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On 07/05/2011 03:07 PM, Paul Berry wrote:
> 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, resulting in sub-optimal output.
>
> Fixes unit test test_lower_breaks_6.
> ---
> src/glsl/lower_jumps.cpp | 53
> +++++++++++++++++++++++++++++++++++++++++++++-
> 1 files changed, 52 insertions(+), 1 deletions(-)
>
> diff --git a/src/glsl/lower_jumps.cpp b/src/glsl/lower_jumps.cpp
> index 2424a54..9b5f8c4 100644
> --- a/src/glsl/lower_jumps.cpp
> +++ b/src/glsl/lower_jumps.cpp
> @@ -343,6 +343,48 @@ 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();
> + lower_break_unconditionally(
> + (ir_instruction *) ir_if->then_instructions.get_tail());
> + lower_break_unconditionally(
> + (ir_instruction *) ir_if->else_instructions.get_tail());
This looks suspicious. How do we know that the tail of the block is
always an if-statement? If as_if returns NULL, this will explode.
> + }
> +
> virtual void visit(class ir_loop_jump * ir)
> {
> /* Eliminate all instructions after each one, since they are
> @@ -618,7 +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:
> @@ -838,6 +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
> @@ -845,7 +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);
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