else 577 unify = false; 578 579 if(unify) { 580 jumps[0]->remove(); 581 jumps[1]->remove(); 582 this->progress = true; 583 584 /* Update jumps[] to reflect the fact that the jumps 585 * are gone, and update block_records[] to reflect the 586 * fact that control can now flow to the next 587 * instruction. 588 */ 589 jumps[0] = 0; 590 jumps[1] = 0; 591 block_records[0].min_strength = strength_none; 592 block_records[1].min_strength = strength_none; 593 594 /* The CONTAINED_JUMPS_LOWERED postcondition is now 595 * satisfied, so we can break out of the loop. 596 */ 597 break; 598 } 599 } 600 601 /* lower a jump: if both need to lowered, start with the strongest one, so that 602 * we might later unify the lowered version with the other one 603 */ 604 bool should_lower[2]; 605 for(unsigned i = 0; i < 2; ++i) 606 should_lower[i] = should_lower_jump(jumps[i]); 607 608 int lower; 609 if(should_lower[1] && should_lower[0]) 610 lower = jump_strengths[1] > jump_strengths[0]; 611 else if(should_lower[0]) 612 lower = 0; 613 else if(should_lower[1]) 614 lower = 1; 615 else 616 /* Neither code path ends in a jump that needs to be 617 * lowered, so the CONTAINED_JUMPS_LOWERED postcondition 618 * is satisfied and we can break out of the loop. 619 */ 620 break; 621 622 if(jump_strengths[lower] == strength_return) { 623 /* To lower a return, we create a return flag (if the 624 * function doesn't have one already) and add instructions 625 * that: 1. store the return value (if this function has a 626 * non-void return) and 2. set the return flag 627 */ 628 insert_lowered_return((ir_return*)jumps[lower]); 629 if(this->loop.loop) { 630 /* If we are in a loop, replace the return instruction 631 * with a break instruction, and then loop so that the 632 * break instruction can be lowered if necessary. 633 */ 634 ir_loop_jump* lowered = 0; 635 lowered = new(ir) ir_loop_jump(ir_loop_jump::jump_break); 636 /* Note: we must update block_records and jumps to 637 * reflect the fact that the control path has been 638 * altered from a return to a break. 639 */ 640 block_records[lower].min_strength = strength_break; 641 jumps[lower]->replace_with(lowered); 642 jumps[lower] = lowered; 643 } else { 644 /* If we are not in a loop, we then proceed as we would 645 * for a continue statement (set the execute flag to 646 * false to prevent the rest of the function from 647 * executing). 648 */ 649 goto lower_continue; 650 } 651 this->progress = true; 652 } else if(jump_strengths[lower] == strength_break) { 653 /* To lower a break, we create a break flag (if the loop 654 * doesn't have one already) and add an instruction that 655 * sets it. 656 * 657 * Then we proceed as we would for a continue statement 658 * (set the execute flag to false to prevent the rest of 659 * the loop body from executing). 660 * 661 * The visit() function for the loop will ensure that the 662 * break flag is checked after executing the loop body. 663 */ 664 jumps[lower]->insert_before(create_lowered_break()); 665 goto lower_continue; 666 } else if(jump_strengths[lower] == strength_continue) { 667 lower_continue: 668 /* To lower a continue, we create an execute flag (if the 669 * loop doesn't have one already) and replace the continue 670 * with an instruction that clears it. 671 * 672 * Note that this code path gets exercised when lowering 673 * return statements that are not inside a loop, so 674 * this->loop must be initialized even outside of loops. 675 */ 676 ir_variable* execute_flag = this->loop.get_execute_flag(); 677 jumps[lower]->replace_with(new(ir) ir_assignment(new (ir) ir_dereference_variable(execute_flag), new (ir) ir_constant(false), 0)); 678 /* Note: we must update block_records and jumps to reflect 679 * the fact that the control path has been altered to an 680 * instruction that clears the execute flag. 681 */ 682 jumps[lower] = 0; 683 block_records[lower].min_strength = strength_always_clears_execute_flag; 684 block_records[lower].may_clear_execute_flag = true; 685 this->progress = true; 686 687 /* Let the loop run again, in case the other branch of the 688 * if needs to be lowered too. 689 */ 690 } 691 } 692 693 /* move out a jump out if possible */ 694 if(pull_out_jumps) { 695 /* If one of the branches ends in a jump, and control cannot 696 * fall out the bottom of the other branch, then we can move 697 * the jump after the if. 698 * 699 * Set move_out to the branch we are moving a jump out of. 700 */ 701 int move_out = -1; 702 if(jumps[0] && block_records[1].min_strength >= strength_continue) 703 move_out = 0; 704 else if(jumps[1] && block_records[0].min_strength >= strength_continue) 705 move_out = 1; 706 707 if(move_out >= 0) 708 { 709 jumps[move_out]->remove(); 710 ir->insert_after(jumps[move_out]); 711 /* Note: we must update block_records and jumps to reflect 712 * the fact that the jump has been moved out of the if. 713 */ 714 jumps[move_out] = 0; 715 block_records[move_out].min_strength = strength_none; 716 this->progress = true; 717 } 718 } 719 720 /* Now satisfy the ANALYSIS postcondition by setting 721 * this->block.min_strength and 722 * this->block.may_clear_execute_flag based on the 723 * characteristics of the two branches. 724 */ 725 if(block_records[0].min_strength < block_records[1].min_strength) 726 this->block.min_strength = block_records[0].min_strength; 727 else 728 this->block.min_strength = block_records[1].min_strength; 729 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; 730 731 /* Now we need to clean up the instructions that follow the 732 * if. 733 * 734 * If those instructions are unreachable, then satisfy the 735 * DEAD_CODE_ELIMINATION postcondition by eliminating them. 736 * Otherwise that postcondition is already satisfied. 737 */ 738 if(this->block.min_strength) 739 truncate_after_instruction(ir); 740 else if(this->block.may_clear_execute_flag) 741 { 742 /* If the "if" instruction might clear the execute flag, then 743 * we need to guard any instructions that follow so that they 744 * are only executed if the execute flag is set. 745 * 746 * If one of the branches of the "if" always clears the 747 * execute flag, and the other branch never clears it, then 748 * this is easy: just move all the instructions following the 749 * "if" into the branch that never clears it. 750 */ 751 int move_into = -1; 752 if(block_records[0].min_strength && !block_records[1].may_clear_execute_flag) 753 move_into = 1; 754 else if(block_records[1].min_strength && !block_records[0].may_clear_execute_flag) 755 move_into = 0; 756 757 if(move_into >= 0) { 758 assert(!block_records[move_into].min_strength && !block_records[move_into].may_clear_execute_flag); /* otherwise, we just truncated */ 759 760 exec_list* list = move_into ? &ir->else_instructions : &ir->then_instructions; 761 exec_node* next = ir->get_next(); 762 if(!next->is_tail_sentinel()) { 763 move_outer_block_inside(ir, list); 764 765 /* If any instructions moved, then we need to visit 766 * them (since they are now inside the "if"). Since 767 * block_records[move_into] is in its default state 768 * (see assertion above), we can safely replace 769 * block_records[move_into] with the result of this 770 * analysis. 771 */ 772 exec_list list; 773 list.head = next; 774 block_records[move_into] = visit_block(&list); 775 776 /* 777 * Then we need to re-start our jump lowering, since one 778 * of the instructions we moved might be a jump that 779 * needs to be lowered. 780 */ 781 this->progress = true; 782 goto retry; 783 } 784 } else { 785 /* If we get here, then the simple case didn't apply; we 786 * need to actually guard the instructions that follow. 787 * 788 * To avoid creating unnecessarily-deep nesting, first 789 * look through the instructions that follow and unwrap 790 * any instructions that that are already wrapped in the 791 * appropriate guard. 792 */ 793 ir_instruction* ir_after; 794 for(ir_after = (ir_instruction*)ir->get_next(); !ir_after->is_tail_sentinel();) 795 { 796 ir_if* ir_if = ir_after->as_if(); 797 if(ir_if && ir_if->else_instructions.is_empty()) { 798 ir_dereference_variable* ir_if_cond_deref = ir_if->condition->as_dereference_variable(); 799 if(ir_if_cond_deref && ir_if_cond_deref->var == this->loop.execute_flag) { 800 ir_instruction* ir_next = (ir_instruction*)ir_after->get_next(); 801 ir_after->insert_before(&ir_if->then_instructions); 802 ir_after->remove(); 803 ir_after = ir_next; 804 continue; 805 } 806 } 807 ir_after = (ir_instruction*)ir_after->get_next(); 808 809 /* only set this if we find any unprotected instruction */ 810 this->progress = true; 811 } 812 813 /* Then, wrap all the instructions that follow in a single 814 * guard. 815 */ 816 if(!ir->get_next()->is_tail_sentinel()) { 817 assert(this->loop.execute_flag); 818 ir_if* if_execute = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.execute_flag)); 819 move_outer_block_inside(ir, &if_execute->then_instructions); 820 ir->insert_after(if_execute); 821 } 822 } 823 } 824 --this->loop.nesting_depth; 825 --this->function.nesting_depth; 826 } 827 828 virtual void visit(ir_loop *ir) 829 { 830 /* Visit the body of the loop, with a fresh data structure in 831 * this->loop so that the analysis we do here won't bleed into 832 * enclosing loops. 833 * 834 * We assume that all code after a loop is reachable from the 835 * loop (see comments on enum jump_strength), so the 836 * DEAD_CODE_ELIMINATION postcondition is automatically 837 * satisfied, as is the block.min_strength portion of the 838 * ANALYSIS postcondition. 839 * 840 * The block.may_clear_execute_flag portion of the ANALYSIS 841 * postcondition is automatically satisfied because execute 842 * flags do not propagate outside of loops. 843 * 844 * The loop.may_set_return_flag portion of the ANALYSIS 845 * postcondition is handled below. 846 */ 847 ++this->function.nesting_depth; 848 loop_record saved_loop = this->loop; 849 this->loop = loop_record(this->function.signature, ir); 850 851 /* Recursively lower nested jumps. This satisfies the 852 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of 853 * an unconditional continue or return at the bottom of the 854 * loop, which are handled below. 855 */ 856 block_record body = visit_block(&ir->body_instructions); 857 858 /* If the loop ends in an unconditional continue, eliminate it 859 * because it is redundant. 860 */ 861 ir_instruction *ir_last 862 = (ir_instruction *) ir->body_instructions.get_tail(); 863 if (get_jump_strength(ir_last) == strength_continue) { 864 ir_last->remove(); 865 } 866 867 /* If the loop ends in an unconditional return, and we are 868 * lowering returns, lower it. 869 */ 870 if (this->function.lower_return) 871 lower_return_unconditionally(ir_last); 872 873 if(body.min_strength >= strength_break) { 874 /* FINISHME: If the min_strength of the loop body is 875 * strength_break or strength_return, that means that it 876 * isn't a loop at all, since control flow always leaves the 877 * body of the loop via break or return. In principle the 878 * loop could be eliminated in this case. This optimization 879 * is not implemented yet. 880 */ 881 } 882 883 if(this->loop.break_flag) { 884 /* We only get here if we are lowering breaks */ 885 assert (lower_break); 886 887 /* If a break flag was generated while visiting the body of 888 * the loop, then at least one break was lowered, so we need 889 * to generate an if statement at the end of the loop that 890 * does a "break" if the break flag is set. The break we 891 * generate won't violate the CONTAINED_JUMPS_LOWERED 892 * postcondition, because should_lower_jump() always returns 893 * false for a break that happens at the end of a loop. 894 * 895 * However, if the loop already ends in a conditional or 896 * unconditional break, then we need to lower that break, 897 * because it won't be at the end of the loop anymore. 898 */ 899 lower_final_breaks(&ir->body_instructions); 900 901 ir_if* break_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.break_flag)); 902 break_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break)); 903 ir->body_instructions.push_tail(break_if); 904 } 905 906 /* If the body of the loop may set the return flag, then at 907 * least one return was lowered to a break, so we need to ensure 908 * that the return flag is checked after the body of the loop is 909 * executed. 910 */ 911 if(this->loop.may_set_return_flag) { 912 assert(this->function.return_flag); 913 /* Generate the if statement to check the return flag */ 914 ir_if* return_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->function.return_flag)); 915 /* Note: we also need to propagate the knowledge that the 916 * return flag may get set to the outer context. This 917 * satisfies the loop.may_set_return_flag part of the 918 * ANALYSIS postcondition. 919 */ 920 saved_loop.may_set_return_flag = true; 921 if(saved_loop.loop) 922 /* If this loop is nested inside another one, then the if 923 * statement that we generated should break out of that 924 * loop if the return flag is set. Caller will lower that 925 * break statement if necessary. 926 */ 927 return_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break)); 928 else 929 /* Otherwise, all we need to do is ensure that the 930 * instructions that follow are only executed if the 931 * return flag is clear. We can do that by moving those 932 * instructions into the else clause of the generated if 933 * statement. 934 */ 935 move_outer_block_inside(ir, &return_if->else_instructions); 936 ir->insert_after(return_if); 937 } 938 939 this->loop = saved_loop; 940 --this->function.nesting_depth; 941 } 942 943 virtual void visit(ir_function_signature *ir) 944 { 945 /* these are not strictly necessary */ 946 assert(!this->function.signature); 947 assert(!this->loop.loop); 948 949 bool lower_return; 950 if (strcmp(ir->function_name(), "main") == 0) 951 lower_return = lower_main_return; 952 else 953 lower_return = lower_sub_return; 954 955 function_record saved_function = this->function; 956 loop_record saved_loop = this->loop; 957 this->function = function_record(ir, lower_return); 958 this->loop = loop_record(ir); 959 960 assert(!this->loop.loop); 961 962 /* Visit the body of the function to lower any jumps that occur 963 * in it, except possibly an unconditional return statement at 964 * the end of it. 965 */ 966 visit_block(&ir->body); 967 968 /* If the body ended in an unconditional return of non-void, 969 * then we don't need to lower it because it's the one canonical 970 * return. 971 * 972 * If the body ended in a return of void, eliminate it because 973 * it is redundant. 974 */ 975 if (ir->return_type->is_void() && 976 get_jump_strength((ir_instruction *) ir->body.get_tail())) { 977 ir_jump *jump = (ir_jump *) ir->body.get_tail(); 978 assert (jump->ir_type == ir_type_return); 979 jump->remove(); 980 } 981 982 if(this->function.return_value) 983 ir->body.push_tail(new(ir) ir_return(new (ir) ir_dereference_variable(this->function.return_value))); 984 985 this->loop = saved_loop; 986 this->function = saved_function; 987 } 988 989 virtual void visit(class ir_function * ir) 990 { 991 visit_block(&ir->signatures); 992 } 993 }; 994 995 bool 996 do_lower_jumps(exec_list *instructions, bool pull_out_jumps, bool lower_sub_return, bool lower_main_return, bool lower_continue, bool lower_break) 997 { 998 ir_lower_jumps_visitor v; 999 v.pull_out_jumps = pull_out_jumps; 1000 v.lower_continue = lower_continue; 1001 v.lower_break = lower_break; 1002 v.lower_sub_return = lower_sub_return; 1003 v.lower_main_return = lower_main_return; 1004 1005 do { 1006 v.progress = false; 1007 visit_exec_list(instructions, &v); 1008 } while (v.progress); 1009 1010 return v.progress; 1011 } 1012
