2 * Copyright © 2010 Luca Barbieri
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21 * DEALINGS IN THE SOFTWARE.
25 * \file lower_jumps.cpp
27 * This pass lowers jumps (break, continue, and return) to if/else structures.
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
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
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.
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".
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.
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.
59 #include "compiler/glsl_types.h"
64 * Enum recording the result of analyzing how control flow might exit
67 * Each possible value of jump_strength indicates a strictly stronger
68 * guarantee on control flow than the previous value.
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.
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
80 * The control flow analysis made by this optimization pass makes two
81 * simplifying assumptions:
83 * - It ignores discard instructions, since they are lowered by a
84 * separate pass (lower_discard.cpp).
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).
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.
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.
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.
112 strength_always_clears_execute_flag
,
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.
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.
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.
140 /* minimum jump strength (of lowered IR, not pre-lowering IR)
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)
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.
149 jump_strength min_strength
;
151 /* can anything clear the execute flag? */
152 bool may_clear_execute_flag
;
156 this->min_strength
= strength_none
;
157 this->may_clear_execute_flag
= false;
163 ir_function_signature
* signature
;
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
;
170 bool may_set_return_flag
;
172 ir_variable
* break_flag
;
173 ir_variable
* execute_flag
; /* cleared to emulate continue */
175 loop_record(ir_function_signature
* p_signature
= 0, ir_loop
* p_loop
= 0)
177 this->signature
= p_signature
;
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;
186 ir_variable
* get_execute_flag()
188 /* also supported for the "function loop" */
189 if(!this->execute_flag
) {
190 exec_list
& list
= this->loop
? this->loop
->body_instructions
: signature
->body
;
191 this->execute_flag
= new(this->signature
) ir_variable(glsl_type::bool_type
, "execute_flag", ir_var_temporary
);
192 list
.push_head(new(this->signature
) ir_assignment(new(this->signature
) ir_dereference_variable(execute_flag
), new(this->signature
) ir_constant(true)));
193 list
.push_head(this->execute_flag
);
195 return this->execute_flag
;
198 ir_variable
* get_break_flag()
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)));
206 return this->break_flag
;
210 struct function_record
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
;
216 unsigned nesting_depth
;
218 function_record(ir_function_signature
* p_signature
= 0,
219 bool lower_return
= false)
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
;
228 ir_variable
* get_return_flag()
230 if(!this->return_flag
) {
231 this->return_flag
= new(this->signature
) ir_variable(glsl_type::bool_type
, "return_flag", ir_var_temporary
);
232 this->signature
->body
.push_head(new(this->signature
) ir_assignment(new(this->signature
) ir_dereference_variable(return_flag
), new(this->signature
) ir_constant(false)));
233 this->signature
->body
.push_head(this->return_flag
);
235 return this->return_flag
;
238 ir_variable
* get_return_value()
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
);
245 return this->return_value
;
249 struct ir_lower_jumps_visitor
: public ir_control_flow_visitor
{
250 /* Postconditions: on exit of any visit() function:
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.
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.
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.
266 * Note that visiting a jump does not lower it. That is the
267 * responsibility of the statement (or function signature) that
273 struct function_record function
;
274 struct loop_record loop
;
275 struct block_record block
;
280 bool lower_sub_return
;
281 bool lower_main_return
;
283 ir_lower_jumps_visitor()
285 pull_out_jumps(false),
286 lower_continue(false),
288 lower_sub_return(false),
289 lower_main_return(false)
293 void truncate_after_instruction(exec_node
*ir
)
298 while (!ir
->get_next()->is_tail_sentinel()) {
299 ((ir_instruction
*)ir
->get_next())->remove();
300 this->progress
= true;
304 void move_outer_block_inside(ir_instruction
*ir
, exec_list
*inner_block
)
306 while (!ir
->get_next()->is_tail_sentinel()) {
307 ir_instruction
*move_ir
= (ir_instruction
*)ir
->get_next();
310 inner_block
->push_tail(move_ir
);
315 * Insert the instructions necessary to lower a return statement,
316 * before the given return instruction.
318 void insert_lowered_return(ir_return
*ir
)
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();
324 new(ir
) ir_assignment(
325 new (ir
) ir_dereference_variable(return_value
),
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;
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.
340 * It is safe to pass NULL to this function.
342 void lower_return_unconditionally(ir_instruction
*ir
)
344 if (get_jump_strength(ir
) != strength_return
) {
347 insert_lowered_return((ir_return
*)ir
);
348 ir
->replace_with(new(ir
) ir_loop_jump(ir_loop_jump::jump_break
));
352 * Create the necessary instruction to replace a break instruction.
354 ir_instruction
*create_lowered_break()
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));
363 * If the given instruction is a break, lower it to an instruction
364 * that sets the break flag, without consulting
365 * should_lower_jump().
367 * It is safe to pass NULL to this function.
369 void lower_break_unconditionally(ir_instruction
*ir
)
371 if (get_jump_strength(ir
) != strength_break
) {
374 ir
->replace_with(create_lowered_break());
378 * If the block ends in a conditional or unconditional break, lower
379 * it, even though should_lower_jump() says it needn't be lowered.
381 void lower_final_breaks(exec_list
*block
)
383 ir_instruction
*ir
= (ir_instruction
*) block
->get_tail();
384 lower_break_unconditionally(ir
);
385 ir_if
*ir_if
= ir
->as_if();
387 lower_break_unconditionally(
388 (ir_instruction
*) ir_if
->then_instructions
.get_tail());
389 lower_break_unconditionally(
390 (ir_instruction
*) ir_if
->else_instructions
.get_tail());
394 virtual void visit(class ir_loop_jump
* ir
)
396 /* Eliminate all instructions after each one, since they are
397 * unreachable. This satisfies the DEAD_CODE_ELIMINATION
400 truncate_after_instruction(ir
);
402 /* Set this->block.min_strength based on this instruction. This
403 * satisfies the ANALYSIS postcondition. It is not necessary to
404 * update this->block.may_clear_execute_flag or
405 * this->loop.may_set_return_flag, because an unlowered jump
406 * instruction can't change any flags.
408 this->block
.min_strength
= ir
->is_break() ? strength_break
: strength_continue
;
410 /* The CONTAINED_JUMPS_LOWERED postcondition is already
411 * satisfied, because jump statements can't contain other
416 virtual void visit(class ir_return
* ir
)
418 /* Eliminate all instructions after each one, since they are
419 * unreachable. This satisfies the DEAD_CODE_ELIMINATION
422 truncate_after_instruction(ir
);
424 /* Set this->block.min_strength based on this instruction. This
425 * satisfies the ANALYSIS postcondition. It is not necessary to
426 * update this->block.may_clear_execute_flag or
427 * this->loop.may_set_return_flag, because an unlowered return
428 * instruction can't change any flags.
430 this->block
.min_strength
= strength_return
;
432 /* The CONTAINED_JUMPS_LOWERED postcondition is already
433 * satisfied, because jump statements can't contain other
438 virtual void visit(class ir_discard
* ir
)
440 /* Nothing needs to be done. The ANALYSIS and
441 * DEAD_CODE_ELIMINATION postconditions are already satisfied,
442 * because discard statements are ignored by this optimization
443 * pass. The CONTAINED_JUMPS_LOWERED postcondition is already
444 * satisfied, because discard statements can't contain other
450 enum jump_strength
get_jump_strength(ir_instruction
* ir
)
453 return strength_none
;
454 else if(ir
->ir_type
== ir_type_loop_jump
) {
455 if(((ir_loop_jump
*)ir
)->is_break())
456 return strength_break
;
458 return strength_continue
;
459 } else if(ir
->ir_type
== ir_type_return
)
460 return strength_return
;
462 return strength_none
;
465 bool should_lower_jump(ir_jump
* ir
)
467 unsigned strength
= get_jump_strength(ir
);
472 lower
= false; /* don't change this, code relies on it */
474 case strength_continue
:
475 lower
= lower_continue
;
478 assert(this->loop
.loop
);
479 /* never lower "canonical break" */
480 if(ir
->get_next()->is_tail_sentinel() && (this->loop
.nesting_depth
== 0
481 || (this->loop
.nesting_depth
== 1 && this->loop
.in_if_at_the_end_of_the_loop
)))
486 case strength_return
:
487 /* never lower return at the end of a this->function */
488 if(this->function
.nesting_depth
== 0 && ir
->get_next()->is_tail_sentinel())
491 lower
= this->function
.lower_return
;
497 block_record
visit_block(exec_list
* list
)
499 /* Note: since visiting a node may change that node's next
500 * pointer, we can't use visit_exec_list(), because
501 * visit_exec_list() caches the node's next pointer before
502 * visiting it. So we use foreach_in_list() instead.
504 * foreach_in_list() isn't safe if the node being visited gets
505 * removed, but fortunately this visitor doesn't do that.
508 block_record saved_block
= this->block
;
509 this->block
= block_record();
510 foreach_in_list(ir_instruction
, node
, list
) {
513 block_record ret
= this->block
;
514 this->block
= saved_block
;
518 virtual void visit(ir_if
*ir
)
520 if(this->loop
.nesting_depth
== 0 && ir
->get_next()->is_tail_sentinel())
521 this->loop
.in_if_at_the_end_of_the_loop
= true;
523 ++this->function
.nesting_depth
;
524 ++this->loop
.nesting_depth
;
526 block_record block_records
[2];
529 /* Recursively lower nested jumps. This satisfies the
530 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
531 * unconditional jumps at the end of ir->then_instructions and
532 * ir->else_instructions, which are handled below.
534 block_records
[0] = visit_block(&ir
->then_instructions
);
535 block_records
[1] = visit_block(&ir
->else_instructions
);
537 retry
: /* we get here if we put code after the if inside a branch */
539 /* Determine which of ir->then_instructions and
540 * ir->else_instructions end with an unconditional jump.
542 for(unsigned i
= 0; i
< 2; ++i
) {
543 exec_list
& list
= i
? ir
->else_instructions
: ir
->then_instructions
;
545 if(!list
.is_empty() && get_jump_strength((ir_instruction
*)list
.get_tail()))
546 jumps
[i
] = (ir_jump
*)list
.get_tail();
549 /* Loop until we have satisfied the CONTAINED_JUMPS_LOWERED
550 * postcondition by lowering jumps in both then_instructions and
554 /* Determine the types of the jumps that terminate
555 * ir->then_instructions and ir->else_instructions.
557 jump_strength jump_strengths
[2];
559 for(unsigned i
= 0; i
< 2; ++i
) {
561 jump_strengths
[i
] = block_records
[i
].min_strength
;
562 assert(jump_strengths
[i
] == get_jump_strength(jumps
[i
]));
564 jump_strengths
[i
] = strength_none
;
567 /* If both code paths end in a jump, and the jumps are the
568 * same, and we are pulling out jumps, replace them with a
569 * single jump that comes after the if instruction. The new
570 * jump will be visited next, and it will be lowered if
571 * necessary by the loop or conditional that encloses it.
573 if(pull_out_jumps
&& jump_strengths
[0] == jump_strengths
[1]) {
575 if(jump_strengths
[0] == strength_continue
)
576 ir
->insert_after(new(ir
) ir_loop_jump(ir_loop_jump::jump_continue
));
577 else if(jump_strengths
[0] == strength_break
)
578 ir
->insert_after(new(ir
) ir_loop_jump(ir_loop_jump::jump_break
));
579 /* FINISHME: unify returns with identical expressions */
580 else if(jump_strengths
[0] == strength_return
&& this->function
.signature
->return_type
->is_void())
581 ir
->insert_after(new(ir
) ir_return(NULL
));
588 this->progress
= true;
590 /* Update jumps[] to reflect the fact that the jumps
591 * are gone, and update block_records[] to reflect the
592 * fact that control can now flow to the next
597 block_records
[0].min_strength
= strength_none
;
598 block_records
[1].min_strength
= strength_none
;
600 /* The CONTAINED_JUMPS_LOWERED postcondition is now
601 * satisfied, so we can break out of the loop.
607 /* lower a jump: if both need to lowered, start with the strongest one, so that
608 * we might later unify the lowered version with the other one
610 bool should_lower
[2];
611 for(unsigned i
= 0; i
< 2; ++i
)
612 should_lower
[i
] = should_lower_jump(jumps
[i
]);
615 if(should_lower
[1] && should_lower
[0])
616 lower
= jump_strengths
[1] > jump_strengths
[0];
617 else if(should_lower
[0])
619 else if(should_lower
[1])
622 /* Neither code path ends in a jump that needs to be
623 * lowered, so the CONTAINED_JUMPS_LOWERED postcondition
624 * is satisfied and we can break out of the loop.
628 if(jump_strengths
[lower
] == strength_return
) {
629 /* To lower a return, we create a return flag (if the
630 * function doesn't have one already) and add instructions
631 * that: 1. store the return value (if this function has a
632 * non-void return) and 2. set the return flag
634 insert_lowered_return((ir_return
*)jumps
[lower
]);
635 if(this->loop
.loop
) {
636 /* If we are in a loop, replace the return instruction
637 * with a break instruction, and then loop so that the
638 * break instruction can be lowered if necessary.
640 ir_loop_jump
* lowered
= 0;
641 lowered
= new(ir
) ir_loop_jump(ir_loop_jump::jump_break
);
642 /* Note: we must update block_records and jumps to
643 * reflect the fact that the control path has been
644 * altered from a return to a break.
646 block_records
[lower
].min_strength
= strength_break
;
647 jumps
[lower
]->replace_with(lowered
);
648 jumps
[lower
] = lowered
;
650 /* If we are not in a loop, we then proceed as we would
651 * for a continue statement (set the execute flag to
652 * false to prevent the rest of the function from
657 this->progress
= true;
658 } else if(jump_strengths
[lower
] == strength_break
) {
659 /* To lower a break, we create a break flag (if the loop
660 * doesn't have one already) and add an instruction that
663 * Then we proceed as we would for a continue statement
664 * (set the execute flag to false to prevent the rest of
665 * the loop body from executing).
667 * The visit() function for the loop will ensure that the
668 * break flag is checked after executing the loop body.
670 jumps
[lower
]->insert_before(create_lowered_break());
672 } else if(jump_strengths
[lower
] == strength_continue
) {
674 /* To lower a continue, we create an execute flag (if the
675 * loop doesn't have one already) and replace the continue
676 * with an instruction that clears it.
678 * Note that this code path gets exercised when lowering
679 * return statements that are not inside a loop, so
680 * this->loop must be initialized even outside of loops.
682 ir_variable
* execute_flag
= this->loop
.get_execute_flag();
683 jumps
[lower
]->replace_with(new(ir
) ir_assignment(new (ir
) ir_dereference_variable(execute_flag
), new (ir
) ir_constant(false)));
684 /* Note: we must update block_records and jumps to reflect
685 * the fact that the control path has been altered to an
686 * instruction that clears the execute flag.
689 block_records
[lower
].min_strength
= strength_always_clears_execute_flag
;
690 block_records
[lower
].may_clear_execute_flag
= true;
691 this->progress
= true;
693 /* Let the loop run again, in case the other branch of the
694 * if needs to be lowered too.
699 /* move out a jump out if possible */
701 /* If one of the branches ends in a jump, and control cannot
702 * fall out the bottom of the other branch, then we can move
703 * the jump after the if.
705 * Set move_out to the branch we are moving a jump out of.
708 if(jumps
[0] && block_records
[1].min_strength
>= strength_continue
)
710 else if(jumps
[1] && block_records
[0].min_strength
>= strength_continue
)
715 jumps
[move_out
]->remove();
716 ir
->insert_after(jumps
[move_out
]);
717 /* Note: we must update block_records and jumps to reflect
718 * the fact that the jump has been moved out of the if.
721 block_records
[move_out
].min_strength
= strength_none
;
722 this->progress
= true;
726 /* Now satisfy the ANALYSIS postcondition by setting
727 * this->block.min_strength and
728 * this->block.may_clear_execute_flag based on the
729 * characteristics of the two branches.
731 if(block_records
[0].min_strength
< block_records
[1].min_strength
)
732 this->block
.min_strength
= block_records
[0].min_strength
;
734 this->block
.min_strength
= block_records
[1].min_strength
;
735 this->block
.may_clear_execute_flag
= this->block
.may_clear_execute_flag
|| block_records
[0].may_clear_execute_flag
|| block_records
[1].may_clear_execute_flag
;
737 /* Now we need to clean up the instructions that follow the
740 * If those instructions are unreachable, then satisfy the
741 * DEAD_CODE_ELIMINATION postcondition by eliminating them.
742 * Otherwise that postcondition is already satisfied.
744 if(this->block
.min_strength
)
745 truncate_after_instruction(ir
);
746 else if(this->block
.may_clear_execute_flag
)
748 /* If the "if" instruction might clear the execute flag, then
749 * we need to guard any instructions that follow so that they
750 * are only executed if the execute flag is set.
752 * If one of the branches of the "if" always clears the
753 * execute flag, and the other branch never clears it, then
754 * this is easy: just move all the instructions following the
755 * "if" into the branch that never clears it.
758 if(block_records
[0].min_strength
&& !block_records
[1].may_clear_execute_flag
)
760 else if(block_records
[1].min_strength
&& !block_records
[0].may_clear_execute_flag
)
764 assert(!block_records
[move_into
].min_strength
&& !block_records
[move_into
].may_clear_execute_flag
); /* otherwise, we just truncated */
766 exec_list
* list
= move_into
? &ir
->else_instructions
: &ir
->then_instructions
;
767 exec_node
* next
= ir
->get_next();
768 if(!next
->is_tail_sentinel()) {
769 move_outer_block_inside(ir
, list
);
771 /* If any instructions moved, then we need to visit
772 * them (since they are now inside the "if"). Since
773 * block_records[move_into] is in its default state
774 * (see assertion above), we can safely replace
775 * block_records[move_into] with the result of this
779 list
.head_sentinel
.next
= next
;
780 block_records
[move_into
] = visit_block(&list
);
783 * Then we need to re-start our jump lowering, since one
784 * of the instructions we moved might be a jump that
785 * needs to be lowered.
787 this->progress
= true;
791 /* If we get here, then the simple case didn't apply; we
792 * need to actually guard the instructions that follow.
794 * To avoid creating unnecessarily-deep nesting, first
795 * look through the instructions that follow and unwrap
796 * any instructions that that are already wrapped in the
799 ir_instruction
* ir_after
;
800 for(ir_after
= (ir_instruction
*)ir
->get_next(); !ir_after
->is_tail_sentinel();)
802 ir_if
* ir_if
= ir_after
->as_if();
803 if(ir_if
&& ir_if
->else_instructions
.is_empty()) {
804 ir_dereference_variable
* ir_if_cond_deref
= ir_if
->condition
->as_dereference_variable();
805 if(ir_if_cond_deref
&& ir_if_cond_deref
->var
== this->loop
.execute_flag
) {
806 ir_instruction
* ir_next
= (ir_instruction
*)ir_after
->get_next();
807 ir_after
->insert_before(&ir_if
->then_instructions
);
813 ir_after
= (ir_instruction
*)ir_after
->get_next();
815 /* only set this if we find any unprotected instruction */
816 this->progress
= true;
819 /* Then, wrap all the instructions that follow in a single
822 if(!ir
->get_next()->is_tail_sentinel()) {
823 assert(this->loop
.execute_flag
);
824 ir_if
* if_execute
= new(ir
) ir_if(new(ir
) ir_dereference_variable(this->loop
.execute_flag
));
825 move_outer_block_inside(ir
, &if_execute
->then_instructions
);
826 ir
->insert_after(if_execute
);
830 --this->loop
.nesting_depth
;
831 --this->function
.nesting_depth
;
834 virtual void visit(ir_loop
*ir
)
836 /* Visit the body of the loop, with a fresh data structure in
837 * this->loop so that the analysis we do here won't bleed into
840 * We assume that all code after a loop is reachable from the
841 * loop (see comments on enum jump_strength), so the
842 * DEAD_CODE_ELIMINATION postcondition is automatically
843 * satisfied, as is the block.min_strength portion of the
844 * ANALYSIS postcondition.
846 * The block.may_clear_execute_flag portion of the ANALYSIS
847 * postcondition is automatically satisfied because execute
848 * flags do not propagate outside of loops.
850 * The loop.may_set_return_flag portion of the ANALYSIS
851 * postcondition is handled below.
853 ++this->function
.nesting_depth
;
854 loop_record saved_loop
= this->loop
;
855 this->loop
= loop_record(this->function
.signature
, ir
);
857 /* Recursively lower nested jumps. This satisfies the
858 * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
859 * an unconditional continue or return at the bottom of the
860 * loop, which are handled below.
862 block_record body
= visit_block(&ir
->body_instructions
);
864 /* If the loop ends in an unconditional continue, eliminate it
865 * because it is redundant.
867 ir_instruction
*ir_last
868 = (ir_instruction
*) ir
->body_instructions
.get_tail();
869 if (get_jump_strength(ir_last
) == strength_continue
) {
873 /* If the loop ends in an unconditional return, and we are
874 * lowering returns, lower it.
876 if (this->function
.lower_return
)
877 lower_return_unconditionally(ir_last
);
879 if(body
.min_strength
>= strength_break
) {
880 /* FINISHME: If the min_strength of the loop body is
881 * strength_break or strength_return, that means that it
882 * isn't a loop at all, since control flow always leaves the
883 * body of the loop via break or return. In principle the
884 * loop could be eliminated in this case. This optimization
885 * is not implemented yet.
889 if(this->loop
.break_flag
) {
890 /* We only get here if we are lowering breaks */
891 assert (lower_break
);
893 /* If a break flag was generated while visiting the body of
894 * the loop, then at least one break was lowered, so we need
895 * to generate an if statement at the end of the loop that
896 * does a "break" if the break flag is set. The break we
897 * generate won't violate the CONTAINED_JUMPS_LOWERED
898 * postcondition, because should_lower_jump() always returns
899 * false for a break that happens at the end of a loop.
901 * However, if the loop already ends in a conditional or
902 * unconditional break, then we need to lower that break,
903 * because it won't be at the end of the loop anymore.
905 lower_final_breaks(&ir
->body_instructions
);
907 ir_if
* break_if
= new(ir
) ir_if(new(ir
) ir_dereference_variable(this->loop
.break_flag
));
908 break_if
->then_instructions
.push_tail(new(ir
) ir_loop_jump(ir_loop_jump::jump_break
));
909 ir
->body_instructions
.push_tail(break_if
);
912 /* If the body of the loop may set the return flag, then at
913 * least one return was lowered to a break, so we need to ensure
914 * that the return flag is checked after the body of the loop is
917 if(this->loop
.may_set_return_flag
) {
918 assert(this->function
.return_flag
);
919 /* Generate the if statement to check the return flag */
920 ir_if
* return_if
= new(ir
) ir_if(new(ir
) ir_dereference_variable(this->function
.return_flag
));
921 /* Note: we also need to propagate the knowledge that the
922 * return flag may get set to the outer context. This
923 * satisfies the loop.may_set_return_flag part of the
924 * ANALYSIS postcondition.
926 saved_loop
.may_set_return_flag
= true;
928 /* If this loop is nested inside another one, then the if
929 * statement that we generated should break out of that
930 * loop if the return flag is set. Caller will lower that
931 * break statement if necessary.
933 return_if
->then_instructions
.push_tail(new(ir
) ir_loop_jump(ir_loop_jump::jump_break
));
935 /* Otherwise, ensure that the instructions that follow are only
936 * executed if the return flag is clear. We can do that by moving
937 * those instructions into the else clause of the generated if
940 move_outer_block_inside(ir
, &return_if
->else_instructions
);
942 /* In case the loop is embedded inside an if add a new return to
943 * the return flag then branch and let a future pass tidy it up.
945 if (this->function
.signature
->return_type
->is_void())
946 return_if
->then_instructions
.push_tail(new(ir
) ir_return(NULL
));
948 assert(this->function
.return_value
);
949 ir_variable
* return_value
= this->function
.return_value
;
950 return_if
->then_instructions
.push_tail(
951 new(ir
) ir_return(new(ir
) ir_dereference_variable(return_value
)));
955 ir
->insert_after(return_if
);
958 this->loop
= saved_loop
;
959 --this->function
.nesting_depth
;
962 virtual void visit(ir_function_signature
*ir
)
964 /* these are not strictly necessary */
965 assert(!this->function
.signature
);
966 assert(!this->loop
.loop
);
969 if (strcmp(ir
->function_name(), "main") == 0)
970 lower_return
= lower_main_return
;
972 lower_return
= lower_sub_return
;
974 function_record saved_function
= this->function
;
975 loop_record saved_loop
= this->loop
;
976 this->function
= function_record(ir
, lower_return
);
977 this->loop
= loop_record(ir
);
979 assert(!this->loop
.loop
);
981 /* Visit the body of the function to lower any jumps that occur
982 * in it, except possibly an unconditional return statement at
985 visit_block(&ir
->body
);
987 /* If the body ended in an unconditional return of non-void,
988 * then we don't need to lower it because it's the one canonical
991 * If the body ended in a return of void, eliminate it because
994 if (ir
->return_type
->is_void() &&
995 get_jump_strength((ir_instruction
*) ir
->body
.get_tail())) {
996 ir_jump
*jump
= (ir_jump
*) ir
->body
.get_tail();
997 assert (jump
->ir_type
== ir_type_return
);
1001 if(this->function
.return_value
)
1002 ir
->body
.push_tail(new(ir
) ir_return(new (ir
) ir_dereference_variable(this->function
.return_value
)));
1004 this->loop
= saved_loop
;
1005 this->function
= saved_function
;
1008 virtual void visit(class ir_function
* ir
)
1010 visit_block(&ir
->signatures
);
1014 } /* anonymous namespace */
1017 do_lower_jumps(exec_list
*instructions
, bool pull_out_jumps
, bool lower_sub_return
, bool lower_main_return
, bool lower_continue
, bool lower_break
)
1019 ir_lower_jumps_visitor v
;
1020 v
.pull_out_jumps
= pull_out_jumps
;
1021 v
.lower_continue
= lower_continue
;
1022 v
.lower_break
= lower_break
;
1023 v
.lower_sub_return
= lower_sub_return
;
1024 v
.lower_main_return
= lower_main_return
;
1026 bool progress_ever
= false;
1029 visit_exec_list(instructions
, &v
);
1030 progress_ever
= v
.progress
|| progress_ever
;
1031 } while (v
.progress
);
1033 return progress_ever
;