/* * Copyright © 2010 Luca Barbieri * * Permission is hereby granted, free of charge, to any person obtaining a * copy of this software and associated documentation files (the "Software"), * to deal in the Software without restriction, including without limitation * the rights to use, copy, modify, merge, publish, distribute, sublicense, * and/or sell copies of the Software, and to permit persons to whom the * Software is furnished to do so, subject to the following conditions: * * The above copyright notice and this permission notice (including the next * paragraph) shall be included in all copies or substantial portions of the * Software. * * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER * DEALINGS IN THE SOFTWARE. */ /** * \file lower_jumps.cpp * * This pass lowers jumps (break, continue, and return) to if/else structures. * * It can be asked to: * 1. Pull jumps out of ifs where possible * 2. Remove all "continue"s, replacing them with an "execute flag" * 3. Replace all "break" with a single conditional one at the end of the loop * 4. Replace all "return"s with a single return at the end of the function, * for the main function and/or other functions * * Applying this pass gives several benefits: * 1. All functions can be inlined. * 2. nv40 and other pre-DX10 chips without "continue" can be supported * 3. nv30 and other pre-DX10 chips with no control flow at all are better * supported * * Continues are lowered by adding a per-loop "execute flag", initialized to * true, that when cleared inhibits all execution until the end of the loop. * * Breaks are lowered to continues, plus setting a "break flag" that is checked * at the end of the loop, and trigger the unique "break". * * Returns are lowered to breaks/continues, plus adding a "return flag" that * causes loops to break again out of their enclosing loops until all the * loops are exited: then the "execute flag" logic will ignore everything * until the end of the function. * * Note that "continue" and "return" can also be implemented by adding * a dummy loop and using break. * However, this is bad for hardware with limited nesting depth, and * prevents further optimization, and thus is not currently performed. */ #include "glsl_types.h" #include #include "ir.h" /** * Enum recording the result of analyzing how control flow might exit * an IR node. * * Each possible value of jump_strength indicates a strictly stronger * guarantee on control flow than the previous value. * * The ordering of strengths roughly reflects the way jumps are * lowered: jumps with higher strength tend to be lowered to jumps of * lower strength. Accordingly, strength is used as a heuristic to * determine which lowering to perform first. * * This enum is also used by get_jump_strength() to categorize * instructions as either break, continue, return, or other. When * used in this fashion, strength_always_clears_execute_flag is not * used. * * The control flow analysis made by this optimization pass makes two * simplifying assumptions: * * - It ignores discard instructions, since they are lowered by a * separate pass (lower_discard.cpp). * * - It assumes it is always possible for control to flow from a loop * to the instruction immediately following it. Technically, this * is not true (since all execution paths through the loop might * jump back to the top, or return from the function). * * Both of these simplifying assumtions are safe, since they can never * cause reachable code to be incorrectly classified as unreachable; * they can only do the opposite. */ enum jump_strength { /** * Analysis has produced no guarantee on how control flow might * exit this IR node. It might fall out the bottom (with or * without clearing the execute flag, if present), or it might * continue to the top of the innermost enclosing loop, break out * of it, or return from the function. */ strength_none, /** * The only way control can fall out the bottom of this node is * through a code path that clears the execute flag. It might also * continue to the top of the innermost enclosing loop, break out * of it, or return from the function. */ strength_always_clears_execute_flag, /** * Control cannot fall out the bottom of this node. It might * continue to the top of the innermost enclosing loop, break out * of it, or return from the function. */ strength_continue, /** * Control cannot fall out the bottom of this node, or continue the * top of the innermost enclosing loop. It can only break out of * it or return from the function. */ strength_break, /** * Control cannot fall out the bottom of this node, continue to the * top of the innermost enclosing loop, or break out of it. It can * only return from the function. */ strength_return }; namespace { struct block_record { /* minimum jump strength (of lowered IR, not pre-lowering IR) * * If the block ends with a jump, must be the strength of the jump. * Otherwise, the jump would be dead and have been deleted before) * * 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 * (e.g. an if with a return in one branch, and a break in the other, while not lowering them) * Note that identical jumps are usually unified though. */ jump_strength min_strength; /* can anything clear the execute flag? */ bool may_clear_execute_flag; block_record() { this->min_strength = strength_none; this->may_clear_execute_flag = false; } }; struct loop_record { ir_function_signature* signature; ir_loop* loop; /* used to avoid lowering the break used to represent lowered breaks */ unsigned nesting_depth; bool in_if_at_the_end_of_the_loop; bool may_set_return_flag; ir_variable* break_flag; ir_variable* execute_flag; /* cleared to emulate continue */ loop_record(ir_function_signature* p_signature = 0, ir_loop* p_loop = 0) { this->signature = p_signature; this->loop = p_loop; this->nesting_depth = 0; this->in_if_at_the_end_of_the_loop = false; this->may_set_return_flag = false; this->break_flag = 0; this->execute_flag = 0; } ir_variable* get_execute_flag() { /* also supported for the "function loop" */ if(!this->execute_flag) { exec_list& list = this->loop ? this->loop->body_instructions : signature->body; this->execute_flag = new(this->signature) ir_variable(glsl_type::bool_type, "execute_flag", ir_var_temporary); list.push_head(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(execute_flag), new(this->signature) ir_constant(true), 0)); list.push_head(this->execute_flag); } return this->execute_flag; } ir_variable* get_break_flag() { assert(this->loop); if(!this->break_flag) { this->break_flag = new(this->signature) ir_variable(glsl_type::bool_type, "break_flag", ir_var_temporary); this->loop->insert_before(this->break_flag); this->loop->insert_before(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(break_flag), new(this->signature) ir_constant(false), 0)); } return this->break_flag; } }; struct function_record { ir_function_signature* signature; ir_variable* return_flag; /* used to break out of all loops and then jump to the return instruction */ ir_variable* return_value; bool lower_return; unsigned nesting_depth; function_record(ir_function_signature* p_signature = 0, bool lower_return = false) { this->signature = p_signature; this->return_flag = 0; this->return_value = 0; this->nesting_depth = 0; this->lower_return = lower_return; } ir_variable* get_return_flag() { if(!this->return_flag) { this->return_flag = new(this->signature) ir_variable(glsl_type::bool_type, "return_flag", ir_var_temporary); 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)); this->signature->body.push_head(this->return_flag); } return this->return_flag; } ir_variable* get_return_value() { if(!this->return_value) { assert(!this->signature->return_type->is_void()); return_value = new(this->signature) ir_variable(this->signature->return_type, "return_value", ir_var_temporary); this->signature->body.push_head(this->return_value); } return this->return_value; } }; struct ir_lower_jumps_visitor : public ir_control_flow_visitor { /* Postconditions: on exit of any visit() function: * * ANALYSIS: this->block.min_strength, * this->block.may_clear_execute_flag, and * this->loop.may_set_return_flag are updated to reflect the * characteristics of the visited statement. * * DEAD_CODE_ELIMINATION: If this->block.min_strength is not * strength_none, the visited node is at the end of its exec_list. * In other words, any unreachable statements that follow the * visited statement in its exec_list have been removed. * * CONTAINED_JUMPS_LOWERED: If the visited statement contains other * statements, then should_lower_jump() is false for all of the * return, break, or continue statements it contains. * * Note that visiting a jump does not lower it. That is the * responsibility of the statement (or function signature) that * contains the jump. */ bool progress; struct function_record function; struct loop_record loop; struct block_record block; bool pull_out_jumps; bool lower_continue; bool lower_break; bool lower_sub_return; bool lower_main_return; ir_lower_jumps_visitor() : progress(false), pull_out_jumps(false), lower_continue(false), lower_break(false), lower_sub_return(false), lower_main_return(false) { } void truncate_after_instruction(exec_node *ir) { if (!ir) return; while (!ir->get_next()->is_tail_sentinel()) { ((ir_instruction *)ir->get_next())->remove(); this->progress = true; } } void move_outer_block_inside(ir_instruction *ir, exec_list *inner_block) { while (!ir->get_next()->is_tail_sentinel()) { ir_instruction *move_ir = (ir_instruction *)ir->get_next(); move_ir->remove(); inner_block->push_tail(move_ir); } } /** * Insert the instructions necessary to lower a return statement, * before the given return instruction. */ void insert_lowered_return(ir_return *ir) { ir_variable* return_flag = this->function.get_return_flag(); if(!this->function.signature->return_type->is_void()) { ir_variable* return_value = this->function.get_return_value(); ir->insert_before( new(ir) ir_assignment( new (ir) ir_dereference_variable(return_value), ir->value)); } ir->insert_before( new(ir) ir_assignment( new (ir) ir_dereference_variable(return_flag), new (ir) ir_constant(true))); this->loop.may_set_return_flag = true; } /** * If the given instruction is a return, lower it to instructions * that store the return value (if there is one), set the return * flag, and then break. * * It is safe to pass NULL to this function. */ void lower_return_unconditionally(ir_instruction *ir) { if (get_jump_strength(ir) != strength_return) { return; } insert_lowered_return((ir_return*)ir); ir->replace_with(new(ir) ir_loop_jump(ir_loop_jump::jump_break)); } /** * Create the necessary instruction to replace a break instruction. */ ir_instruction *create_lowered_break() { void *ctx = this->function.signature; return new(ctx) ir_assignment( new(ctx) ir_dereference_variable(this->loop.get_break_flag()), new(ctx) ir_constant(true), 0); } /** * If the given instruction is a break, lower it to an instruction * that sets the break flag, without consulting * should_lower_jump(). * * It is safe to pass NULL to this function. */ void lower_break_unconditionally(ir_instruction *ir) { if (get_jump_strength(ir) != strength_break) { return; } ir->replace_with(create_lowered_break()); } /** * If the block ends in a conditional or unconditional break, lower * it, even though should_lower_jump() says it needn't be lowered. */ void lower_final_breaks(exec_list *block) { ir_instruction *ir = (ir_instruction *) block->get_tail(); lower_break_unconditionally(ir); ir_if *ir_if = ir->as_if(); if (ir_if) { lower_break_unconditionally( (ir_instruction *) ir_if->then_instructions.get_tail()); lower_break_unconditionally( (ir_instruction *) ir_if->else_instructions.get_tail()); } } virtual void visit(class ir_loop_jump * ir) { /* Eliminate all instructions after each one, since they are * unreachable. This satisfies the DEAD_CODE_ELIMINATION * postcondition. */ truncate_after_instruction(ir); /* Set this->block.min_strength based on this instruction. This * satisfies the ANALYSIS postcondition. It is not necessary to * update this->block.may_clear_execute_flag or * this->loop.may_set_return_flag, because an unlowered jump * instruction can't change any flags. */ this->block.min_strength = ir->is_break() ? strength_break : strength_continue; /* The CONTAINED_JUMPS_LOWERED postcondition is already * satisfied, because jump statements can't contain other * statements. */ } virtual void visit(class ir_return * ir) { /* Eliminate all instructions after each one, since they are * unreachable. This satisfies the DEAD_CODE_ELIMINATION * postcondition. */ truncate_after_instruction(ir); /* Set this->block.min_strength based on this instruction. This * satisfies the ANALYSIS postcondition. It is not necessary to * update this->block.may_clear_execute_flag or * this->loop.may_set_return_flag, because an unlowered return * instruction can't change any flags. */ this->block.min_strength = strength_return; /* The CONTAINED_JUMPS_LOWERED postcondition is already * satisfied, because jump statements can't contain other * statements. */ } virtual void visit(class ir_discard * ir) { /* Nothing needs to be done. The ANALYSIS and * DEAD_CODE_ELIMINATION postconditions are already satisfied, * because discard statements are ignored by this optimization * pass. The CONTAINED_JUMPS_LOWERED postcondition is already * satisfied, because discard statements can't contain other * statements. */ (void) ir; } enum jump_strength get_jump_strength(ir_instruction* ir) { if(!ir) return strength_none; else if(ir->ir_type == ir_type_loop_jump) { if(((ir_loop_jump*)ir)->is_break()) return strength_break; else return strength_continue; } else if(ir->ir_type == ir_type_return) return strength_return; else return strength_none; } bool should_lower_jump(ir_jump* ir) { unsigned strength = get_jump_strength(ir); bool lower; switch(strength) { case strength_none: lower = false; /* don't change this, code relies on it */ break; case strength_continue: lower = lower_continue; break; case strength_break: assert(this->loop.loop); /* never lower "canonical break" */ if(ir->get_next()->is_tail_sentinel() && (this->loop.nesting_depth == 0 || (this->loop.nesting_depth == 1 && this->loop.in_if_at_the_end_of_the_loop))) lower = false; else lower = lower_break; break; case strength_return: /* never lower return at the end of a this->function */ if(this->function.nesting_depth == 0 && ir->get_next()->is_tail_sentinel()) lower = false; else lower = this->function.lower_return; break; } return lower; } block_record visit_block(exec_list* list) { /* Note: since visiting a node may change that node's next * pointer, we can't use visit_exec_list(), because * visit_exec_list() caches the node's next pointer before * visiting it. So we use foreach_list() instead. * * foreach_list() isn't safe if the node being visited gets * removed, but fortunately this visitor doesn't do that. */ block_record saved_block = this->block; this->block = block_record(); foreach_list(node, list) { ((ir_instruction *) node)->accept(this); } block_record ret = this->block; this->block = saved_block; return ret; } virtual void visit(ir_if *ir) { if(this->loop.nesting_depth == 0 && ir->get_next()->is_tail_sentinel()) this->loop.in_if_at_the_end_of_the_loop = true; ++this->function.nesting_depth; ++this->loop.nesting_depth; block_record block_records[2]; ir_jump* jumps[2]; /* Recursively lower nested jumps. This satisfies the * CONTAINED_JUMPS_LOWERED postcondition, except in the case of * unconditional jumps at the end of ir->then_instructions and * ir->else_instructions, which are handled below. */ block_records[0] = visit_block(&ir->then_instructions); block_records[1] = visit_block(&ir->else_instructions); retry: /* we get here if we put code after the if inside a branch */ /* Determine which of ir->then_instructions and * ir->else_instructions end with an unconditional jump. */ for(unsigned i = 0; i < 2; ++i) { exec_list& list = i ? ir->else_instructions : ir->then_instructions; jumps[i] = 0; if(!list.is_empty() && get_jump_strength((ir_instruction*)list.get_tail())) jumps[i] = (ir_jump*)list.get_tail(); } /* Loop until we have satisfied the CONTAINED_JUMPS_LOWERED * postcondition by lowering jumps in both then_instructions and * else_instructions. */ for(;;) { /* Determine the types of the jumps that terminate * ir->then_instructions and ir->else_instructions. */ jump_strength jump_strengths[2]; for(unsigned i = 0; i < 2; ++i) { if(jumps[i]) { jump_strengths[i] = block_records[i].min_strength; assert(jump_strengths[i] == get_jump_strength(jumps[i])); } else jump_strengths[i] = strength_none; } /* If both code paths end in a jump, and the jumps are the * same, and we are pulling out jumps, replace them with a * single jump that comes after the if instruction. The new * jump will be visited next, and it will be lowered if * necessary by the loop or conditional that encloses it. */ if(pull_out_jumps && jump_strengths[0] == jump_strengths[1]) { bool unify = true; if(jump_strengths[0] == strength_continue) ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_continue)); else if(jump_strengths[0] == strength_break) ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_break)); /* FINISHME: unify returns with identical expressions */ else if(jump_strengths[0] == strength_return && this->function.signature->return_type->is_void()) ir->insert_after(new(ir) ir_return(NULL)); else unify = false; if(unify) { jumps[0]->remove(); jumps[1]->remove(); this->progress = true; /* Update jumps[] to reflect the fact that the jumps * are gone, and update block_records[] to reflect the * fact that control can now flow to the next * instruction. */ jumps[0] = 0; jumps[1] = 0; block_records[0].min_strength = strength_none; block_records[1].min_strength = strength_none; /* The CONTAINED_JUMPS_LOWERED postcondition is now * satisfied, so we can break out of the loop. */ break; } } /* lower a jump: if both need to lowered, start with the strongest one, so that * we might later unify the lowered version with the other one */ bool should_lower[2]; for(unsigned i = 0; i < 2; ++i) should_lower[i] = should_lower_jump(jumps[i]); int lower; if(should_lower[1] && should_lower[0]) lower = jump_strengths[1] > jump_strengths[0]; else if(should_lower[0]) lower = 0; else if(should_lower[1]) lower = 1; else /* Neither code path ends in a jump that needs to be * lowered, so the CONTAINED_JUMPS_LOWERED postcondition * is satisfied and we can break out of the loop. */ break; if(jump_strengths[lower] == strength_return) { /* To lower a return, we create a return flag (if the * function doesn't have one already) and add instructions * that: 1. store the return value (if this function has a * non-void return) and 2. set the return flag */ insert_lowered_return((ir_return*)jumps[lower]); if(this->loop.loop) { /* If we are in a loop, replace the return instruction * with a break instruction, and then loop so that the * break instruction can be lowered if necessary. */ ir_loop_jump* lowered = 0; lowered = new(ir) ir_loop_jump(ir_loop_jump::jump_break); /* Note: we must update block_records and jumps to * reflect the fact that the control path has been * altered from a return to a break. */ block_records[lower].min_strength = strength_break; jumps[lower]->replace_with(lowered); jumps[lower] = lowered; } else { /* If we are not in a loop, we then proceed as we would * for a continue statement (set the execute flag to * false to prevent the rest of the function from * executing). */ goto lower_continue; } this->progress = true; } else if(jump_strengths[lower] == strength_break) { /* To lower a break, we create a break flag (if the loop * doesn't have one already) and add an instruction that * sets it. * * Then we proceed as we would for a continue statement * (set the execute flag to false to prevent the rest of * the loop body from executing). * * The visit() function for the loop will ensure that the * break flag is checked after executing the loop body. */ jumps[lower]->insert_before(create_lowered_break()); goto lower_continue; } else if(jump_strengths[lower] == strength_continue) { lower_continue: /* To lower a continue, we create an execute flag (if the * loop doesn't have one already) and replace the continue * with an instruction that clears it. * * Note that this code path gets exercised when lowering * return statements that are not inside a loop, so * this->loop must be initialized even outside of loops. */ ir_variable* execute_flag = this->loop.get_execute_flag(); jumps[lower]->replace_with(new(ir) ir_assignment(new (ir) ir_dereference_variable(execute_flag), new (ir) ir_constant(false), 0)); /* Note: we must update block_records and jumps to reflect * the fact that the control path has been altered to an * instruction that clears the execute flag. */ jumps[lower] = 0; block_records[lower].min_strength = strength_always_clears_execute_flag; block_records[lower].may_clear_execute_flag = true; this->progress = true; /* Let the loop run again, in case the other branch of the * if needs to be lowered too. */ } } /* move out a jump out if possible */ if(pull_out_jumps) { /* If one of the branches ends in a jump, and control cannot * fall out the bottom of the other branch, then we can move * the jump after the if. * * Set move_out to the branch we are moving a jump out of. */ int move_out = -1; if(jumps[0] && block_records[1].min_strength >= strength_continue) move_out = 0; else if(jumps[1] && block_records[0].min_strength >= strength_continue) move_out = 1; if(move_out >= 0) { jumps[move_out]->remove(); ir->insert_after(jumps[move_out]); /* Note: we must update block_records and jumps to reflect * the fact that the jump has been moved out of the if. */ jumps[move_out] = 0; block_records[move_out].min_strength = strength_none; this->progress = true; } } /* Now satisfy the ANALYSIS postcondition by setting * this->block.min_strength and * this->block.may_clear_execute_flag based on the * characteristics of the two branches. */ if(block_records[0].min_strength < block_records[1].min_strength) this->block.min_strength = block_records[0].min_strength; else this->block.min_strength = block_records[1].min_strength; 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; /* Now we need to clean up the instructions that follow the * if. * * If those instructions are unreachable, then satisfy the * DEAD_CODE_ELIMINATION postcondition by eliminating them. * Otherwise that postcondition is already satisfied. */ if(this->block.min_strength) truncate_after_instruction(ir); else if(this->block.may_clear_execute_flag) { /* If the "if" instruction might clear the execute flag, then * we need to guard any instructions that follow so that they * are only executed if the execute flag is set. * * If one of the branches of the "if" always clears the * execute flag, and the other branch never clears it, then * this is easy: just move all the instructions following the * "if" into the branch that never clears it. */ int move_into = -1; if(block_records[0].min_strength && !block_records[1].may_clear_execute_flag) move_into = 1; else if(block_records[1].min_strength && !block_records[0].may_clear_execute_flag) move_into = 0; if(move_into >= 0) { assert(!block_records[move_into].min_strength && !block_records[move_into].may_clear_execute_flag); /* otherwise, we just truncated */ exec_list* list = move_into ? &ir->else_instructions : &ir->then_instructions; exec_node* next = ir->get_next(); if(!next->is_tail_sentinel()) { move_outer_block_inside(ir, list); /* If any instructions moved, then we need to visit * them (since they are now inside the "if"). Since * block_records[move_into] is in its default state * (see assertion above), we can safely replace * block_records[move_into] with the result of this * analysis. */ exec_list list; list.head = next; block_records[move_into] = visit_block(&list); /* * Then we need to re-start our jump lowering, since one * of the instructions we moved might be a jump that * needs to be lowered. */ this->progress = true; goto retry; } } else { /* If we get here, then the simple case didn't apply; we * need to actually guard the instructions that follow. * * To avoid creating unnecessarily-deep nesting, first * look through the instructions that follow and unwrap * any instructions that that are already wrapped in the * appropriate guard. */ ir_instruction* ir_after; for(ir_after = (ir_instruction*)ir->get_next(); !ir_after->is_tail_sentinel();) { ir_if* ir_if = ir_after->as_if(); if(ir_if && ir_if->else_instructions.is_empty()) { ir_dereference_variable* ir_if_cond_deref = ir_if->condition->as_dereference_variable(); if(ir_if_cond_deref && ir_if_cond_deref->var == this->loop.execute_flag) { ir_instruction* ir_next = (ir_instruction*)ir_after->get_next(); ir_after->insert_before(&ir_if->then_instructions); ir_after->remove(); ir_after = ir_next; continue; } } ir_after = (ir_instruction*)ir_after->get_next(); /* only set this if we find any unprotected instruction */ this->progress = true; } /* Then, wrap all the instructions that follow in a single * guard. */ if(!ir->get_next()->is_tail_sentinel()) { assert(this->loop.execute_flag); ir_if* if_execute = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.execute_flag)); move_outer_block_inside(ir, &if_execute->then_instructions); ir->insert_after(if_execute); } } } --this->loop.nesting_depth; --this->function.nesting_depth; } virtual void visit(ir_loop *ir) { /* Visit the body of the loop, with a fresh data structure in * this->loop so that the analysis we do here won't bleed into * enclosing loops. * * We assume that all code after a loop is reachable from the * loop (see comments on enum jump_strength), so the * DEAD_CODE_ELIMINATION postcondition is automatically * satisfied, as is the block.min_strength portion of the * ANALYSIS postcondition. * * The block.may_clear_execute_flag portion of the ANALYSIS * postcondition is automatically satisfied because execute * flags do not propagate outside of loops. * * The loop.may_set_return_flag portion of the ANALYSIS * postcondition is handled below. */ ++this->function.nesting_depth; loop_record saved_loop = this->loop; this->loop = loop_record(this->function.signature, ir); /* Recursively lower nested jumps. This satisfies the * CONTAINED_JUMPS_LOWERED postcondition, except in the case of * an unconditional continue or return at the bottom of the * loop, which are handled below. */ block_record body = visit_block(&ir->body_instructions); /* If the loop ends in an unconditional continue, eliminate it * because it is redundant. */ ir_instruction *ir_last = (ir_instruction *) ir->body_instructions.get_tail(); if (get_jump_strength(ir_last) == strength_continue) { ir_last->remove(); } /* If the loop ends in an unconditional return, and we are * lowering returns, lower it. */ if (this->function.lower_return) lower_return_unconditionally(ir_last); if(body.min_strength >= strength_break) { /* FINISHME: If the min_strength of the loop body is * strength_break or strength_return, that means that it * isn't a loop at all, since control flow always leaves the * body of the loop via break or return. In principle the * loop could be eliminated in this case. This optimization * is not implemented yet. */ } if(this->loop.break_flag) { /* We only get here if we are lowering breaks */ assert (lower_break); /* If a break flag was generated while visiting the body of * the loop, then at least one break was lowered, so we need * to generate an if statement at the end of the loop that * does a "break" if the break flag is set. The break we * generate won't violate the CONTAINED_JUMPS_LOWERED * postcondition, because should_lower_jump() always returns * false for a break that happens at the end of a loop. * * However, if the loop already ends in a conditional or * unconditional break, then we need to lower that break, * because it won't be at the end of the loop anymore. */ lower_final_breaks(&ir->body_instructions); ir_if* break_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.break_flag)); break_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break)); ir->body_instructions.push_tail(break_if); } /* If the body of the loop may set the return flag, then at * least one return was lowered to a break, so we need to ensure * that the return flag is checked after the body of the loop is * executed. */ if(this->loop.may_set_return_flag) { assert(this->function.return_flag); /* Generate the if statement to check the return flag */ ir_if* return_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->function.return_flag)); /* Note: we also need to propagate the knowledge that the * return flag may get set to the outer context. This * satisfies the loop.may_set_return_flag part of the * ANALYSIS postcondition. */ saved_loop.may_set_return_flag = true; if(saved_loop.loop) /* If this loop is nested inside another one, then the if * statement that we generated should break out of that * loop if the return flag is set. Caller will lower that * break statement if necessary. */ return_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break)); else /* Otherwise, all we need to do is ensure that the * instructions that follow are only executed if the * return flag is clear. We can do that by moving those * instructions into the else clause of the generated if * statement. */ move_outer_block_inside(ir, &return_if->else_instructions); ir->insert_after(return_if); } this->loop = saved_loop; --this->function.nesting_depth; } virtual void visit(ir_function_signature *ir) { /* these are not strictly necessary */ assert(!this->function.signature); assert(!this->loop.loop); bool lower_return; if (strcmp(ir->function_name(), "main") == 0) lower_return = lower_main_return; else lower_return = lower_sub_return; function_record saved_function = this->function; loop_record saved_loop = this->loop; this->function = function_record(ir, lower_return); this->loop = loop_record(ir); assert(!this->loop.loop); /* Visit the body of the function to lower any jumps that occur * in it, except possibly an unconditional return statement at * the end of it. */ visit_block(&ir->body); /* If the body ended in an unconditional return of non-void, * then we don't need to lower it because it's the one canonical * return. * * If the body ended in a return of void, eliminate it because * it is redundant. */ if (ir->return_type->is_void() && get_jump_strength((ir_instruction *) ir->body.get_tail())) { ir_jump *jump = (ir_jump *) ir->body.get_tail(); assert (jump->ir_type == ir_type_return); jump->remove(); } if(this->function.return_value) ir->body.push_tail(new(ir) ir_return(new (ir) ir_dereference_variable(this->function.return_value))); this->loop = saved_loop; this->function = saved_function; } virtual void visit(class ir_function * ir) { visit_block(&ir->signatures); } }; } /* anonymous namespace */ bool do_lower_jumps(exec_list *instructions, bool pull_out_jumps, bool lower_sub_return, bool lower_main_return, bool lower_continue, bool lower_break) { ir_lower_jumps_visitor v; v.pull_out_jumps = pull_out_jumps; v.lower_continue = lower_continue; v.lower_break = lower_break; v.lower_sub_return = lower_sub_return; v.lower_main_return = lower_main_return; bool progress_ever = false; do { v.progress = false; visit_exec_list(instructions, &v); progress_ever = v.progress || progress_ever; } while (v.progress); return progress_ever; }