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glsl: Add explanatory comments to lower_jumps.cpp.

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No functional change.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
This commit is contained in:
Paul Berry
2011-06-29 10:28:40 -07:00
parent f4830be938
commit e2c748aec5
1 changed files with 322 additions and 14 deletions
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src/glsl/lower_jumps.cpp
+322 -14
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@@ -60,12 +60,76 @@
#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
};
@@ -180,6 +244,27 @@ struct function_record
};
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;
@@ -220,18 +305,57 @@ struct ir_lower_jumps_visitor : public ir_control_flow_visitor {
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.
*/
}
enum jump_strength get_jump_strength(ir_instruction* ir)
@@ -304,18 +428,34 @@ struct ir_lower_jumps_visitor : public ir_control_flow_visitor {
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 */
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();
}
/* 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) {
@@ -326,7 +466,12 @@ retry: /* we get here if we put code after the if inside a branch */
jump_strengths[i] = strength_none;
}
/* move both jumps out if possible */
/* 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)
@@ -344,10 +489,19 @@ retry: /* we get here if we put code after the if inside a branch */
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;
}
}
@@ -367,9 +521,18 @@ retry: /* we get here if we put code after the if inside a branch */
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
*/
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();
@@ -378,29 +541,58 @@ retry: /* we get here if we put code after the if inside a branch */
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) {
/* 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
} 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) {
/* We can't lower to an actual continue because that would execute the increment.
/* To lower a break, we create a break flag (if the loop
* doesn't have one already) and add an instruction that
* sets it.
*
* 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.
* 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).
*
* 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.
* The visit() function for the loop will ensure that the
* break flag is checked after executing the loop body.
*/
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:
/* 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;
@@ -411,6 +603,12 @@ lower_continue:
/* 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;
@@ -421,22 +619,46 @@ lower_continue:
{
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;
@@ -451,14 +673,34 @@ lower_continue:
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();)
{
@@ -479,6 +721,9 @@ lower_continue:
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));
@@ -493,29 +738,87 @@ lower_continue:
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.
*/
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 */
/* 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) {
/* 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.
*/
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);
}
@@ -536,6 +839,11 @@ lower_continue:
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(this->function.return_value)