/*
* 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 <string.h>
#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
};
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()
{
this->progress = 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);
}
};
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;
do {
v.progress = false;
visit_exec_list(instructions, &v);
} while (v.progress);
return v.progress;
}