/*
* 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 jump_strength
{
strength_none,
strength_always_clears_execute_flag,
strength_continue,
strength_break,
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 is_main;
unsigned nesting_depth;
function_record(ir_function_signature* p_signature = 0)
{
this->signature = p_signature;
this->return_flag = 0;
this->return_value = 0;
this->nesting_depth = 0;
this->is_main = this->signature && (strcmp(this->signature->function_name(), "main") == 0);
}
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 {
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);
}
}
virtual void visit(class ir_loop_jump * ir)
{
truncate_after_instruction(ir);
this->block.min_strength = ir->is_break() ? strength_break : strength_continue;
}
virtual void visit(class ir_return * ir)
{
truncate_after_instruction(ir);
this->block.min_strength = strength_return;
}
virtual void visit(class ir_discard * 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 if (this->function.is_main)
lower = lower_main_return;
else
lower = lower_sub_return;
break;
}
return lower;
}
block_record visit_block(exec_list* list)
{
block_record saved_block = this->block;
this->block = block_record();
visit_exec_list(list, 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];
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 */
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();
}
for(;;) {
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;
}
/* move both jumps out if possible */
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;
jumps[0] = 0;
jumps[1] = 0;
block_records[0].min_strength = strength_none;
block_records[1].min_strength = strength_none;
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
break;
if(jump_strengths[lower] == strength_return) {
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();
jumps[lower]->insert_before(new(ir) ir_assignment(new (ir) ir_dereference_variable(return_value), ((ir_return*)jumps[lower])->value, NULL));
}
jumps[lower]->insert_before(new(ir) ir_assignment(new (ir) ir_dereference_variable(return_flag), new (ir) ir_constant(true), NULL));
this->loop.may_set_return_flag = true;
if(this->loop.loop) {
ir_loop_jump* lowered = 0;
lowered = new(ir) ir_loop_jump(ir_loop_jump::jump_break);
block_records[lower].min_strength = strength_break;
jumps[lower]->replace_with(lowered);
jumps[lower] = lowered;
} else
goto lower_continue;
this->progress = true;
} else if(jump_strengths[lower] == strength_break) {
/* We can't lower to an actual continue because that would execute the increment.
*
* In the lowered code, we instead put the break check between the this->loop body and the increment,
* which is impossible with a real continue as defined by the GLSL IR currently.
*
* Smarter options (such as undoing the increment) are possible but it's not worth implementing them,
* because if break is lowered, continue is almost surely lowered too.
*/
jumps[lower]->insert_before(new(ir) ir_assignment(new (ir) ir_dereference_variable(this->loop.get_break_flag()), new (ir) ir_constant(true), 0));
goto lower_continue;
} else if(jump_strengths[lower] == strength_continue) {
lower_continue:
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));
jumps[lower] = 0;
block_records[lower].min_strength = strength_always_clears_execute_flag;
block_records[lower].may_clear_execute_flag = true;
this->progress = true;
break;
}
}
/* move out a jump out if possible */
if(pull_out_jumps) {
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]);
jumps[move_out] = 0;
block_records[move_out].min_strength = strength_none;
this->progress = true;
}
}
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;
if(this->block.min_strength)
truncate_after_instruction(ir);
else if(this->block.may_clear_execute_flag)
{
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);
exec_list list;
list.head = next;
block_records[move_into] = visit_block(&list);
this->progress = true;
goto retry;
}
} else {
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;
}
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)
{
++this->function.nesting_depth;
loop_record saved_loop = this->loop;
this->loop = loop_record(this->function.signature, ir);
block_record body = visit_block(&ir->body_instructions);
if(body.min_strength >= strength_break) {
/* FINISHME: turn the this->loop into an if, or replace it with its body */
}
if(this->loop.break_flag) {
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(this->loop.may_set_return_flag) {
assert(this->function.return_flag);
ir_if* return_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->function.return_flag));
saved_loop.may_set_return_flag = true;
if(saved_loop.loop)
return_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
else
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);
function_record saved_function = this->function;
loop_record saved_loop = this->loop;
this->function = function_record(ir);
this->loop = loop_record(ir);
assert(!this->loop.loop);
visit_block(&ir->body);
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;
}