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glsl: Lower break instructions when necessary at the end of a loop.

Browse source 
Normally lower_jumps.cpp doesn't need to lower a break instruction
that occurs at the end of a loop, because all back-ends can produce
proper GPU instructions for a break instruction in this "canonical"
location.  However, if other break instructions within the loop are
already being lowered, then a break instruction at the end of the loop
needs to be lowered too, since after the optimization is complete a
new conditional break will be inserted at the end of the loop.

Without this patch, lower_jumps.cpp may require multiple passes in
order to lower all jumps.  This results in sub-optimal output because
lower_jumps.cpp produces a brand new set of temporary variables each
time it is run, and the redundant temporary variables are not
guaranteed to be eliminated by later optimization passes.

Fixes unit test test_lower_breaks_6.
This commit is contained in:
Paul Berry 2011-07-01 17:29:35 -07:00
parent e71b4ab8a6
commit 067c9d7bd7
1 changed files with 54 additions and 1 deletions
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55
src/glsl/lower_jumps.cpp
View file

@ -341,6 +341,50 @@ struct ir_lower_jumps_visitor : public ir_control_flow_visitor {
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
@ -616,7 +660,7 @@ retry: /* we get here if we put code after the if inside a branch */
* 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));
jumps[lower]->insert_before(create_lowered_break());
goto lower_continue;
} else if(jump_strengths[lower] == strength_continue) {
lower_continue:
@ -836,6 +880,9 @@ lower_continue:
}
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
@ -843,7 +890,13 @@ lower_continue:
* 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);